Abstract
It is expected that in humans, the lowered fitness of inbred offspring has produced a sexual aversion between close relatives. Generally, the strength of this aversion depends on the degree of relatedness between two individuals, with closer relatives inciting greater aversion than more distant relatives. Individuals are also expected to oppose acts of inbreeding that do not include the self, as inbreeding between two individuals posits fitness costs not only to the individuals involved in the sexual act, but also to their biological relatives. Thus, the strength of inbreeding aversion should be predicted by the fitness costs an inbred child posits to a given individual, irrespective of this individual’s actual involvement in the sexual act. To test this prediction, we obtained information about the family structures of 663 participants, who reported the number of same-sex siblings, opposite-sex siblings, opposite-sex half siblings and opposite-sex cousins. Each participant was presented with three different types of inbreeding scenarios: 1) Participant descriptions, in which participants themselves were described as having sex with an actual opposite-sex relative (sibling, half sibling, or cousin); 2) Related third-party descriptions, in which participants’ actual same-sex siblings were described as having sex with their actual opposite-sex relatives; 3) Unrelated third-party descriptions, in which individuals of the same sex as the participants but unrelated to them were described as having sex with opposite-sex relatives. Participants rated each description on the strength of sexual aversion (i.e., disgust-reaction). We found that unrelated third-party descriptions elicited less disgust than related third-party and participant descriptions. Related third-party and participant descriptions elicited similar levels of disgust suggesting that the strength of inbreeding aversion is predicted by inclusive fitness costs. Further, in the related and unrelated conditions alike, the strength of inbreeding aversion was positively associated with the degree of relatedness between those described in the descriptions.
Highlights
Offspring born to first-degree relatives are 17%–40% more likely to suffer disease and death as compared to children born to non-relatives [1,2,3]
We found a significant effect of sex on disgust reactions to inbreeding descriptions overall (Wald x2(1) = 32.23, p,.001) with women (M = 8.04, SE = 0.54) being more disgusted than men (M = 7.21, SE = 1.36)
We studied the reactions of 663 Finnish students and postgraduate students to various descriptions of inbreeding and provide the first extensive test of the association between the strength of inbreeding aversions and associated inclusive fitness costs
Summary
Offspring born to first-degree relatives are 17%–40% more likely to suffer disease and death as compared to children born to non-relatives [1,2,3]. Reproductive success is measured by the total number of allele copies that an individual is able to transmit further due to their own (in)actions, either through direct descendants or through offspring of related individuals such as the individual’s siblings or cousins This logic is captured by inclusive-fitness theory [6]. In the case of father-daughter incest, by not acting, an uninvolved brother or sister bears the cost of losing a potentially healthy niece/nephew of r = 1/4 (had the daughter had a child with an unrelated male) for an inbred sibling-niece/nephew with an r = 3/ 4, but suffering from inbreeding depression Whether this pays off in fitness terms depends on the magnitude of the inbreeding depression and the certainties of relatedness between the individuals involved. The closer the relatedness of the incestuous relatives to each other and to oneself, the greater the costs of inbreeding, and the greater the opposition to such acts
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