Alloparenting in humans: fitness consequences of aunts and uncles on survival in historical Finland

Offspring success depends on parental investment in humans, but it can also be improved by investment from other genetically related "helpers," known as "alloparents." Kin selection theory predicts that individuals with a low current reproductive value should be more likely to exhibit such helping behavior to improve their inclusive fitness. In humans, nonreproductive adult uncles and aunts are often expected to improve their nephews’ and nieces’ fitness, but few studies exist to test this hypothesis in detail. Using an extensive (N = 4145) demographic data set from 18th- to 19th-century Finland, we investigate whether the presence of adult aunts/uncles benefitted their nieces or nephews. Specifically, we use discrete time survival analysis to assess the effect of having adult aunts/uncles residing in the same parish in a given year, on a child’s risk of dying at each age from birth to 15, while controlling for the effect of other types of helpers (e.g., grandmothers and siblings). We also compare the effects of reproductive versus nonreproductive aunts/uncles and lineage (paternal vs. maternal). Overall, we show that contrary to predictions, the presence of nonreproductive uncles and aunts in the parish was not correlated with offspring survival, with only old childless uncles weakly improving their niece’s survival, young childless uncles decreasing their nephew’s survival, and old childless aunts decreasing their niece’s survival. This study is the first to directly investigate the fitness consequences of having childless adult aunts and uncles in a human family and has implications for understanding the evolution of family dynamics and cooperative breeding strategies in humans.