Despite overwhelming evidence for a negative effect of inbreeding on fitness in plants and nonhuman animals, the exact nature of its effect in humans remains subject to debate. To obtain a better understanding of the effects of inbreeding on reproductive success in humans, we reconstructed the genealogies of the current inhabitants of a small and isolated Swiss village and used these to estimate the level of inbreeding of both members of all married couples, as well as their relatedness (i.e. the level of inbreeding of their offspring). Although there was no effect of parental relatedness on the number of children a couple had, we found that inbred mothers, but not inbred fathers, had significantly fewer children. Thus, although related couples did not have fewer children themselves, their inbred daughters did leave them with fewer grandchildren. Thereby, we provide evidence for the existence of inbreeding depression in human fertility, also in relatively outbred and egalitarian communities.
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