Wealth modifies relationships between kin and women's fertility in high-income countries

Kin are generally expected to behave more cooperatively with their relatives than with unrelated individuals, and this cooperative behavior may result in positive effects on fitness. Such kin effects are likely to be modified by resource availability: in contexts of resource stress, cooperation among kin may disappear or weaken as more energy is required for investment in self. We use the Generations and Gender Survey, a large, multinational demographic survey, to test the following: firstly, how kin availability measures (parental survival status and coresidence with parents) affect measures of women's fitness (timing of first birth, total fertility, and probability of childlessness); and, secondly, whether wealth (an indicator of resource stress or abundance) modifies kin effects in a high-income, low-fertility setting. We find differing effects of survival status of, and coresidence with, parents on fertility outcomes. Having a living mother tends to be correlated with higher fitness: women with living mothers have earlier first births, and mothers' death in early life is correlated with a higher probability of childlessness. Fathers' survival has no effect on any outcome. Coresidence with parents, on the other hand, delays first births and results in lower total fertility and higher probability of childlessness. We additionally find that the negative effects of coresidence on reproductive outcomes are exaggerated for poor women. Our results speak of the role of environment in modifying the relationship between kin and fertility.