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An extensive literature theorizes the role of repartnering for cohort fertility and whether union dissolution can be an engine for fertility. A large share of higher order unions are nonmarital cohabitations, but most previous studies on completed cohort fertility have analyzed only marital unions, and none have incorporated nonmarital cohabitations using population-level data. To analyze the relationship between the number of unions and cohort fertility for men and women, we use Poisson regression with Finnish register data to enumerate every birth, marriage, and cohabitation among the 1969–1972 birth cohorts at ages 18–46. We show that dissolutions of first cohabitations are the main pathway to repartnering and that most higher order unions are cohabitations. Nonmarital repartnering is a strong predictor of low fertility. In contrast, remarriage is positively associated with cohort fertility. Because the bulk of first-union dissolutions and higher order unions are nonmarital, repartnering is not an efficient engine for fertility at the aggregate level. Marriage and cohabitation are far from indistinguishable in a country often described as a second demographic transition forerunner.

Darwin was the first to offer an evolutionary explanation for widespread sex differences in mating strategies across animal taxa. However, initial applications of evolutionary approaches to the study of human behavior were met with resistance. In response, social scientists began systematically applying rigorous evaluations of evolutionarily informed hypotheses by way of testable predictions. Mate choice research was a key area for this early work, focused on demonstrating sex differences in mating behavior. Subsequent research has tempered some earlier conclusions, increasingly relating mating strategies to aspects of socioecology. In particular, the adult sex ratio has emerged as an essential demographic variable contributing to behavioral variability within and between the sexes, leading to frequency-dependent rethinking for the patterning of mating competition and parental investment. Empirical examination of the association between adult sex ratio and aggression, family formation, partnership stability, and risk-taking is a growing field of research cross-cutting various literatures. In this chapter, we first describe early theory motivating research on sex differentiated behavior, followed by recent frequency-dependent reexaminations of sexual selection. Second, we introduce the adult sex ratio as a central determinant of fitness payoffs to differing reproductive strategies. Third, we review empirical studies on the consequences of sex ratio variation, highlighting outcomes related to sociosexuality, family formation, and violence. Fourth, we detail the maturation of the field, and where theoretical and empirical work has unveiled complexities regarding reproductive strategies. Finally, we reflect on future directions, where theoretical refinement is under way or needed, and highlight promising and novel research approaches.

Bateman’s principles continue to influence the understanding of human reproductive behaviour despite enduring criticism. However, few rigorous studies on Bateman’s principles in contemporary industrialized populations exist, and previous studies are hampered by small sample size and exclusion of non-marital unions. Here we address these shortcomings by assessing mating success and reproductive success using population-wide Finnish register data of marital and non-marital co-habitations and children born. These unique data allow us to compare different operationalizations of mating success, namely, co-residential unions and cumulated union duration; and to examine variability across social strata. Our data support Bateman’s first and second principles but the association between mating success and reproductive success is less clear. The number of unions is somewhat more positively associated with reproductive success for men than women, and the mating success–reproductive success association turns negative when adjusting for union duration. Having had more mates is associated with lower reproductive success than having had one union. More unions are positively associated with higher reproductive success only for men in the lowest income quartile. We conclude that union duration, controlling number of unions, is associated with higher male reproductive success and should be incorporated as an important dimension of mating success.

Extensive literature theorizes the role of re-partnering on cohort fertility and whether union dissolution can be an engine for fertility. A large share of higher-order unions is non-marital cohabitations. Yet, most previous completed cohort fertility studies on the topic analyze marital unions only and none have measured cohabitations using population-level data. We use Finnish register data to enumerate every birth, marriage, and cohabitation from ages 18-46 in the 1969–1972 birth cohorts, and analyze the relationship between the number of unions and cohort fertility for men and women using Poisson regression. We show that re-partnering is driven by cohabitations. Re-marriage is positively associated with cohort fertility, compared to individuals in a single intact marriage. However, when measured using marriages as well as non-marital cohabitations, re-partnering is negatively associated with fertility, compared to individuals in a single intact union. This negative association increases with socioeconomic status. “Serial cohabitation” is a strong predictor of low fertility. Men see a slight “re-marriage premium” in fertility and a (non-marital) “re-partnering penalty,” compared to women. Thus, re-partnering is likely not an efficient engine for fertility. Further, marriage and cohabitation are far from indistinguishable in a country often described as a second demographic transition forerunner.

This is the first study to use longitudinal population register data to illustrate that marital stability may relate to the adoption of the spouse's language. The paper draws on theories of boundary shifting and boundary crossing to examine two main ancestral groups in Finland, Finnish speakers and Swedish speakers, between whom intermarriage is common. Administrative changes in how the question about language/ethnicity was registered between the censuses of 1975 and 1980 make it possible to distinguish between persons who are concordant or discordant on the main language used and ethnic affiliation. Using data on the entire married population, and adjusting for several individuals and couple characteristics, we estimate the couples’ divorce risk as a function of language convergence with Cox regressions. Discordance in terms of adopting the Finnish language used by the partner is associated with a higher divorce risk, as compared with couples who are exogamous in terms of both language use and ethnic affiliation. Adopting the Swedish language, on the other hand, is associated with a slightly reduced divorce risk. Thus, these data provide some evidence that marital stability may relate to language convergence.

Separation often leads to worse economic consequences for women than for men. However, little is known about how economic consequences of separation play out for different groups of women. Women who are younger than their male partner are generally assumed to have lesser agency, but evidence mostly comes from contexts with low gender equality. Here, we examine women's benefit recipiency as a function of the partner age gap of their dissolved union. Using register data from Sweden, we examine whether women from man-older unions suffer greater economic disadvantage after separation, and whether patterns differ for ancestral Swedes and women with migrant background. Results from logistic regression models suggest that, post-separation, the uptake of social and housing benefits increases for nearly all groups of women. However, these data do not show any consistent disadvantages of women from man-older unions. Among ancestral Swedes, patterns differed by benefit type, and among women of African/Middle Eastern origin, benefit recipiency increases were inversely U-shaped to the age gap. Social norms do not appear to explain economic costs of separation, but may explain why the risk of separation itself differed between ancestral Swedish women and women with migrant background.

An important source of inequality in mortality is regional variation. However, studies that investigate regional mortality patterns within and across national borders are extremely rare. We respond by carrying out a comparative study of Finland and Sweden: two welfare states that share many attributes, with one exception being their mortality trajectories. While Finland has risen rapidly in the global life expectancy rankings, Sweden has lost its historical place among the top ten. Using individual-level register data, we study regional trends in life expectancy and life-span variation by sex. Although all regions, in both countries, have experienced substantial improvements in life expectancy and lifespan inequality from 1990-2014, considerable differences between regions have remained unchanged, suggesting the existence of persistent inequality. In particular, Swedish-speaking minority regions in Finland had maintained their mortality advantage over Finnish-speaking regions. Nevertheless, there is some evidence of convergence between the regions of Finland and Sweden.

Work from social and biological sciences has shown that adult sex ratios are associated with relationship behaviours. When partners are abundant, opportunities for mate switching may increase and relationship stability decrease. To date, most of the human literature has used regional areas at various levels of aggregation to define partner markets. But, in developed countries, many individuals of reproductive age spend a considerable amount of time outside their residential areas, and other measures may better capture the opportunities to meet a (new) partner. Here, we use Danish register data to test whether the sex ratio of the occupational sector is linked to divorce. Our data cover individuals in Denmark who married during 1981–2002 and we control for age at and duration of marriage, education and parity. Results support the prediction that a higher proportion of opposite-sex individuals in one's occupational sector is associated with higher divorce risk. This holds for both men and women, but associations are somewhat stronger for men and vary by education. Our results highlight the need to study demographic behaviours of men and women simultaneously, and to consider partner markets beyond geographical areas so that differing strategies for males and females may be examined.

Final version published 2017 in Evolution and Human Behavior. Abstract: Whether polygyny is harmful for women and their children is a long-standing question in anthropology. Few studies, however, have explored whether the effect of polygyny varies for women of different wife order, and whether there are different outcomes for their sons and daughters. Because males have higher reproductive variance, especially when they are allowed to take multiple wives, parents may have higher fitness returns from investing in sons over daughters in polygynous households. Moreover, previous studies have found that first wives and their children are advantaged over monogamous and second order wives (who marry into unions later). Here we test the predictions that children of first wives will have an advantage over children to monogamous or second wives, and that sex-biased investment will be strongest among first wives. Using data from the Arsi Oromo of Ethiopia (n~6200 children) we test whether associations with mother’s wife order extend beyond childhood into adulthood by examining simultaneously child survival, education and age at marriage. We find that polygynous first wives have no child survival disadvantage, first wives’ sons benefit in terms of longer education and daughters have an earlier age at marriage than daughters of monogamous women. Second wives have lower child survival than monogamous women, but surviving children experience advantages in later life outcomes, particularly marriage. These findings challenge the view that polygynous women are always doing the ‘best of a bad job’. Rather, our results suggest that via their surviving sons and daughters there may be long-term benefits for some polygynous women.

BackgroundWe explore how mortality is related to unemployment and intragenerational social mobility in Finland. Unemployment and social mobility are two labour market experiences that are largely studied separately, despite the fact that selection processes into unemployment and downward mobility are intertwined. Because both causal and health selection mechanisms may vary depending on the timing of these experiences, we consider heterogeneity by age and economic context.MethodsWe run discrete time event history analysis for death (at age 30–75 years) in two periods (economic recession and growth) and analyse younger and older individuals and men and women separately.ResultsThe odds of mortality were particularly high for individuals experiencing unemployment and when unemployment occurred during economic growth (OR ranging between 1.39 and 2.77). Younger men had high odds of mortality following unemployment (OR 1.86–2.77). In contrast, downward mobility was associated with higher odds of mortality only among older men and women and only during economic growth. The benefits of upward mobility were experienced mainly by younger men (OR ranging between 0.86 and 0.87) and were not experienced by women at all.ConclusionResults show that when in an individual’s life and the economic cycle unemployment and social mobility occur matters for whether these experiences are associated with mortality.

Variation in preventable death can be understood from an evolutionary life history perspective. However, previous studies have failed to isolate ecological effects on preventable death. We use population-wide Census data from Northern Ireland and find that extrinsic mortality rate and crime, but not sex ratio, impacts on male preventable death., Background and objectives: Individual investment in health varies greatly within populations and results in significant differences in the risk of preventable death. Life history theory predicts that individuals should alter their investment in health (somatic maintenance) in response to ecological cues that shift the perceived fitness payoffs to such investments. However, previous research has failed to isolate the effects of different ecological factors on preventable death, and has often relied on macro-level data without individual controls. Here, we test some key predictions concerning the local ecology—that higher extrinsic mortality rate (EMR), crime rate and mate-scarcity (male/female-biased sex ratio) at the ward-level—will be associated with a higher risk of preventable death. Methodology: We use census-based data from Northern Ireland (n = 927 150) on preventable death during an 8.7-year period from the 2001 Census and run Cox regressions for (i) accident/suicide or alcohol-related death and (ii) deaths from preventable diseases, for men and women separately, controlling for a wide range of individual variables. Results: We find evidence of ward-level EMR and crime rate being positively associated with preventable death among men, particularly men with low socioeconomic position. There was a tentative relationship between male-biased sex ratio and preventable death among women, but not among men. Conclusion and implications: Both behaviours that might lead to ‘risky’ death and health neglect might be adaptive responses to local ecologies. Efforts to reduce crime might be as effective as those to reduce extrinsic mortality, and both could have positive effects on various health behaviours.

Evolutionary models of human life-history predict that ecological characteristics drive variability in reproductive timing by altering anticipated returns to inclusive fitness. Local extrinsic mortality rate (EMR), crime (CR), and female-biased sex ratios have all been predicted to accelerate reproduction. However, previous research has failed to isolate the impact of these ecological characteristics from individual factors, such as wealth. Here, we utilize a unique longitudinal dataset from Northern Ireland (570 electoral wards; 62339 individuals) that enables us to address this issue and to apply a novel measure of extrinsic mortality based on a definition from public health. We demonstrate that high ward-level EMR, CR, and female-biased sex ratios have additive positive impact on the risk of early motherhood and that CR and EMR predict early fatherhood. These effects remained significant after adjustment for potentially confounding factors but were greatly attenuated when individual-level socioeconomic characteristics were adjusted for. Our findings suggest that young individuals in this population are sensitive to several ecological cues, including local crime and adult sex ratio, which speed up first birth over and above the strong effects of individual wealth.

How to allocate resources between somatic maintenance and reproduction in a manner that maximizes inclusive fitness is a fundamental challenge for all organisms. Life history theory predicts that effort put into somatic maintenance (health) should vary with sex, mating and parenting status because men and women have different costs of reproduction, and because life transitions such as family formation alter the fitness payoffs from investing in current versus future reproduction. However, few tests of how such life history parameters influence behaviours closely linked to survival exist. Here we examine whether specific forms of preventable death (accidents/suicides, alcohol-related causes, and other preventable diseases) are predicted by marital status and dependent offspring in a modern developed context; that of Northern Ireland. We predict that men, non-partnered individuals and individuals who do not have dependent offspring will be at higher risk of preventable death. Running survival analyses on the entire adult population (aged 16–59, n=927,134) controlling for socioeconomic position (SEP) and other potential confounds, we find that being single (compared to cohabiting/married) increases risk of accidental/suicide death for men (but not for women), whereas having dependent children is associated with lower risk of preventable mortality for women but less so for men. We also find that the protective effect of partners is larger for men with low SEP than for high SEP men. Findings support life history predictions and suggest that individuals respond to variation in fitness costs linked to their mating and parenting status.

Pepper & Nettle make a compelling case for how evolutionary thinking can help explain behaviours that cluster with deprivation. The role of extrinsic mortality risk in driving behaviour is probably important, but strong evidence is still lacking. By thinking carefully about behaviours seemingly at odds with an evolutionary life history perspective, we can gain important insights that will help refine theory.

Parents face trade-offs between investing in child health and other fitness enhancing activities. In humans, parental investment theory has mostly been examined through the analysis of differential child outcomes, with less emphasis on the actions parents take to further a particular offspring’s condition. Here, we make use of household data on health-seeking for children in a high mortality context where such behaviours are crucial for offspring survival. Using Demographic and Health Survey (DHS) data from 17 sub-Saharan African countries, we examine whether maternal factors (age, health, marital status) and child factors (birth order, health, sex, age) independently influence parental investment in health-seeking behaviours: two preventative behaviours (malaria net use and immunization) and two curative ones (treating fever and diarrhoea). Results indicate that children with lower birth order, older mothers and mothers with better health status have higher odds of investment. The effects of a child’s sex and health status and whether the mother is polygynously married vary depending on the type of health-seeking behaviour (preventative versus curative). We discuss how these results square with predictions from parental investment theory pertaining to the state of mothers and children, and reflect on some potential mechanisms and directions for future research.

Evidence from animal species indicates that a male-biased adult sex ratio (ASR) can lead to higher levels of male parental investment and that there is heterogeneity in behavioural responses to mate scarcity depending on mate value. In humans, however, there is little consistent evidence of the effect of the ASR on pair-bond stability and parental investment and even less of how it varies by an individual's mate value. In this paper we use detailed census data from Northern Ireland to test the association between the ASR and pair-bond stability and parental investment by social status (education and social class) as a proxy for mate value. We find evidence that female, but not male, cohabitation is associated with the ASR. In female-biased areas women with low education are less likely to be in a stable pair-bond than highly educated women, but in male-biased areas women with the lowest education are as likely to be in a stable pair-bond as their most highly educated peers. For both sexes risk of separation is greater at female-biased sex ratios. Lastly, our data show a weak relationship between parental investment and the ASR that depends on social class. We discuss these results in the light of recent reformulations of parental investment theory. This article is part of the themed issue ‘Adult sex ratios and reproductive decisions: a critical re-examination of sex differences in human and animal societies’.

In this chapter, we consider what evolutionary anthropology contributes to the study of population health, focusing specifically on family structure and its relationship to child and adult physical health. Evolutionary anthropologists have now built a substantial body of literature on relationships between family structure and various dimensions of human wellbeing, particularly in the context of small-scale ‘traditional’ societies and developing rural communities most at risk of poor health outcomes. Crucially, they have also constructed theoretical models to account for variation in key dimensions of family structure in relation to individual, ecological, and cultural factors. Thus, evolutionary anthropologists have much to say, not only about the extent to which specific family structures may influence health but also why certain family forms may persist or change across time and space. Here, focusing on studies primarily conducted in sub-Saharan Africa, we review the literature on three interrelated dimensions of family structure and their relationship to health: (1) family size, (2) polygynous versus monogamous marriage, and (3) the role of extended kin. Using these examples, we highlight the theoretical and empirical contributions of evolutionary anthropology and draw out implications for population policy and related initiatives seeking to improve family health in the developing world.