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There is a large bias against incest involving first-degree relatives in primates including humans. Both non-human great apes and humans avoid relations with first-degree relatives (i.e. parents with offspring, or siblings). Incest avoidance is considered a evolutionary mechanism to avoid undesirable alleles and phenotypes from remaining in the population (Pusey, 1990). There are many mechanisms for incest avoidance, both social and biological, including sex-based dispersal, MHC haplotypes, and olfactory cues. Westermarck (1891) proposed that close association during childhood (such as between siblings), reduces sexual attraction later in life. This mechanism was originally proposed for human incest avoidance, but is applicable to non-human great apes as primates have extended infant and juvenile developmental periods.
Among primates and most mammals, there is a large bias against relations between first- degree relatives. In fact, incest avoidance is seen as an evolutionary mechanism to avoid undesirable alleles and phenotypes from remaining in the population (Pusey, 1990). In human populations, incest has been shown to lead to higher incidence of genetic disorders (https://www.psychologytoday.com/blog/animals-and-us/201210/the-problem-i...), demonstrating the evolutionary importance of mating with non-relatives. Here, we will define incest as sexual relations between first-degree relatives (i.e. between parents and offspring or siblings).
While incest avoidance is common among humans and non-human primates, only humans have social repercussions for incest. In most Western societies, there are clear and strict laws against incest between first-order relatives (parents and children and between siblings. However, laws against incest are not universal; for example, in China, consensual incest between adults is legal ("Limits on the Protection of Legal Interest in the Criminalization of Incest (page 29)" (PDF). Max Planck Institute for Foreign and International Criminal Law. 14 March 2008. Retrieved 30 August 2012.).
Laws against incest vary from country to country, though most have laws protecting minors from incestuous relations. In the United States, despite clear laws against incest, it is estimated that 100,000 to one million cases of incest occur annually (http://family.jrank.org/pages/846/Incest-Prevalence-Incest.html).
In non-human primates, one important mechanism for incest avoidance is sex-based dispersal. Most primate social groups consist of several breeding females and one to several males. In virtually all groups, breeding males or females leave their natal groups (Pusey & Packer, 1987). Additionally, in primates, there are extended infant and juvenile developmental periods; familiarity during upbringing is a proxy for genetic relatedness. Thus, females and offspring or siblings are not likely to breed. Westermarck (1891) proposed, that for humans, close association during childhood, such as between siblings, reduces sexual attraction later in life. One good example of this in humans is Taiwanese “minor” marriages in the 1800s, where families would arrange marriages by giving their daughters over to the future groom’s family as infants. In “major” marriages, couples would meet right before the wedding (http://www.livescience.com/2226-incest-taboo-nature.html). Minor marriages, compared to major marriages, had far fewer offspring, thus providing some support for Westermarck’s hypothesis.
Jane Goodall, in her longterm field research (1986), showed that females and their male offspring and siblings hardly ever mated. Kuester et al. (1994) examined Barbary macaques; they found that matings between paternal kin were more frequent than those between maternal kin. However, they found that these matings rarely led to viable offspring.
One possible biological mechanism for incest avoidance is the major histocompatibility complex (MHC). The MHC are cell-surface molecules that control immune function; MHC molecules bind to protein fragments from pathogens and display them on the cell surface for recognition by immune cells (Jordan & Bruford, 1998). Similarity in the MHC, for humans, mediates such things as organ transplant rejection. Mating with an MHC-dissimilar individual would reduce the risk of mating with a relative; thus, one would expect primates and other animals to choose MHC-dissimilar mates (Grob et al., 1998). One study looked at MHC haplotypes in human mate choice in 411 Hutterite couples (North American isolated reproductive group of European ancestry). The researchers found fewer matches between MHC haplotype between spouses as expected using the geneology of the 411 couples and population genotype frequencies (Ober et al., 1997), providing support for the notion that MHC genotype plays a role in selection of non-related mates.
Another proposed biological mechanism of incest avoidance is through olfactory-mediation kin recognition (Boulet et al., 2009). Boulet et al. (2009) sampled labial and scrotal secretions from female and male ring-tailed lemurs throughout their different reproductive seasons. They found, using a combination of microsatellite and biochemical makers, that during the competitive breeding season, the odor profiles of related individuals converged. Thus, olfactory cues could provide a biological basis for rejecting related mates. Other animals, such as birds, also prefer non-related mates (Bateson, 1978). Bateson showed that male quail individuals prefer to mate with unfamiliar females, not with those that they were raised with.
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