Resources modify the associations between kin and fertility in historical Finland.

Inclusive fitness theory predicts that to maximize fitness, individuals should invest in kin, but relatives can also compete for resources. These opposing forces can have opposite effects on fertility, which was further constrained by marital and inheritance norms in traditional Western societies. How available resources influence this balance between helping and competing remains poorly understood. To investigate whether kin help or harm fertility, we used genealogical records from a population registry to study how relatives alive at age 15 were associated with marriage and lifetime number of children in Finnish people born 1752-1879, comparing landowning and landless individuals. Our results show that for focals born to landowning families, both older and younger siblings were associated with a higher age at first marriage, but the maternal grandmother and younger sisters were positively associated with fertility. These associations also differed by the focal's sex. Among the landless, siblings were also relatively weakly associated with poor marital outcomes, but not with the lifetime number of children. Relatives were not associated with lifetime childlessness. These results suggest that a high level of resources may intensify both positive and negative associations between kin and fertility, possibly through inheritance patterns, competition for resources and childcare help.