Radical declines in fertility and postponement of first reproduction during the recent human demographic transitions have posed a challenge to interpreting human behaviour in evolutionary terms. This challenge has stemmed from insufficient evolutionary insight into individual reproductive decision-making and the rarity of datasets recording individual long-term reproductive success throughout the transitions. We use such data from about 2,000 Finnish mothers (first births: 1880s to 1970s) to show that changes in the maternal risk of breeding failure (no offspring raised to adulthood) underlay shifts in both fertility and first reproduction. With steady improvements in offspring survival, the expected fertility required to satisfy a low risk of breeding failure became lower and observed maternal fertility subsequently declined through an earlier age at last reproduction. Postponement of the age at first reproduction began when this risk approximated zero–even for mothers starting reproduction late. Interestingly, despite vastly differing fertility rates at different stages of the transitions, the number of offspring successfully raised to breeding per mother remained relatively constant over the period. Our results stress the importance of assessing the long-term success of reproductive strategies by including measures of offspring quality and suggest that avoidance of breeding failure may explain several key features of recent life-history shifts in industrialized societies.

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Objectives This study provides an evolutionary perspective to a classic topic in demography, that is, the discrepancy between reproductive intention and subsequent behavior, in the context of China's two‐child policy. Methods We conduct an event history analysis of longitudinal data from the 2015 and 2018 waves of the Xi'an Fertility Survey (sample size = 321 followed one‐child mothers) to test the hypotheses of how within‐family support/conflict affects women's fertility behavior. Results Only 50% of positive intentions (i.e., intending to have a second child) led to another (live) birth within the 3‐year interval; meanwhile, 15% of uncertain intentions and 5% of negative intentions resulted in a birth. Husband's and the firstborn's emotional support raised the hazard of second childbirth along maternal life course, which cannot be fully mediated by mother's fertility intention and thus, contributed to an intention‐behavior gap. Husband's sibship size had dual effects on female childbearing behavior: A positive indirect effect mediated by fertility intention, but a negative direct effect presumably due to sibling competition for intergenerational support. Finally, after controlling for fertility intention, having a firstborn son was still associated significantly with a lower second‐childbirth hazard, presumably due to son preference as well as concern over parental investment. Conclusions Our study identifies a discrepancy between maternal fertility intention and realized childbearing, which was partly explained by (lack of) support from other (multiple) stakeholders in family reproduction.

The change in benefits of high socioeconomic status to fertility in humans during the demographic transition from high to low fertility has interested both demographers and evolutionary biologists. Evolutionary analyses add to demographic analyses by considering also males and status-related differential in male mating success, but they have been limited to time cross-sections and have not linked this differential to differentials in other determinants of male fertility. We use life-history records of males (n = 3791) entering marriage market before (1810s-1880s) and during (1890s-1960s) the Finnish fertility transition to investigate associations between socioeconomic status and chance and timing of marriage, choice for spouse, and lifetime fertility. Low status invariantly brought a lower marriage chance throughout these 160 years, which partly explained why ever-married high-status men lost advantage at early marriage when the system of achieving a high status shifted from inheritance to self-effort. The loss, coupled with assortative mating by age, promoted disappearance of differential in wife's age at marriage and thus, disappearance of differential in fertility between ever-married high-and low-status men in the Finnish fertility transition. Consequently, among all men (married and unmarried), statusrelated differential in lifetime fertility-not selection coefficient-declined over the transition. This study is among the first to show the interrelated dynamics of status-related differentials in male mating and reproductive traits; by doing so, it contributes to an evolutionary understanding of change of status-fertility relationship in human fertility transition and confirms continuing phenotypic selection on male status/wealth in modern societies.