Incest: An evolutionary model

• Published in: Ethology and sociobiology 1990, Vol.11 (2), p.97-111
• Main Author: Welham, Clive V.J.
• Format: Article
• Language: English
• Subjects: Cuckoldry; Human sociobiology; Incest; Opposite Sex Relations; Paternity confidence; Sexuality; Victim Offender Relations
• ISSN: 0162-3095
• DOI: 10.1016/0162-3095(90)90031-Z

A majority of cultures have taboos against incestuous behavior (defined as heterosexual intercourse between individuals related by r ≥ 1 4 ). However, conservative estimates for modern industrial societies suggest that as many as 1 person in 100 will encounter incest in their lifetime. Females comprise the majority of victims, while the majority of perpetrators are adult males (usually fathers). Onset of incest varies nonuniformly with respect to female age and is highly correlated with age of menarche. Here, I apply evolutionary theory to the problem of incest, with particular emphasis on the male perpetrator/female victim dyad. Two assumptions underlie my hypothesis: 1) males promote their fitness by mating with as many females as possible; and 2) given the high parental investment required to rear human offspring, confidence of paternity is the primary factor mediating the risk of incest. A male who believes he has been cuckolded can abandon his mate (and her offspring), practice infanticide, or, if neither option is available, sire his own offspring and/or attempt to mate with any unrelated female offspring, once they reach reproductive age. A model is developed that explores the trade-off for the male between his confidence in lack of paternity, and the costs of an incestuous mating. Risk of sexual abuse is shown to be inversely related to the cost of incest and positively related to the probability of cuckoldry. Maternal relatives always pose less risk to females than their paternal counterparts, and if incest costs are sufficiently high, putative degree of relatedness may be a poor predictor of abuse risk. Female reproductive tactics will be at cross-purposes to those of the male, which result in a female attempting to increase the paternity confidence of her mate and/or limiting his access to her offspring. Finally, the model makes several predictions that are testable in preindustrial human societies: 1) For an equivalent degree of outbreeding, societies with low confidence of paternity will have higher rates of incest relative to those with higher confidence; and 2) if, on the other hand, paternity confidence is equivalent, then societies with higher coefficients of inbreeding (through the sanctioning of cross and parallel cousin marriages) will have higher rates of incest compared with more outbred populations.