Abstract
Due to the intense selection pressure against inbreeding, humans are expected to possess psychological adaptations that regulate mate choice and avoid inbreeding. From a gene’s-eye perspective, there is little difference in the evolutionary costs between situations where an individual him/herself is participating in inbreeding and inbreeding among other close relatives. The difference is merely quantitative, as fitness can be compromised via both routes. The question is whether humans are sensitive to the direct as well as indirect costs of inbreeding. Using responses from a large population-based sample (27,364 responses from 2,353 participants), we found that human motivations to avoid inbreeding closely track the theoretical costs of inbreeding as predicted by inclusive fitness theory. Participants were asked to select in a forced choice paradigm, which of two acts of inbreeding with actual family members they would want to avoid most. We found that the estimated fitness costs explained 83.6% of participant choices. Importantly, fitness costs explained choices also when the self was not involved. We conclude that humans intuit the indirect fitness costs of mating decisions made by close family members and that psychological inbreeding avoidance mechanisms extend beyond self-regulation.
Highlights
From a gene’s-eye perspective, there is little evolutionary difference between the costs of selecting fitness-jeopardizing sexual partners oneself and the costs when other family members do the same (Hamilton, 1964)
Given the intense selection pressures posed by short generation pathogens (Hamilton, 1980; Tooby, 1982) and deleterious recessive mutations (Bittles and Neel, 1994; Charlesworth and Charlesworth, 1999), humans (Westermarck, 1891; Lieberman and Antfolk, 2015) and a wide range of other species (Manson and Perry, 1993; Pusey and Wolf, 1996; Bretman et al, 2004; Lemaitre et al, 2012) have evolved systems promoting the avoidance of the fitness costs associated with inbreeding
Of the 27,364 choices, 83.6% were classified correctly based on the difference between the inclusive fitness costs of the two scenarios
Summary
From a gene’s-eye perspective, there is little evolutionary difference between the costs of selecting fitness-jeopardizing sexual partners oneself and the costs when other family members do the same (Hamilton, 1964). The likelihood that a given allele will be passed onto future generations can be substantially compromised via either route. One mating arrangement that imposes substantial fitness costs is inbreeding between close kin (i.e., sex between closely related biological kin). Given the intense selection pressures posed by short generation pathogens (Hamilton, 1980; Tooby, 1982) and deleterious recessive mutations (Bittles and Neel, 1994; Charlesworth and Charlesworth, 1999), humans (Westermarck, 1891; Lieberman and Antfolk, 2015) and a wide range of other species (Manson and Perry, 1993; Pusey and Wolf, 1996; Bretman et al, 2004; Lemaitre et al, 2012) have evolved systems promoting the avoidance of the fitness costs associated with inbreeding.
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