Reduced reproductive success is associated with selective constraint on human genes.

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.

Birth Spacing and Parents' Physical and Mental Health: An Analysis Using Individual and Sibling Fixed Effects.

An extensive literature has examined the relationship between birth spacing and subsequent health outcomes for parents, particularly for mothers. However, this research has drawn almost exclusively on observational research designs, and almost all studies have been limited to adjusting for observable factors that could 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 mothers' and fathers' physical and mental health concerns immediately after childbirth (1-5 and 6-11 months after childbirth), in the medium term (5-6 years after childbearing), and in the long term (10-11 years after childbearing). To examine short-term health outcomes, we estimate individual fixed-effects models: we hold constant factors that could influence parents' birth spacing behavior and their health, comparing 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 mothers' and fathers' family backgrounds. The results from our analyses that do not apply individual or sibling fixed effects are consistent with much of the previous literature: shorter and longer birth intervals are associated with worse health outcomes than birth intervals of approximately 2-3 years. 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 might modestly influence short-, medium-, and long-term physical health outcomes. Overall, these results are consistent with small to negligible effects of birth spacing behavior on (non-pregnancy-related) parental health outcomes.

The Swedish Kinship Universe: A Demographic Account of the Number of Children, Parents, Siblings, Grandchildren, Grandparents, Aunts/Uncles, Nieces/Nephews, and Cousins Using National Population Registers.

Given that surprisingly little is known about the demography of human kinship, we provide a demographic account of the kinship networks of individuals in Sweden in 2017 across sex and cohort between ages 0 and 102. We used administrative register data of the full population of Sweden to provide the first kinship enumeration for a complete population based on empirical data. We created ego-focused kinship networks of children, parents, siblings, grandchildren, grandparents, aunts and uncles, nieces and nephews, and cousins. We show the average number of kin of different types, the distribution of the number of kin, and changes in dispersion over time. A large share of all kin of an individual are horizontal kin, such as cousins. We observe the highest number of kin-on average, roughly 20-around age 35. We show differences between matrilineal and patrilineal kin and differences in the kinship structure arising from fertility with more than one childbearing partner, such as half-siblings. The results demonstrate substantial variability in kinship within a population. We discuss our findings in the context of other methods to estimate kinship.

The relationship between life-course accumulated income and childbearing of Swedish men and women born 1940-70.

This study uses income accumulated over ages 20-60 to examine whether richer or poorer individuals have more children. Income histories are calculated using yearly administrative register data from contemporary Sweden for cohorts born 1940-70. Differences by parity and income distribution are examined separately by sex. There is a strong positive gradient between accumulated disposable income (and to a lesser extent earnings) and fertility for men in all cohorts and a gradual transformation from a negative to a positive gradient for women. In particular, accumulated incomes are substantially lower for childless men and women than those with children. For men, fertility increases monotonically with increasing income, whereas for women much of the positive gradient results from low fertility among women with very low accumulated incomes in later cohorts. Most of the positive income-fertility gradient can be explained by the high incomes of men and women with two to four children.

Kinship and socio-economic status: Social gradients in frequencies of kin across the life course in Sweden.

The influence of kin on various outcomes is heavily debated. However, kinship size itself conditions the probability of potential effects. Socio-economic gradients in the prevalence, variance, and types of kin are, therefore, a vital aspect of the functions of kin. Unfortunately, these parameters are largely unknown. We used Swedish register data to enumerate consanguine and in-law kin across the life course of the 1975 birth cohort. We calculated differences in kinship size between this cohort's income quartiles and educational groups. We decomposed how specific kin relations, generations, and demographic behaviours contributed to these differences. Among low socio-economic status (SES) groups, higher fertility in earlier generations resulted in more kin compared with high-SES groups. Low-SES groups had more horizontal consanguine kin, while high-SES groups had more in-laws. Lower fertility and higher union instability among low-SES men substantially narrowed SES differences in kinship size. Kinship size varied substantially within SES groups.

How Does Mortality Contribute to Lifetime Pension Inequality? Evidence From Five Decades of Swedish Taxation Data.

As with many social transfer schemes, pension systems around the world are often progressive: individuals with lower incomes receive a higher percentage of their income as a subsequent pension. On the other hand, those with lower earnings have higher mortality and thus accumulate fewer years of pension income. Both of these opposing factors influence the progressiveness of pension systems. Empirical efforts to disentangle the effects of mortality inequality on lifetime pension inequality have been scarce. Using Swedish taxation data linked with death registers for 1970-2018, we study how education and preretirement earnings relate to lifetime pensions from age 60 onward and how mortality inequalities contribute to overall inequalities in lifetime pensions. The results show that a progressive replacement structure and mortality differences contribute to the overall distribution of pension payments over the life course. Up to one quarter of lifetime pension inequality is attributable to the greater longevity of socially advantaged groups-particularly among men. Hence, mortality inequalities are an important determinant of the overall degree of between-group income transfers in a pension system, but they are not as important as inequalities in prior earnings.

Women's Experience of Child Death Over the Life Course: A Global Demographic Perspective.

The death of a child affects the well-being of parents and families worldwide, but little is known about the scale of this phenomenon. Using a novel methodology from formal demography applied to data from the 2019 Revision of the United Nations World Population Prospects, we provide the first global overview of parental bereavement, its magnitude, prevalence, and distribution over age for the 1950-2000 annual birth cohorts of women. We project that the global burden of parental bereavement will be 1.6 times lower for women born in 2000 than for women born in 1955. Accounting for compositional effects, we anticipate the largest improvements in regions of the Global South, where offspring mortality continues to be a common life event. This study quantifies an unprecedented shift in the timing of parental bereavement from reproductive to retirement ages. Women in the 1985 cohort and subsequent cohorts will be more likely to lose an adult child after age 65 than to lose a young child before age 50, reversing a long-standing global trend. "Child death" will increasingly come to mean the death of adult offspring. We project persisting regional inequalities in offspring mortality and in the availability of children in later life, a particular concern for parents dependent on support from their children after retirement. Nevertheless, our analyses suggest a progressive narrowing of the historical gap between the Global North and South in the near future. These developments have profound implications for demographic theory and highlight the need for policies to support bereaved older parents.

Two Decades of Same-Sex Marriage in Sweden: A Demographic Account of Developments in Marriage, Childbearing, and Divorce.

In this study, we provide demographic insight into the still relatively new family form of same-sex marriage. We focus on period trends in same-sex marriage formation and divorce during 1995-2012 in Sweden and the role of childbearing in same-sex unions. The period begins with the introduction of registered partnership for same-sex couples and also covers the introduction of formal same-sex marriage in 2009. We use register data for the complete population of Sweden to contrast patterns in male and female same-sex marriage formation and divorce. We show that female same-sex union formation increased rapidly over the period, while trends for male same-sex unions increased less. The introduction of same-sex marriage legislation in 2009 appears to have had little effect on the pace of formation of same-sex unions. In contrast, legal changes supporting parental rights in same-sex unions may have fueled the formation of female same-sex marriages as well as parenthood in such unions. Further, we show that divorce risks in the marital unions of two women are much higher than in other types of marriages. We find some convergence of divorce risks across union types at the end of our study period: male same-sex unions have the same divorce risk levels as opposite-sex marriages, and the elevated risks of divorce in female same-sex unions appear to have stabilized at somewhat lower levels than those observed in the late 1990s.

Interpregnancy intervals and perinatal and child health in Sweden: A comparison within families and across social groups.

A large body of research has shown that children born after especially short or long birth intervals experience 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 child 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 (less than seven months) and very long intervals (>60 months). We find few differences in the patterns by maternal educational level or maternal country of origin after stratifying by the mother's highest educational attainment.

Cognitive ability and fertility among Swedish men born 1951-1967: evidence from military conscription registers.

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 1951-1967. 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 0.56 fewer children, and men with the highest category have 0.09 more children.

When and Where Birth Spacing Matters for Child Survival: An International Comparison Using the DHS.

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 after unobserved maternal heterogeneity is controlled for, intervals shorter than 36 months substantially increase the probability of infant death. However, 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 toward 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 are much less consequential in high-income settings.

Parity and Mortality: An Examination of Different Explanatory Mechanisms Using Data on Biological and Adoptive Parents.

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 with 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 45-97. 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.

Birth Intervals and Health in Adulthood: A Comparison of Siblings Using Swedish Register Data.

A growing body of research has examined whether birth intervals influence perinatal 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: for men, height, physical fitness, and the probability of falling into different body mass index categories; and for men and women, mortality. In models that 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, although 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.

The Long-Term Cognitive and Socioeconomic Consequences of Birth Intervals: A Within-Family Sibling Comparison Using Swedish Register Data.

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 appears 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.

Demographic research has paid much attention to the impact of childhood conditions on adult mortality. We focus on one of the key aspects of early life conditions, sibling group size, and examine the causal effect of growing up in a large family on mortality. While previous studies have focused on low- or middle-income countries, we examine whether growing up in a large family is a disadvantage in Sweden, a context where most parents have adequate resources, which are complemented by a generous welfare state. We used Swedish register data and frailty models, examining all-cause and cause-specific mortality between the ages of 40 and 74 for the 1938-72 cohorts, and also a quasi-experimental approach that exploited multiple births as a source of exogenous variation in the number of siblings. Overall our results do not indicate that growing up in a large family has a detrimental effect on longevity in Sweden.

Socioeconomic status and sex ratios at birth in Sweden: No evidence for a Trivers-Willard effect for a wide range of status indicators.

OBJECTIVES: This study examines if there exists a positive association between socioeconomic status and the proportion of male births in humans, as proposed by Trivers and Willard in 1973, using individual-level data drawn from the complete population of Sweden. METHODS: We examine more than 3,000,000 births between 1960 and 2007 using administrative register data with comprehensive information on various dimensions of socioeconomic status. We use six different operationalizations of socioeconomic status, including earnings, post-transfer income (including government allowances), wealth, parental wealth, educational level, and occupational class. We apply regression models that compare both changes in status for the same woman over time and differences in status across different women. We also measure socioeconomic status both at the year of child birth and the year of conception. RESULTS: Our results show the absence of any relationship between socioeconomic status and sex ratios, using a large number of different operationalizations of status. CONCLUSIONS: We conclude that no substantive relationship between socioeconomic status and sex ratios exists for the population and period of our study.

Correlations in fertility across generations: can low fertility persist?

Correlations in family size across generations could have a major influence on human population size in the future. Empirical studies have shown that the associations between the fertility of parents and the fertility of children are substantial and growing over time. Despite their potential long-term consequences, intergenerational fertility correlations have largely been ignored by researchers. We present a model of the fertility transition as a cultural process acting on new lifestyles associated with fertility. Differences in parental and social influences on the acquisition of these lifestyles result in intergenerational correlations in fertility. We show different scenarios for future population size based on models that disregard intergenerational correlations in fertility, models with fertility correlations and a single lifestyle, and models with fertility correlations and multiple lifestyles. We show that intergenerational fertility correlations will result in an increase in fertility over time. However, present low-fertility levels may persist if the rapid introduction of new cultural lifestyles continues into the future.

Multigenerational transmission of family size in contemporary Sweden.

The study of the intergenerational transmission of fertility has a long history in demography, but until now research has focused primarily on parents' influence on their children's fertility patterns and has largely overlooked the possible influence of other kin. This study examines the transmission of fertility patterns from parents, grandparents, uncles, and aunts, using event history models to determine the risk of first, second, and third births. Swedish register data are used to study the 1970-82 birth cohorts. The findings indicate strong associations between the fertility of index persons and that of their parents, and also independent associations between the completed fertility of index persons and that of their grandparents and parents' siblings. The results suggest that, when examining background effects in fertility research, it is relevant to take a multigenerational perspective and to consider the characteristics of extended kin.

Religion and Fertility: A Longitudinal Register Study Examining Differences by Sex, Parity, Partner's Religion, and Religious Conversion in Finland.

We use longitudinal data on religious affiliation in Finland to examine childbearing behavior. All analyses are based on detailed fertility information from the Finnish national register of each person's religious denomination for men and women born in 1956-1975. We identify higher fertility according to parity among members of the Evangelical Lutheran state church and other Protestant churches, and lower fertility among individuals with no religious affiliation. Most other religious groups-Orthodox Christians, Jews, Muslims, and adherents of Eastern religions-have intermediate levels of fertility. We also find that religious converts, that is, those observed with more than one religious denomination over their life course, typically are similar to the non-converts of the group they convert to, though with more distinct deviations from the Finnish population. Women show larger differences by religious affiliation than men. We find the largest differences across religions when we examine the proportion of childless men and women. Overall, differences between religious groups are rather modest, and childbearing patterns are quite similar. Our results provide, to our knowledge, the first examination of religion and fertility using national-level longitudinal data.

Religion and mental health in young adulthood: a register-based study on differences by religious affiliation in sickness absence due to mental disorders in Finland.

BACKGROUND: Religiosity and spirituality are known to be positively correlated with health. This is the first study to analyse the interrelation between religious denomination and sickness absence due to mental disorders using population register data with detailed ICD codes. METHODS: The follow-up study was based on the entire population born in Finland between 1984 and 1996 (N=794 476). Each person was observed from age 20 over the period from 2004 to 2018. Cox proportional hazards models were applied to analyse the association between religious denomination and first-time sickness allowance receipts for any cause and mental disorder. Mental disorders were categorised as severe mental illness (F20-F31), depression (F32-F33), anxiety (F40-F48) and any other mental disorder (all other F codes). Men and women were analysed separately. RESULTS: The differences in sickness absence due to mental disorder were substantial between religious affiliations. Compared with members of the Evangelical Lutheran state church, the relative hazard for mental disorders among non-affiliated women was 1.34 (95% CI 1.30 to 1.39), while that among women with other religions was 1.27 (95% CI 1.19 to 1.35), after adjusting for own and parental characteristics. The corresponding numbers for men were 1.45 (95% CI 1.39 to 1.50) and 1.42 (95% CI 1.30 to 1.54), respectively. The gradient was larger for severe mental illness and depression than for anxiety and other mental disorders. For any cause of sickness absence, there was no difference between Lutherans, non-affiliated individuals and those with other religions. CONCLUSIONS: Epidemiologists and public health practitioners should further examine the association between mental disorders and church membership using administrative registers.