Whether couples agree on having a(nother) child is crucial for both individuals and society. While fertility research has long focused on women, recent studies emphasize the need to incorporate both partners’ perspectives. However, analyses that jointly consider men’s and women’s fertility intentions remain scarce. This focus on women has been partly driven by homogamy—the tendency for individuals to select partners with similar traits and values. Given that couples with mixed backgrounds have higher dissolution rates, they may also be less likely to share family-related beliefs. This study examines how agreement on fertility intentions varies among mixed and homogamous couples in Sweden. Using the 2021 Swedish Generation and Gender Survey (GGS) and stratifying by respondents’ gender, we find that most couples agree not to have a(nother) child, reflecting recent fertility declines. Couples where both partners are migrants exhibit the highest agreement, while mixed couples show the most disagreement and the strongest gender asymmetries in reported intentions. However, these differences are small and vary by the gender of the reporting partner. The higher disagreement among mixed couples aligns with broader research on their elevated dissolution risks. However, reverse causality is possible—value differences may be linked to other stressors, making childbearing less desirable. By highlighting the role of couple composition in fertility decision-making, our findings contribute to understanding how family formation dynamics vary across different couple types.

Objective: This study analyzes heterogeneity in divorce rates after the 1987 transition from mutual consent to unilateral no-fault divorce in Finland.

Background: Marriage and divorce legislation can impact divorce rates. However, some groups may be more responsive to changes in legal context than others. We propose that unilateral no-fault divorce laws either (a) increase divorce more in more or less divorce-prone groups, or (b) increase divorce equally across these groups.

Methods: We use population-wide individual-level register data from Finland to identify salient social groups with different divorce propensity, including ethno-linguistic and religious affiliations with divergent divorce propensity and couples of different parental status, marriage length, and marital history. We use piecewise constant exponential survival models to estimate the association with divorce proneness before and after the introduction of mutual consent divorce laws.

Results: Divorce rates increase in all studied subgroups by about 60% in the years following unilateral divorce. We found no support for the hypothesis that groups that were either more or less divorce-prone prior to the reform would be particularly responsive to divorce liberalization in the short-to-medium term.

Conclusions: The findings speak toward a universal rather than heterogeneous effect of divorce law liberalization.

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.

Despite increasing diversity within many societies, ethnically endogamous unions remain common. In contexts where one ethnic minority has lived alongside the majority for centuries, understanding who partners with whom is central to understanding how ethnic boundaries are maintained or dissolved. This study examines the role of own and parental ethnolinguistic affiliation for the first partner choice in Finland. We provide a unique test of the relevance of ethnic endogamy across two generations, in a context where both groups are native, but one (Finnish speakers) overwhelmingly outnumbers the other (Swedish speakers). Using register data on the total population, we examine how a person's ethnolinguistic affiliation and background affect the choice of the first cohabiting partner in terms of the partner's ethnolinguistic affiliation and background. We apply discrete-time competing risk models for men and women born 1970-1983. Results indicate that Swedish-registered individuals with two Swedish-registered parents are, by far, the most likely to partner with another Swedish-registered person with endogamous background. Partnering with a Swedish-registered person with exogamous background is most likely among individuals who themselves come from mixed unions. Patterns are remarkably consistent across gender, and adjustments for education and residential area only marginally alter the results.

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 frequencydependent 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.

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.

Age gaps between partners have undergone dramatic changes in high-income countries over the past century. Yet, there has been little focus on age gaps for immigrants and their descendants. This is an important omission because age gaps can be interpreted as a macro-level indicator of intergenerational adaptation. We examine the age gaps of biological parents (childbearing partners) among immigrants and their descendants in Sweden, a country with high gender equality and a stable mean age gap. Using longitudinal, whole-population data, we examine changes in age gaps for cohorts born 1950-86. Cohort trends in age gaps often follow very different patterns for male and female immigrants, with limited evidence of adaptation across cohorts. However, there is considerable evidence of adaptation towards the Swedish norm among the second generation, including from direct comparison between immigrants and their children. The largest differences between women and men are seen among the first generation with a Swedish-born partner.

Converging lines of inquiry from across the social and biological sciences target the adult sex ratio (ASR; the proportion of males in the adult population) as a fundamental population-level determinant of behavior. The ASR, which indicates the relative number of potential mates to competitors in a population, frames the selective arena for competition, mate choice, and social interactions. Here we review a growing literature, focusing on methodological developments that sharpen knowledge of the demographic variables underlying ASR variation, experiments that enhance understanding of the consequences of ASR imbalance across societies, and phylogenetic analyses that provide novel insights into social evolution. We additionally highlight areas where research advances are expected to make accelerating contributions across the social sciences, evolutionary biology, and biodiversity conservation. A detailed Review across animal and human societies provides insight on the causes and consequences of adult sex ratio skew.

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.

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 holds particularly 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. We discuss the implications with regard to the theoretical debate and problems of unobserved heterogeneity in the sex ratio literature.