Publications

Birth order and upper-secondary school tracking in Sweden: a mechanism for birth order inequality in educational attainment.​
​Marco Santacroce and Kieron Barclay (2025) Social Science Research

Abstract
Using Swedish register data, this study investigates the association between birth order and upper-secondary school track choice. A large body of research has shown that ordinal position within the sibling group matters for development trajectories and attainment processes. Researchers have also long been interested in the effects of secondary school tracking, showing that it can reinforce the effect of social origins. Using data for over 2 million pupils transitioning from compulsory to non-compulsory upper-secondary school from 1996 to 2019, and sibling fixed-effects, we find that later birth order is negatively associated with the probability of enrolling in university-preparatory academic tracks, known for having higher expected earnings and professional opportunities. These findings persist net of earlier educational performance, gender, parental education, or migration background. Later-born children are more likely to complete vocational programs. These findings shed light on some of the potential mechanisms driving the higher educational attainment, earnings, and employment stability of first- and earlier-born children, as they tend to complete secondary school tracks that provide greater future opportunities. The influence of birth order on completed years of education at age 30 diminishes by half when adjusting for track choices (i.e., secondary effects) and loses statistical significance when GPA (i.e., primary effects) is introduced as an additional control. While an unequivocal explanation for the origins of divergent tracking choices eludes us, existing literature suggests variation in parenting practices, child investments, and the familial environment contribute to these aspirational differences.

Leadership skills and completed fertility among males: a study based on data from Swedish registries.​
​Steffen Peters and Kieron Barclay (2025) Vienna Yearbook of Population Research

Abstract
Having leadership skills (LS) may increase an individual’s chances of ascending to a higher rank in hierarchical social structures, which can, in turn, provide the resources needed to support a partner and/or a child. Nevertheless, research on the association between LS and family formation processes (marriage, fertility) is scarce. We explore the prospective association between LS and marriage/completed fertility for 650941 Swedish males. Poisson regression and linear probabilitymodels are applied, including sibling fixed effects models. Our findings demonstrate a positive association between men’s LS, as measured at the age of assignment to military service (17–20 years), and their probability of marrying by age 39 or older (depending on the birth cohort). Furthermore, among the men in our sample, we find that LS are positively linked with the number of children, and are negatively linked with the probability of remaining childless. These associations are only partially explained by education and income.

More twins expected in low-income countries with later maternal ages at birth and population growth.
D. Susie Lee and Kieron Barclay (2025) Human Reproduction

Abstract
STUDY QUESTION
How are the changing maternal age structure and population growth expected to shape future twinning rates in low-income countries?
SUMMARY ANSWER
With maternal age at birth projected to shift toward older ages, twinning rates are also estimated to increase in most low-income countries by 2050 and even more by 2100.
WHAT IS KNOWN ALREADY
Many of the sub-Saharan African and South Asian countries are undergoing, and projected to further experience, the shift of maternal age at birth to older ages. Advanced maternal age is a well-established predictor of multiple births at the individual level, but currently, it is unknown how the changes in maternal age distribution are associated with the changes in twinning rates at the population level in low-income countries.
STUDY DESIGN, SIZE, DURATION
We first estimated age-specific twinning probability based on Demographic Health Surveys and World Fertility Surveys data. We then scaled up the age-specific twinning probability at the population level to estimate changes in the number of twin births in 2050 and 2100 attributable to the estimated shifts in maternal age toward older ages as projected by the UN World Population Prospects (WPP).
PARTICIPANTS/MATERIALS, SETTING, METHODS
We analyzed ∼3.19 million births that occurred within 10 years before the interview. Majority of the births in our data took place between 1980 and 2015 across 39 countries, where the uptake of medically assisted reproduction (MAR) is known to have been low during the observation period. We estimated country fixed-effects models to obtain country-specific twinning rates and age-specific twinning probability. We applied these estimates to the future number of births projected by the UN WPP, to estimate the number of twin births in 2050 and 2100.
MAIN RESULTS AND THE ROLE OF CHANCE
With maternal age at birth projected to shift toward older ages, twinning rates are also estimated to increase in most countries by 2050 compared to 2010 (increases from 0.3% to 63% depending on countries), and even more in all studied countries by 2100 (increases from 3.5% to 79%). Due to its large population size, India will continue to have among the largest share of twin births despite its estimated decline of twin births by 10.5% by 2100. Nigeria, due to its not only large and growing population size but also high twinning rate, is expected to contribute the second largest number of twin births.
LIMITATIONS, REASONS FOR CAUTION
Although the accuracy in maternal recall of multiple births tends to be high, our use of data based on recalled births from the past nonetheless imply a potential bias in our estimation of twinning rates.
WIDER IMPLICATIONS OF THE FINDINGS
The present study suggests that, even if the spread of MAR remains slow in many low-income countries, twinning rates and number of twin births are expected to grow as an increase in maternal age at birth and population growth continue. Our findings call for more public health attention and societal support to be paid to twins and their families in low-income countries, given that twins are at higher risk of developmental challenges and health disadvantages.

Pathways from education to mortality, mediated through income.
​Govert Bijwaard and Kieron Barclay (2025) Health Economics

Abstract
Less education is associated with higher mortality. Less education is also associated with lower income, which is also associated with higher mortality. We investigate the impact of education on mortality and the extent to which this is mediated through income over the life course. We account for both selective educational attainment and selective income over the life course, through inverse propensity weighting (IPW) of the mortality hazard. We decompose the educational gain, i.e. the decrease in mortality from more education, in the hazard into an indirect effect of education, running through changes in income and a direct effect of education, running through other factors. We use Swedish conscription data (men only), linked to parental information and individual annual income for the period 1968 till 2012. Our empirical results indicate large educational gains in mortality. We also find that this educational gradient is running through changes in income, especially for the more educated, and not running through other factors related to education. We conduct several robustness and sensitivity check that indicate that are results are robust.

The midlife health of only children: chronic disease indicators and biomarkers by sibship size in three nationally representative UK cohorts.
​Jenny Chanfreau, Katherine Keenan, Kieron Barclay and Alice Goisis (2024) International Journal of Epidemiology

Abstract
Background
Despite persistent concerns about only children’s disadvantage relative to individuals with siblings, existing health-related evidence is inconsistent. Recent evidence from Nordic countries about only children having poorer health outcomes may not apply elsewhere because selection processes differ across contexts. We investigate the midlife health of only children in the UK where one-child families tend to be socio-economically advantaged relative to large families.
Methods
Using the 1946, 1958 and 1970 British birth cohort studies, we examine various biomarkers and self-reported measures of chronic disease by sibship size when respondents are aged in their mid-40s, mid-50s and mid-60s. We estimate separate linear probability models for each cohort, age and outcome, adjusting for childhood and early adulthood circumstances.
Results
We found no evidence of only children differing from those with one, two or three or more siblings, at any age, in any of the cohorts, on: heart problems, hypertension, high triglycerides, high glycated haemoglobin or high C-reactive protein. However, compared with only children, the probability for cancer (0.019, 95% confidence interval [CI]: 0.002, 0.035; age 46/1970) and poor general health (0.060, CI: 0.015, 0.127; age 55/1958; and 0.110, CI: 0.052, 0.168; age 63/1946) was higher among those with three or more siblings.
Conclusions
There is no consistent pattern of only child health disadvantage for midlife chronic disease outcomes across ages or cohorts in the UK. Research should focus on better understanding how sibship size differentials are contingent on context.

The influence of extended kin on educational achievement: an examination of cousin order and cousin group size
​Kieron Barclay and Dalton Conley (2024) American Journal of Sociology

Abstract
Despite growing interest in the potential influence of grandparents on grandchild status attainment, research has not addressed whether the ordinal position or number of grandchildren affects outcomes. We apply sibling- and cousin-fixed effects analyses to Swedish population data to examine how cousin order and cousin group size influence grade point average (GPA) percentile rank at the end of compulsory school. We study cohorts born 1972-2003 (N=2,326,309). In sibling fixed effects analyses, second-born, fifth-born, and tenth or later born maternal cousins achieve GPA ranked scores 0.66, 1.04, and 1.29 percentile points lower than first-born cousins, respectively. Amongst paternal cousins the differences relative to the first-born cousin are 0.25, 0.47, and 0.20 percentile points low-er, respectively—suggesting the greater influence of the mother’s extended family. In further analyses we examine whether an arguably exogenous shock to cousin group size, a twin birth to an aunt or un-cle, has any impact on GPA percentile rank. Instrumental variable analyses indicate that an increase in maternal cousin group size has a statistically significant negative effect on GPA rank at low parities, while the effects of an increase in paternal cousin group size are less consistent.

Birth spacing and parents’ physical and mental health: an analysis using individual and sibling fixed effects
​Kieron Barclay, Martin Kolk, and Øystein Kravdal (2024) Demography

Abstract
There is a large literature examining the relationship between birth spacing and subsequent health outcomes for parents, and particularly for mothers. However, research on this topic draws almost exclusively on observational research designs, and almost all studies have been limited to adjusting for observable factors that may confound the relationship between birth spacing and health outcomes. In this study we use Norwegian register data to examine the relationship between birth spacing and the number of general practitioner consultations for physical and mental health concerns for both mothers and fathers in the period immediately after childbirth (1–5 and 6–11 months after the birth), the medium-term (5-6 years after childbearing) as well as the long-term (10-11 years after childbearing). To examine short-term health outcomes, we estimate individual fixed-effects models. This means that we hold constant factors that may influence the birth spacing behavior of mothers and fathers as well as their health, and compare health outcomes after different births to the same parent. We apply sibling fixed effects in our analysis of medium- and long-term outcomes, holding constant the family background of the mothers and fathers that we study. The results from our analyses that do not apply individual or sibling fixed effects yield results consistent with much of the previous literature, where both shorter and longer birth intervals are associated with worse health outcomes than birth intervals approximately 2-3 years long. Estimates from individual fixed effects models suggest that particularly short intervals have a modest negative effect on maternal mental health in the short-term, with more ambiguous evidence that particularly short- or long intervals may modestly influence physical health outcomes in the short-, medium- and long-term. Overall, these results are consistent with small to negligible effects of birth spacing behavior on (non-pregnancy related) parental health outcomes.

Maternal age and the risk of low birth weight and preterm delivery: a pan-Nordic comparison
​Siddartha Aradhya, Anna Tegunimataka, Øystein Kravdal, Pekka Martikainen, Mikko Myrskylä, Kieron Barclay, and Alice Goisis (2023) International Journal of Epidemiology

Abstract
Background
Advanced maternal age at birth is considered a risk factor for adverse birth outcomes. A recent study applying a sibling design has shown, however, that the association might be confounded by unobserved maternal characteristics.
Methods
Total population register data on all live singleton births during the period 1999-2012 in Denmark (N=580,133; 90% population coverage), Norway (N=540,890), and Sweden (N=941,403), and from 2001-2014 in Finland (N=568,026). We test whether advanced maternal age at birth independently increases the risk of low birth weight (<2,500 g) and preterm birth (<37 weeks’ gestation). We estimated within-family models to reduce confounding by unobserved maternal characteristics shared by siblings using three model specifications: Model 0 examines the bivariate association; Model 1 adjusts for parity and sex; Model 2 for parity, sex, and birth year.
Results
The main results (Model 1) show an increased risk of LBW and preterm delivery with increasing maternal ages. For example, compared with maternal ages of 26-27 years, maternal ages of 38-39 years display a 2.2, 0.9, 2.1, and 2.4 percentage point increase in the risk of LBW in Denmark, Finland, Norway, and Sweden respectively. The same patterns hold for pre-term delivery.
Conclusions
Advanced maternal age is independently associated with higher risk of poor perinatal health outcomes even after adjusting for all observed and unobserved factors shared between siblings.

Body mass index during early adulthood and first births: racial/ethnic and sex differences in the US NLSY79 cohort​
​Susie D. Lee, Natalie Nitsche and Kieron Barclay (2023) Population Studies

Abstract
Studies show that body mass index during early adulthood (“early BMI”) predicts first birth rates, but early childbearers tend to be omitted. This sample selection distorts the prevalence of childlessness and particularly the racial/ethnic heterogeneity therein because first birth timing differs by race/ethnicity. We imputed pre-parental early BMI for a larger sample that includes early childbearers for the same US NLSY79 data used in a previous study, and simulated differences in the probability of childlessness at age 40+ using posterior distributions based on the Bayesian framework. Obesity was consistently associated with higher childlessness across race/ethnic groups in both sexes, but only amongst obese women were first births delayed until after early adulthood. The overall higher childlessness among the underweight appeared largely driven by Black women. Our findings on the intersectionality of race/ethnicity and sex in the BMI-childlessness pathways encourage research on the underlying mechanisms and on more recent cohorts across different societies

Preterm births and educational disadvantage: heterogeneous effects across families and schools
​Anna Baranowska-Rataj, Kieron Barclay, Joan Costa-i-Font, Mikko Myrskylä and Berkay Özcan (2023) ​Population Studies

Abstract
Although preterm births are the leading cause of perinatal morbidity and mortality in advanced economies, evidence about the consequences of such births later in life is limited. Using Swedish register data on cohorts born 1982-1994 (N=1,087,750), we examine the effects of preterm births on school grades measured at age 16 using sibling fixed-effect models. We further examine how preterm births are affected by the degree of prematurity and the compensating role of family socioeconomic resources and characteristics of school districts. Our results show that the negative effects of preterm births are confined to children born extremely preterm (<28 weeks). Children born moderately preterm (32-<37 weeks) suffer no ill effects. We do not find any evidence for the moderating effect of parental socioeconomic resources. Extremely preterm children in the top decile of school districts have as good grades as full-term children in an average school district.

Health outcomes of only children across the life course: an investigation using Swedish register data​
Katherine Keenan, Kieron Barclay and Alice Goisis (2023​​) ​Population Studies

Abstract
​Only children – children with no full biological siblings – are a growing subgroup in many high-income settings, but we know little about their life course health outcomes. We address this research gap using Swedish population register data for cohorts born 1940-1975 and introducing conceptual and methodological innovations to the literature. We compare the health of only children with children from multi-child sibling groups, taking into account birth order, family size and the presence of half-siblings. Only children had lower height and fitness scores, were more likely to be overweight or obese in late adolescence, and had higher mortality, than those with 1 or 2 siblings. However, only children without half-siblings were always healthier than those with half-siblings, suggesting that parental disruption confers additional disadvantages. The health disadvantage was attenuated but not fully explained by adjustment for parental characteristics and after employing within-family maternal cousin comparison designs.

Birth order and health events attributable to alcohol and narcotics in midlife: A 25-year follow-up of a national Swedish birth cohort and their siblings
​Lauren Bishop and Kieron Barclay (2022) SSM – Population Health

Abstract
Higher birth order is associated with increased risks of adverse health outcomes attributable to alcohol or narcotics in adolescence, but it remains unclear whether these observed birth order effects are also present in midlife. Drawing on a national Swedish cohort born in 1953 and their siblings, we estimate associations between birth order and alcohol- or narcotics-attributable hospitalization or death with a 25-year follow-up to assess whether birth order differences are observed during this life course period. Health events attributable to alcohol or narcotics use were identified using the Swedish National Patient and Cause of Death registers, respectively. We apply Cox proportional hazards models to estimate average birth order differences in hazards for alcohol- or narcotics-attributable hospitalization or death between ages 30 and 55. We estimate birth order differences between families, and use two fixed-effects approaches to estimate birth order differences within families and within families of the same type. Bivariate results indicate increased hazards for both outcomes with higher birth order; however, these results are no longer observed after adjustment for familial background characteristics in all models. Our results thereby show limited evidence for birth order differences in midlife. This study highlights that shared factors within the family of origin may be stronger predictors of adverse health outcomes attributable to substance use among siblings during this life course period. Future research should disentangle the contributions of the social environment within the family of origin for adverse health outcomes attributable to alcohol or narcotics among siblings.

Sibling group size and BMI over the life course: evidence from four British cohort studies
​​Jenny Chanfreau, Kieron Barclay, Katherine Keenan and Alice Goisis (2022​​) Advances in Life Course Research

Abstract
Only children, here defined as individuals growing up without siblings, are a small but growing demographic subgroup. Existing research has consistently shown that, on average, only children have higher body mass index (BMI) than individuals who grow up with siblings. How this difference develops with age is unclear and existing evidence is inconclusive regarding the underlying mechanisms. We investigate BMI trajectories for only children and those with siblings up to late adolescence for four British birth cohorts and across adulthood for three cohorts. We use data on BMI from ages 2–63 years (cohort born 1946); 7–55 years (born 1958); 10–46 (born 1970) and 3–17 years (born 2000–2002). Using mixed effects regression separately for each cohort, we estimate the change in BMI by age comparing only children and those with siblings. The results show higher average BMI among only children in each cohort, yet the difference is substantively small and limited to school age and adolescence. The association between sibling status and BMI at age 10/11 is not explained by differential health behaviours (physical activity, inactivity and diet) or individual or family background characteristics in any of the cohorts. Although persistent across cohorts, and despite the underlying mechanism remaining unexplained, the substantively small magnitude of the observed difference and the convergence of the trajectories by early adulthood in all cohorts raises doubts about whether the difference in BMI between only children and siblings in the UK context should be of research or clinical concern. Future research could usefully be directed more at whether only children experience elevated rates of disease, for which high BMI is a risk factor, at different stages of the life course and across contexts.

Reduced reproductive success is associated with selective constraint on human genes
​Eugene J. Gardner, Matthew D. C. Neville, Kaitlin E. Samocha, Kieron Barclay, Martin Kolk, Mari E. K. Niemi, George Kirov, Hilary C. Martin and Matthew E. Hurles (2022​​) Nature

Abstract
​Genome-wide sequencing of human populations has revealed substantial variation among genes in the intensity of purifying selection acting on damaging genetic variants1. Although genes under the strongest selective constraint are highly enriched for associations with Mendelian disorders, most of these genes are not associated with disease and therefore the nature of the selection acting on them is not known2. Here we show that genetic variants that damage these genes are associated with markedly reduced reproductive success, primarily owing to increased childlessness, with a stronger effect in males than in females. We present evidence that increased childlessness is probably mediated by genetically associated cognitive and behavioural traits, which may mean that male carriers are less likely to find reproductive partners. This reduction in reproductive success may account for 20% of purifying selection against heterozygous variants that ablate protein-coding genes. Although this genetic association may only account for a very minor fraction of the overall likelihood of being childless (less than 1%), especially when compared to more influential sociodemographic factors, it may influence how genes evolve over time.

Genetic influences on educational achievement in comparative perspective
​Tina Baier, Volker Lang, Michael Grätz, Kieron Barclay, Dalton Conley, Chris Dawes, Thomas Laidley and Torkild H. Lyngstad (2022​​) ​European Sociological Review

Abstract
There is a growing interest in how social conditions moderate genetic influences on education (gene-environment interactions (GxE)). Previous research has focused on the family, specifically parents’ social background, and has neglected the institutional environment. To assess the impact of macro-level influences we compare genetic influences on educational achievement and their social stratification across Germany, Norway, Sweden, and the US. We combine well-established GxE-conceptualizations with the comparative stratification literature and propose that educational systems and welfare state regimes affect the realization of genetic potential. We analyze population-representative survey data on twins (Germany and the US) and twin registers (Norway and Sweden), and estimate genetically sensitive variance decomposition models. Our comparative design yields three main findings. First, Germany stands out with comparatively weak genetic influences on educational achievement suggesting that early tracking limits the realization thereof. Second, in the US genetic influences are comparatively strong and similar in size to the Nordic countries. Third, in Sweden genetic influences are stronger among disadvantaged families supporting the expectation that challenging and uncertain circumstances promote genetic expression. That we observe this pattern only in Sweden indicates that the underlying processes cannot be linked to institutional features that do not vary between Norway and Sweden.

Birth spacing and health and socioeconomic outcomes across the life course: evidence from the Utah Population Database
​Kieron Barclay and Ken R. Smith (2022​​) ​Demography

Abstract
​The relationship between the length of birth intervals and child outcomes has received increased attention in recent years, but few studies have examined offspring outcomes across the life course in North America. In this study we examine the relationship between birth intervals and a range of short- and long-term outcomes, including preterm birth, low birth weight, infant mortality, college graduation, occupational attainment, and adult mortality, using data from the Utah Population Database (UPDB). To study infant outcomes we use data on cohorts born 1947--2019, to study educational and occupational outcomes we use data on cohorts born 1950--1990, and to study adult mortality we use data on cohorts born 1900--1949, with mortality outcomes followed until 2019. We use linear regression, linear probability models, and survival analysis, and compare the results from models with and without sibling comparisons. Children born after a birth interval of 9-12 months have a higher probability of low birth weight, preterm birth, and infant mortality both with and without sibling comparisons, while longer intervals are associated with a lower probability of these outcomes. Short intervals before the birth of the next youngest sibling are also associated with LBW, preterm birth, and infant mortality both with and without sibling comparisons. This pattern raises concerns that the sibling comparison models are not fully adjusting for within-family factors predicting both spacing and perinatal outcomes. Considering long-term outcomes, not even the very shortest birth intervals are negatively associated with educational or occupational outcomes, nor long-term mortality, in sibling comparison analyses. These findings suggest that extremely short birth intervals may increase the probability of poor perinatal outcomes, but that any such disadvantages disappear over the extended life course.

Does the impact of parental death vary by parental socioeconomic status? A study of children’s educational and occupational attainment
Kieron Barclay and Martin Hällsten (2022​​) ​Journal of Marriage and Family

Abstract
Objective: We examine whether parental death differentially affects educational and occupational attainment by the socioeconomic status of the parent who dies and the socioeconomic status of the surviving parent and extended kin.
Background: An extensive literature has explored the main effect of parental death on offspring attainment, but few studies have examined socioeconomic differentials in the impact of parental death. Understanding the potential role of socioeconomic resources in compensating for disadvantage is important for understanding whether parental death, and disadvantageous events more generally, have an equalizing or exacerbating effect on socioeconomic differences in offspring socioeconomic attainment.
Method: Using Swedish population register data on cohorts born 1973—1982 we examine grade point average at age 16, the transition from lower to upper-secondary education, the transition to tertiary education, overall educational attainment, and occupational status by age 30. We match families using antemortem parental socioeconomic trajectories. We also employ sibling fixed effects models.
Results: We observe inconsistent results in our between-family regression analyses adjusting for observables. In sibling fixed effects models, we see zero results for moderation by parents’ occupations.
Conclusion: We find little clear or convincing evidence that there are socioeconomic differentials in the impact of parental death in Sweden.
​Implications: The Swedish welfare state may reduce socioeconomic differentials in the impact of parental death. However, the lack of socioeconomic variation may also be influenced by factors such as compensatory agency.

The production of inequalities within families and across generations: the intergenerational effects of birth order on educational attainment
Kieron Barclay, Torkild H. Lyngstad, and Dalton Conley (2021​​) ​European Sociological Review

Abstract
There has long been interest in the extent to which effects of social stratification extend and persist across generations. We take a novel approach to this question by asking whether birth order in the parental generation influences the educational attainment of their children. To address this question, we use Swedish population data on cohorts born 1960-1982. To study the effects of parental birth order we use cousin fixed effects comparisons. In analyses where we compare cousins who share the same biological grandparents to adjust for unobserved factors in the extended family, we find that having a later-born parent reduces educational attainment to a small extent. For example, a second- or fifth-born mother reduces educational attainment by 0.09 and 0.18 years, respectively, while having a second- or fifth-born father reduces educational attainment by 0.04 and 0.11 years, respectively. After adjusting for attained parental education and social class, the parental birth order effect is practically attenuated to zero. Overall our results suggest that parental birth order influences offspring educational and socioeconomic outcomes through the parents own educational and socioeconomic attainment. We cautiously suggest that parental birth order may have potential as an instrument for parental socioeconomic status in social stratification research more generally.

Sibling similarity in education across and within societies
Michael Grätz, Kieron Barclay, Øyvind Wiborg, Torkild H. Lyngstad, Aleksi Karhula, Jani Erola, Patrick Präg, Thomas M. Laidley, and Dalton Conley (​2021​) ​Demography

Abstract
The extent to which siblings resemble each other measures the omnibus impact of family background on life chances. We study sibling similarity in cognitive skills, school grades, and educational attainment in Finland, Germany, Norway, Sweden, the United Kingdom, and the United States. We also compare sibling similarity by parental education and occupation within these societies. The comparison of sibling correlations across and within societies allows us to characterize the omnibus impact of family background on education across social landscapes. Across countries, we find larger differences in sibling similarity at the population level for final educational attainment than for cognitive skills and for school grades. In general, sibling similarity in education varies less across countries than sibling similarity in earnings. In addition, rankings of countries in sibling resemblance differ across the three educational outcomes we study. We find socioeconomic differences in sibling similarity to vary across parental resources, countries, and measures of educational success. In particular, Sweden and the United States show a higher sibling similarity in educational attainment in families with a high level of father’s education and Finland, Germany, and Norway show a higher sibling similarity in educational attainment in families with a low level of father’s education. We discuss the implications of our results for theories about the impact of institutions and income inequality on and the mechanisms underlying educational inequality.

Do income and marriage mediate the relationship between cognitive ability and fertility? Data from Swedish taxation and conscriptions registers for men born 1951-1967
​Martin Kolk and Kieron Barclay (2021​) ​Intelligence

Abstract
Recent evidence suggests a positive association between fertility and cognitive ability among Swedish men. In this study we use data on 18 birth cohorts of Swedish men to examine whether and how the relationship between cognitive ability and patterns of childbearing are mediated by income, education and marriage histories. We examine whether the expected positive associations between cognitive ability and life course income, can explain this positive association. We also explore the role of marriage for understanding the positive gradient between cognitive ability and fertility. To address these questions we use Swedish population administrative data that holds information on fertility histories, detailed taxation records, and data from conscription registers. We also identify siblings in order to adjust for confounding by shared family background factors. Our results show that while cognitive ability, education, income, marriage, and fertility, are all positively associated with each other, income only explains a part of the observed positive gradient between fertility and cognitive ability. We find that much of the association between cognitive ability and fertility can be explained by marriage, but that a positive association exists among both ever-married and never-married men. Both low income and low cognitive ability are strong predictors of childlessness and low fertility in our population. The results from the full population persist in the sub-sample of brothers.

Are skewed sex ratios associated with violent crime? A longitudinal analysis using Swedish register data
Andreas Filser, Kieron Barclay, Amber Beckley, Caroline Uggla and Sebastian Schnettler (2021​) ​Evolution and Human Behavior

Abstract
There is widespread concern in both the popular and academic literature that a surplus of men in a population intensifies mating competition between men, particularly unpartnered men, resulting in increased violence towards both men and women. Recent contributions challenge this perspective and argue that male mating competition and levels of violence will be higher when sex ratios are female-skewed. Existing empirical evidence remains inconclusive. We argue that this empirical ambiguity results from analyses of aggregate-level data, which put inferences at risk of ecological fallacies. Our analysis circumvents such problems by using individual-level, longitudinal demographic register and police data for the Stockholm metropolitan area, Sweden (1990-2003, n=758,498). These data allow us to investigate the association between municipality-level sex ratios and violent offending (homicide, assault, threat, and sexual crimes) while adjusting for sociodemographic factors. Results suggest that aggregated offending rates are negatively associated with male-skewed sex ratios, whereas individual-level violent offending correlates positively with male-skews. We find that the more-men-more-violence association particularly holds for male violence against other men, but is insignificant for violence against women. Moreover, the association is significant among childless men, but not among fathers. However, robustness checks question the causality of these associations. Female violent offending is positively, albeit due to a low number of cases, insignificantly associated with male-skews. Moreover, both male and female non-violent offending is higher in male-skewed municipalities. These results contradict theoretical predictions. We discuss the implications with regard to the theoretical debate and problems of unobserved heterogeneity in the sex ratio literature.

The influence of health in early adulthood on male fertility
Kieron Barclay and Martin Kolk (2020​) ​Population and Development Review

Abstract
Despite the large literature examining predictors of fertility, previous research has not offered a population-level perspective on how health in early adulthood is related to male fertility. Using Swedish population and military conscription registers, we study how body mass index (BMI), physical fitness and height are associated with total fertility and parity transitions by 2012 among 405,427 Swedish men born 1965-1972, meaning we observe fertility up to age 40 or older. Applying linear regression and sibling fixed effects, we find that these anthropometric measures are strong predictors of fertility, even after accounting for education and cumulative income. Men with a ‘normal’ BMI and in the highest decile of physical fitness have the most children. Men who were obese at ages 17-20 had a relative probability of childlessness almost twice as high as men who had a ‘normal’ BMI, and men in the bottom decile of physical fitness had a relatively probability of childlessness more than 50% higher than men in the top decile. In sibling comparison models the tallest men have the most children and men in the lowest two deciles of height have significantly lower fertility. Further analyses show that the strong associations persist even among men who married.

The effects of marital status, fertility, and bereavement on adult mortality in polygamous and monogamous households: evidence from the Utah Population Database
Kieron Barclay, Robyn Donrovich Thorén, Heidi Hanson and Ken R. Smith (2020​) ​Demography

Abstract
Although the associations between marital status, fertility, bereavement and adult mortality have been widely studied, much less is known about these associations in polygamous households, which remain prevalent across much of the world. We use data from the Utah Population Database on 110,890 women and 106,979 men born up to 1900, with mortality follow-up into the 20th century. We examine how the number of wife deaths affects male mortality in polygamous marriages, how sister wife deaths affect female mortality in polygamous marriages relative to the death of a husband, and how marriage order affects the mortality of women in polygamous marriages. We also examine how number of children ever born and child deaths affect the mortality of men and women, and variation across monogamous and polygamous unions. Our analyses of women show that the death of a husband and the death of a sister wife have similar effects on mortality. Marriage order does not play a role in the mortality of women in polygamous marriages. For men, the death of one wife in a polygamous marriage increases mortality to a lesser extent than it does for men in monogamous marriages. For polygamous men there is a dose-response effect to losing additional wives. Child deaths and lower fertility are both associated with higher mortality. We consistently find that the presence of other kin in the household, whether they be a second wife, a sister wife, or children, serves to mitigate the negative effects of bereavement.

Interpregnancy intervals and perinatal and child health in Sweden: a comparison within families and across social groups
​Kieron Barclay, Anna Baranowska-Rataj, Martin Kolk and Anneli Ivarsson (2020​) ​Population Studies

Abstract
A large body of research has shown that children born after especially short or long birth intervals have an elevated risk of poor perinatal outcomes, but recent work suggests this may be explained by confounding by unobserved family characteristics. We use Swedish population data on cohorts born 1981-2010 and sibling fixed effects to examine whether the length of the birth interval preceding the index person influences the risk of preterm birth, low birth weight, and hospitalization during childhood. We also present analyses stratified by salient social characteristics such as maternal educational level and maternal country of birth. We find few effects of birth intervals on our outcomes except for very short intervals (<7 months), and very long intervals (>60 months). We find few differences in the patterns by maternal educational level, or by maternal country of origin after stratifying by the mother’s highest attained education. ​

When and where birth spacing matters for child survival: an international comparison using the DHS
​Joseph Molitoris, Kieron Barclay and Martin Kolk (2019​) ​Demography

Abstract
A large body of research has found an association between short birth intervals and the risk of infant mortality in developing countries, but recent work on other perinatal outcomes from highly developed countries has called these claims into question, arguing that previous studies have failed to adequately control for unobserved heterogeneity. Our study addresses this issue by estimating within-family models on a sample of 4.5 million births from 77 countries at various levels of development. We show that even after controlling for unobserved maternal heterogeneity, intervals less than 24 months substantially increase the probability of infant death. We do show, however, that the importance of birth intervals as a determinant of infant mortality varies inversely with maternal education and the strength of the relationship varies regionally. Finally, we demonstrate that the mortality-reducing effects of longer birth intervals are strong at low levels of development but decline steadily towards zero at higher levels of development. These findings offer a clear way to reconcile previous research showing that birth intervals are important for perinatal outcomes in low-income countries, but much less consequential in high-income settings.

Cognitive ability and fertility among Swedish men born 1951-1967. Evidence from military conscription registers.
Martin Kolk and Kieron Barclay (2019​) ​Proceedings of the Royal Society B: Biological Sciences

Abstract
We examine the relationship between cognitive ability and childbearing patterns in contemporary Sweden using administrative register data. The topic has a long history in the social sciences and has been the topic of a large number of studies, many reporting a negative gradient between intelligence and fertility. We link fertility histories to military conscription tests with intelligence scores for all Swedish men born 1951 to 1967. We find a positive relationship between intelligence scores and fertility, and this pattern is consistent across the cohorts we study. The relationship is most pronounced for the transition to a first child, and men with the lowest categories of IQ-scores have the fewest children. Using fixed effects models we additionally control for all factors that are shared by siblings, and after such adjustments we find a stronger positive relationship between IQ and fertility. Furthermore, we find a positive gradient within groups at different levels of education. Compositional differences of this kind are therefore not responsible for the positive gradient we observe - instead the relationship is even stronger after controlling for both educational careers and parental background factors. In our models where we compare brothers to one another we find that, relative to men with IQ 100, the group with the lowest category of cognitive ability have 0.58 fewer children, and men with the highest category have 0.14 more children. 

Parity and mortality: an examination of different explanatory mechanisms using data on biological and adoptive parents
​​Kieron Barclay and Martin Kolk (2019​) ​European Journal of Population

Abstract
A growing literature has demonstrated a relationship between parity and mortality, but the explanation for that relationship remains unclear. This study aims to pick apart physiological and social explanations for the parity-mortality relationship by examining the mortality of parents who adopt children but who have no biological children in comparison to the mortality of parents with biological children. Using Swedish register data, we study post-reproductive mortality amongst women and men from cohorts born between 1915 and 1960, over ages 45 to 97. Our results show the relative risks of mortality for adoptive parents are always lower than those of parents with biological children. Mortality amongst adoptive parents is lower for those who adopt more than one child, while for parents with biological children we observe a U-shaped relationship, where parity-two parents have the lowest mortality. Our discussion considers the relative importance of physiological and social depletion effects, and selection processes.

Paternal age and the risk of low birth weight and preterm delivery: a Finnish register-based study
​Alice Goisis, Hanna Remes, Kieron Barclay, Pekka Martikainen and Mikko Myrsklyä (2018​) ​
​Journal of Epidemiology and Community Health

Abstract
Background: Based on existing studies, there is no conclusive evidence as to whether and why paternal age matters for birth outcomes.
Methods: We used Finnish Population Registers on 106,652 children born 1987-2000. We first document the unadjusted association between paternal age and the risk of low birth weight (<2500g; LBW) and preterm birth (<37 weeks gestation). Second, we investigate whether the unadjusted association is attenuated on adjustment for child’s, maternal and parental socio-economic characteristics. Third, by adopting a within-family design which involves comparing children born to the same father at different ages, we additionally adjust for unobserved parental characteristics shared between siblings.
Results: The unadjusted results show that being born to a father aged 40+, as opposed to a father 30-34, is associated with an increased risk of LBW of 0.96 percent (95% CI: 0.5%-1.3%) and to a younger father (<25) with a 1 percent (95% CI: 0.6%-1.3%) increased risk. The increased risk at younger paternal ages is halved on adjustment for the child’s characteristics and fully attenuated on adjustment for child/parental characteristics. The increased risk at paternal ages 40+ is partially attenuated on adjustment for maternal characteristics (β=0.62%; 95% CI: 0.13%-1.1%). Adjustment for unobserved parental characteristics shared by siblings further attenuates the 40+ coefficient (β=0.4%; 95% CI: -0.5%-1.2%). Results for preterm delivery are similar.
Conclusions: the results underscore the importance of considering paternal age as a potential risk factor for adverse birth outcomes and of expanding research on its role and the mechanisms linking it to birth outcomes. 

The birth order paradox: sibling differences in educational attainment
​​Kieron Barclay (2018​) ​Research in Social Stratification and Mobility

Abstract
This study uses population register data to examine the relationship between birth order and educational attainment in Sweden, and demonstrates that while the net effect of birth order on educational attainment is negative, later-born children often spend longer in education. The explanation for this finding is due to educational expansion in Sweden in the 20th century, which outweighs the negative causal effect of birth order for the affected cohorts. This is particularly true for women due to the fact that the rate of increasing educational enrolment has been greater for women than for men. These results also show that later-borns in large families particularly benefit from educational expansion due to the longer average birth interval between the first and last child in large families, meaning that the supply of educational opportunities increased to a greater extent in the intervening period. However, in periods where education is not expanding, later-born siblings continue to fare worse than first-borns.
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Birth intervals and health in adulthood: a comparison of siblings using Swedish register data
​​Kieron Barclay and Martin Kolk (2018​) ​Demography

Abstract
A growing body of research has examined whether birth intervals influence peri-natal outcomes and child health, as well as long-term educational and socioeconomic outcomes; to date, however, very little research has examined whether birth spacing influences long-term health. We use contemporary Swedish population register data to examine the relationship between birth-to-birth intervals and a variety of health outcomes in adulthood, including, for men: height, physical fitness, and the probability of falling into different body mass index (BMI) categories, and, for both men and women: mortality. In models where we do not adjust carefully for family background we find that short and long birth intervals are clearly associated with height, physical fitness, being overweight or obese, and mortality. However, after carefully adjusting for family background using a within-family sibling comparison design, we find that birth spacing is generally not associated with long-term health, though we find that men born after very long birth intervals have a higher probability of being overweight or obese in early adulthood. Overall we conclude that birth intervals have little independent effect on long-term health outcomes.

Parental age and offspring mortality: negative effects of reproductive aging may be counterbalanced by secular increases in longevity
​​Kieron Barclay and Mikko Myrsklyä (2018​) ​Population Studies

Abstract
As parental ages at birth continue to rise, concerns about the effects of fertility postponement on offspring are increasing as well. Advanced parental ages have been associated with negative health outcomes for offspring, including decreased longevity, due to reproductive ageing. The literature, however, has neglected to examine the potential benefits of being born at a later date. Secular declines in mortality mean that later birth cohorts are living longer. We analyse mortality over ages 30-74 among 1.9 million Swedish men and women born 1938-1960, and use a sibling comparison design that accounts for all time-invariant factors shared by the siblings. When incorporating cohort improvements in mortality, we find that those born to older mothers do not suffer any significant mortality disadvantage, and that those born to older fathers have lower mortality. These findings are likely to be explained by secular declines in mortality counterbalancing the negative effects of reproductive ageing.

Birth order and college major in Sweden
​Kieron Barclay, Martin Hällsten and Mikko Myrsklyä (2017​) ​Social Forces

Abstract
Previous research on birth order has consistently shown that later-borns have lower educational attainment than first-borns, however it is not known whether there are birth order patterns in college major. Given empirical evidence that parents disproportionately invest in first-born children, there are likely to be birth order patterns attributable to differences in both opportunities and preferences, related to ability, human capital specialization through parent-child transfers of knowledge, and personality. Birth order patterns in college major specialization may shed light on these explanatory mechanisms, and may also account for long-term birth order differences in educational and labour market outcomes. Using Swedish population register data and sibling fixed effects we find large birth order differences in university applications. First-borns are more likely to apply to, and graduate from, medicine and engineering programs at university, while later-borns are more likely to study journalism and business programs, and to attend art school. We also find that these birth order patterns are stronger in high SES families, and that differences in college major explain approximately half of the within-family birth order differences in long-term earnings. These results indicate that early life experiences and parental investment shapes sibling differences in ability, preferences, and ambitions even within the shared environment of the family.

Advanced maternal age is not an independent risk factor for low birth weight or preterm birth
​Alice Goisis, Hanna Remes, Kieron Barclay, Pekka Martikainen and Mikko Myrsklyä (2017​) ​American Journal of Epidemiology

Abstract
Advanced maternal age at birth is considered a major risk factor for birth outcomes. It is unclear to what extent this association is confounded by maternal characteristics. To test whether advanced maternal age at birth independently increases the risk of low birth weight ( < 2500g) and preterm birth ( < 37 weeks gestation), we compare between-family models (comparing children born to different mothers at different ages) with within-family models (comparing children who are born to the same mother at different ages). The latter procedure reduces confounding by unobserved parental characteristics that are shared by siblings. We use Finnish population registers of 124,098 children born 1987-2000. In between-family models, when compared with maternal ages 25-29, maternal ages 35-39 and 40 and above are associated with 1.1 percentage point (95% confidence intervals: 0.8, 1.4) and 2.2 percentage point (95% confidence intervals: 1.4, 2.9) increases, respectively, in the probability of low birth weight. The associations are similar for risk of preterm birth. In within-family models, the relationship between advanced maternal age and low birth weight or preterm is statistically and substantively negligible. In Finland, advanced maternal age is not independently associated with the risk of low birth weight and preterm birth among mothers who have at least two live births.

The long-term cognitive and socioeconomic consequences of birth intervals: a within-family sibling comparison using Swedish register data
​Kieron Barclay and Martin Kolk (2017​) ​Demography

Abstract
​We examine the relationship between birth-to-birth intervals and a variety of mid- and long-term cognitive and socioeconomic outcomes, including high school GPA, cognitive ability, educational attainment, earnings, unemployment status, and receiving government welfare support. Using contemporary Swedish population register data and a within-family sibling comparison design, we find that neither the birth interval preceding the index person, nor the birth interval following the index person, are associated with any substantively meaningful changes in mid- or long-term outcomes. This is true even for individuals born before or after birth-to-birth intervals of less than 12 months. We conclude that in a contemporary high income welfare state there appear to be no relationship between unusually short or long birth intervals and adverse long-term outcomes.

The effect of number of siblings on adult mortality: evidence from Swedish registers for cohorts born between 1938 and 1972​
​Anna Baranowska-Rataj, Kieron Barclay and Martin Kolk (2017​) ​Population Studies

Abstract
​Demographic research has paid much attention to the impact of childhood conditions on adult mortality. This paper focused on one of the key aspects of early life conditions, family size, and examined the causal effect of growing up in a large family on mortality. We used Swedish register data and frailty models examining all-cause and cause-specific mortality between the ages of 40 and 74 for the 1938-1972 cohorts, as well as a quasi-experimental approach that exploited multiple births as a source of exogenous variation in the number of siblings. While previous studies have focused on low or middle-income countries, we examined whether growing up in a large family is a disadvantage in Sweden, a context where most parents have adequate resources and are complemented by a generous welfare state. Overall our results do not indicate that growing up in a large family has a detrimental effect on longevity.

Advantages of later motherhood
​Mikko Myrskylä, Kieron Barclay and Alice Goisis (2017​) ​Der Gynäkologe

Abstract
In this review we summarise recent research examining how childbearing at older ages affects the health and wellbeing of both the mother and child. Although most research on advanced maternal age focuses on the risks associated with reproductive aging, recent research suggests that older mothers today differ from those in the past, and that there are also benefits to postponing childbearing to older ages. Older mothers today observe better health behaviours during pregnancy, are more socioeconomically advantaged, and seem to be happier after childbearing. Furthermore, the children of older parents in high-income countries have better health and educational outcomes.

Birth order and hospitalization for alcohol and narcotics use in Sweden
Kieron Barclay, Mikko Myrskylä, Per Tynelius, Daniel Berglind and Finn Rasmussen
​(2016​) ​Drug and Alcohol Dependence

Abstract
​Background: Previous studies have shown that birth order is an important predictor of later life health as well as socioeconomic attainment. In this study we examine the relationship between birth order and hospitalization for alcohol and narcotics use in Sweden.
Methods: We study the relationship between birth order and hospitalization related to alcohol and narcotics use before and after the age of 20 using Swedish register data for cohorts born 1987-1994. We apply Cox proportional hazard models and use sibling fixed effects, eliminating confounding by factors shared by the siblings.
Results: Before age 20 we find that later born siblings are hospitalized for alcohol use at a higher rate than first-borns, and there is a monotonic increase in the hazard of hospitalization with increasing birth order. Second-borns are hospitalized at a rate 47% higher than first-borns, and third-borns at a rate 65% higher. Similar patterns are observed for hospitalization for narcotics use. After age 20 the pattern is similar, but the association is weaker. These patterns are consistent across various sibling group sizes.
Conclusions: Later born siblings are more likely to be hospitalized for both alcohol and narcotics use in Sweden. These birth order effects are substantial in size, and larger than the estimated sex differences for the risk of hospitalization related to alcohol and drug use before age 20, or previous estimates for socioeconomic status differences in alcohol and drug abuse.

Reproductive history and post-reproductive mortality: a sibling comparison analysis using Swedish register data
Kieron Barclay, Katherine Keenan, Emily Grundy, Martin Kolk and Mikko Myrskylä
​(2016) ​Social Science & Medicine

Abstract
A growing body of evidence suggests that reproductive history influences post-reproductive mortality. A potential explanation for this association is confounding by socioeconomic status in the family of origin, as socioeconomic status is related to both fertility behaviours and to long-term health. We examine the relationship between age at first birth, completed parity, and post-reproductive mortality and address the potential confounding role of family of origin. We use Swedish population register data for men and women born 1932-1960, and examine both all-cause and cause-specific mortality. The contributions of our study are the use of a sibling comparison design that minimizes residual confounding from shared family background characteristics and assessment of cause-specific mortality that can shed light on the mechanisms linking reproductive history to mortality. Our results were entirely consistent with previous research on this topic, with teenage first time parents having higher mortality, and the relationship between parity and mortality following a U-shaped pattern where childless men and women and those with five or more children had the highest mortality. These results indicate that selection into specific fertility behaviours based upon socioeconomic status and experiences within the family of origin does not explain the relationship between reproductive history and post-reproductive mortality. Additional analyses where we adjust for other lifecourse factors such as educational attainment, attained socioeconomic status, and post-reproductive marital history do not change the results. Our results add an important new level of robustness to the findings on reproductive history and mortality by showing that the association is robust to confounding by factors shared by siblings. However it is still uncertain whether reproductive history causally influences health, or whether other confounding factors such as childhood health or risk-taking propensity could explain the association. 

Maternal age and offspring health and health behaviours in late adolescence in Sweden
Kieron Barclay and Mikko Myrskylä (2016) ​Social Science & Medicine - Population Health

​Abstract
In this study we investigate the relationship between maternal age at the time of birth and a variety of health behaviours and measures of health amongst young adults in contemporary Sweden. Previous research has shown that those born to younger and older mothers tend to have worse perinatal outcomes, and worse health in middle- and later adulthood. However, previous work has not examined health in early adulthood, and no studies have explored whether maternal age is related to health behaviours. Using survey data on 1,236 19-year olds born in Sweden in 1990, we find that those born to older mothers have lower self-rated health, are more likely to smoke, more likely to drink alcohol regularly, and less likely to exercise regularly. We discuss potential explanations for these findings, such as older parents exerting lower social control due to greater levels of workplace responsibilities and time demands, long-term consequences of the poor peri-natal outcomes of those born to older mothers, as well as the potential role of parental health behaviours. Our findings suggest that health behaviours may play an important mediating role in explaining the worse long-term health of those born to younger and older mothers.
Advanced maternal age and offspring outcomes: reproductive aging and counterbalancing period trends
Kieron Barclay and Mikko Myrskylä (2016) ​Population and Development Review
​
​Abstract
Women are having children later in the developed world. The mean age at first birth has increased in each of the 23 OECD countries since 1970, now averaging 28, and advanced-age fertility has also been increasing. Advanced maternal age is associated with increased risk of poor perinatal outcomes, as well as an increased risk of mortality and cancer in adulthood. The research documenting these negative outcomes, however, neglects the potential benefits of being born at a later date. For a prospective parent, delaying parenthood means that the child is born in a later birth cohort. This is beneficial, as for many important outcomes from health to educational attainment, secular trends are positive. We illustrate this general principle using data from Sweden, and show that the macro level contextual trends outweigh the individual risk factors. As a result, fertility postponement even beyond age 40 is positively associated with long-term offspring outcomes. ​

Birth order and mortality: a population-based cohort study
Kieron Barclay and Martin Kolk (2015) Demography​

​Abstract
This study uses Swedish population register data to investigate the relationship between birth order and mortality at ages 30 to 69 for Swedish cohorts born between 1938 and 1960, using a within-family comparison. The main analyses are conducted with discrete-time survival analysis using a within-family comparison, and the estimates are adjusted for age, mother’s age at the time of birth, and cohort. Focusing on sibships ranging in size from two to six, we find that mortality risk in adulthood increases with later birth order. The results show that the relative effect of birth order is greater among women than among men. This pattern is consistent for all the major causes of death but is particularly pronounced for mortality attributable to cancers of the respiratory system and to external causes. Further analyses in which we adjust for adult socioeconomic status and adult educational attainment suggest that social pathways only mediate the relationship between birth order and mortality risk in adulthood to a limited degree.

A within-family analysis of birth order and intelligence using population conscription data on Swedish men
Kieron Barclay (2015) Intelligence​

Abstract 
​This study examines the relationship between birth order and intelligence in Sweden. This research question has been of interest for decades, but only one study using a sibling comparison design has found that birth order has a negative effect on intelligence. The data used in this study is Swedish administrative register data, with data on cognitive ability drawn from the military conscription register for men born 1965 to 1977. Within-family comparison linear regression models are used to estimate the difference in cognitive ability by birth order amongst brothers. I find that there is a negative relationship between birth order and cognitive ability. This is consistent in sibling-group-size-specific analyses of sibling groups with two through to six children. Further analyses demonstrate that this negative relationship between birth order and intelligence is consistent in different socioeconomic status groups, and amongst individuals born in the 1960s and 1970s. Analyses of brothers in two-child sibling groups show that the relationship between birth order and intelligence varies by the birth interval. Second borns have a statistically significantly lower cognitive ability score if the birth interval is up to six years, but not if it is longer.

Birth order and educational attainment: evidence from fully adopted sibling groups
Kieron Barclay (2015) Intelligence​

Abstract 
This study uses data on fully adopted sibling groups to test whether the explanation for the consistently observed negative effects of birth order are physiological or social in origin. Swedish administrative register data is used to construct full sibling data for cohorts born 1960–1982. Using a within-family comparison approach, I compare adopted siblings of different adopted birth order to one another to see whether birth order amongst adopted children (N = 6968) is associated with educational attainment by age 30, and the likelihood of having entered tertiary education by age 30. These same within-family comparison analyses are also performed on siblings in fully biologically related sibling groups (N= 1,588,401). I find that there is a negative relationship between adopted birth order and both educational attainment and the likelihood of entering tertiary education in fully adopted sibling sets. These findings strongly suggest that differences in educational attainment by birth order are driven by intrafamily social dynamics. I also conduct additional analyses in fully adopted sibling groups where age order and adoption order are reversed to test whether there is evidence for tutoring by siblings. These results do not indicate clear support for any tutoring effect.

Birth order and physical fitness in early adulthood: evidence from Swedish military conscription data
Kieron Barclay and Mikko Myrskylä (2014) Social Science & Medicine​

Abstract 
Physical fitness at young adult ages is an important determinant of physical health, cognitive ability, and mortality. However, few studies have addressed the relationship between early life conditions and physical fitness in adulthood. An important potential factor influencing physical fitness is birth order, which prior studies associate with several early- and later-life outcomes such as height and mortality. This is the first study to analyse the association between birth order and physical fitness in late adolescence. We use military conscription data on 218,873 Swedish males born between 1965 and 1977. Physical fitness is measured by a test of maximal working capacity, a measure of cardiovascular fitness closely related to V02max. We use linear regression with sibling fixed effects, meaning a within-family comparison, to eliminate the confounding influence of unobserved factors that vary between siblings. To understand the mechanism we further analyse whether the association between birth order and physical fitness varies by sibship size, parental socioeconomic status, birth cohort or length of the birth interval. We find a strong, negative and monotonic relationship between birth order and physical fitness. For example, third-born children have a maximal working capacity approximately 0.1 (p < 0.000) standard deviations lower than first-born children. The association exists both in small (3 or less children) and large families (4 or more children), in high and low socioeconomic status families, and amongst cohorts born in the 1960s and the 1970s. While in the whole population the birth order effect does not depend on the length of the birth intervals, in two-child families a longer birth interval strengthens the advantage of the first-born. Our results illustrate the importance of birth order for physical fitness, and suggest that the first-born advantage already arises in late adolescence.

Workplace sex composition and ischaemic heart disease: a longitudinal analysis using Swedish register data
Kieron Barclay and Kirk Scott (2014) Scandinavian Journal of Public Health

Abstract 
​Aims: The aim of this study is to follow-up on previous research indicating that the sex composition of workplaces is related to a number of health outcomes, including sickness absenteeism and mortality. We test two hypotheses. The first is Kanter’s theory of tokenism, which suggests that minority group members suffer from an increased risk of stress. Secondly, we test the hypothesis that workplaces with a higher proportion of men will have a higher incidence rate of ischaemic heart disease (IHD), as men are more likely to engage in negative health behaviours, and through peer effects this will result in a workplace culture that is detrimental to health over the long term. Methods: Large-scale, longitudinal Swedish administrative register data are used to study the risk of overnight hospitalization for IHD amongst 67,763 men over the period 1990 to 2001. Discrete-time survival analyses were estimated in the form of logistic regression models.
Results: Men have an elevated risk of suffering from IHD in non-gender-balanced workplaces, but this association was only statistically significant in workplaces with 61–80% and 81–100% males. However, after adjusting for occupation no clear pattern of association could be discerned. No pattern of association was observed for women.
Conclusions: This study suggests that the gender composition of workplaces is not strongly associated with the risk of suffering from IHD.

Sex ratios at sexual maturity and longevity: evidence from Swedish register data
Kieron Barclay (2013) Demographic Research

Abstract 
This study tests the recently proposed hypothesis that the contextual sex ratio at sexual maturity is related to longevity. Previous empirical research in the United States has shown that a higher proportion of males at the age of sexual maturity increases the risk of mortality for males both before and after the age of 65. I use Swedish administrative register data, linking the 1960 census to individual-level mortality data over the period 1960 to 2007. I calculate the sex ratio at two geographic levels, municipalities and parishes. Two different specifications of the sex ratio are calculated: males aged 18 to 27 over females aged 15 to 24, and males aged 18 to 22 over females aged 16 to 20. I conduct piece-wise constant survival analyses over the period from 1960 to 2007 to analyze the risk of mortality before age 65. I run separate analyses for males and females, using cohorts born in 1941 and 1942. For males, the results generally show that for both males and females a higher proportion of males was associated with a lower relative risk of mortality before age 65. The results were not statistically significant. The lack of a consistent statistically significant association for either males or females, and the trend for males being in the opposite direction of what was hypothesized, suggests that support for the hypothesis in Sweden is very weak.

Peer clustering of exercise and eating behaviours among young adults in Sweden: a cross-sectional study of egocentric network data
Kieron Barclay, Christofer Edling and Jens Rydgren (2013) BMC Public Health

Abstract 
​Research suggests that the growing prevalence of obesity may be related to the influence of the health behaviours of peers. We look at clustering of exercise and eating behaviours amongst a previously unstudied group, young adults in Sweden. Previous research has mainly been conducted in the United States and Britain, countries that have relatively high rates of obesity. Using ego-alter dyads from the egocentric network data as the unit of analysis, we conduct logistic regressions to investigate the association between ego and alter exercise and eating behaviours. Respondents have a significantly greater probability of engaging in regular exercise and eating healthily if a nominated peer also does so. Furthermore, the degree to which this behavior is shared is modulated by the strength of the relationship between the two individuals, with a greater probability of engaging in these behaviours observed when the relationship with the nominated peer is strong relative to when the relationship is weak. However, we find that ego-alter homogeneity in terms of gender and migration status was not associated with a significantly greater probability of behaving in a similar manner to a nominated peer. Furthermore, the status of the nominated peer as a relative or not did not impact the probability that the ego would engage in similar health behaviours to that alter. We observe strong associations between ego and alter health behaviours for young adults, consistent with previous research. Although we cannot draw causal inferences, these results suggest that the health behaviours of an individual’s peers may play a role in shaping their own health behaviours.

Sex composition of the workplace and mortality risk
Kieron Barclay (2013) Journal of Biosocial Science

Abstract 
This study uses Swedish occupational register data to examine whether the proportion of men in administrative workplaces in the Swedish public service affects all-cause mortality risks amongst both males and females of working age. Using piecewise constant survival models to analyse occupational data from the Swedish administrative registers from 1995 to 2007, it was found that for males, a 1% increase in the proportion of males was associated with a 1.3% increase in mortality risk (hazard ratio, HR 1.013, 95% CI 1.007–1.020, p<0.001), but no association was found for females (HR 1.004, 95% CI 0.996–1.012, p=0.297). Adjustments were made for age, family status, education, occupational status, occupational segregation by sex, the total number of individuals in the workplace, level of government, region, period and variables reflecting the workplace structure by age, age by sex, occupation and education. A higher proportion of males may be related to (i) an increased exposure to risky health behaviours such as alcohol consumption and unhealthy dietary patterns, (ii) a tendency towards sickness presenteeism, and (iii) an increase in the levels of several well-established emotional stressors in the workplace, leading to an increased level of psychosocial stress. The findings and potential extensions of this research are discussed.