Incest

Incest is sexual activity or marriage between very close family members. It is a taboo, as well as a criminal offense, in virtually all societies. In many societies premarital sex is allowed or encouraged; in most such societies, the same restrictions apply to sexual unions as to marriage.

Genetics

Today, the general availability of birth control methods has made inbreeding a largely separate issue from incest. Opponents of incest argue, however, that incest should still be restricted since birth control is not always 100% effective.

Related Topics:
Birth control - Inbreeding

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Some have suggested that the incest taboo is a social mechanism to reduce the chances of congenital birth-defects that can result from inbreeding. Scientists have generally rejected this as an explanation for the incest taboo for two reasons. First, in many societies partners with whom marriage is forbidden and partners with whom marriage is preferred are equally related in genetic terms; the inbreeding argument would not explain the incest taboo in these societies. Second, the inbreeding argument oversimplifies the consequences of inbreeding in a population. Inbreeding leads to an increase in homozygocity, that is, the same allele at the same locus on both members of a chromosome pair. This occurs because close relatives are more likely to share more alleles than nonrelated individuals. If an individual has an allele linked to a congenital birth defect, it is likely that close relatives also have this allele; a homozygote would express the congenital birth defect. If an individual does not have such an allele, a homozygote would be healthy. Thus, the frequency of a defect-carrying gene in a population may go up, or down, when inbreeding occurs.

Related Topics:
Homozygocity - Allele - Congenital birth defect

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Thus, in small populations this dynamic would lead to an initial increase in birth defects. But if health care is limited, it is likely that such children would not reproduce; consequently, the frequencies for the allele in question would go down. Ultimately the result would be a population with a large number of homozygotes and a small number of birth defects. In large populations with good health care, however, it is likely that there will be consistently high levels of heterozygosity despite periodic inbreeding. Consequently the alleles linked to congenital birth defects will remain in the population, with a significant chance of a homozygote with the linked allele. In ancient Hawaiian society, negative genetic consequences of incest were circumvented by rigorous physical and mental exams required of children of nobility (ali'i) as proof of their social status, for example races over bare lava rock which would result in injuries fatal to hemophiliacs.

Related Topics:
Heterozygosity - Ali'i - Hemophilia

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Recent research on the mechanisms of human adaptive immunity suggests that there is a strong evolutionary pressure to maintain as diverse an array of antibody genes as possible. This may provide a biological explanation as to why opposite-sex siblings tend not to be attracted to one another and generally prefer to seek other partners. There is some observational evidence for this: in what is now a key study of the Westermarck effect, the anthropologist Melford E. Spiro, in a cohort study of children raised as communal siblings in the Kiryat Yedidim kibbutz in the 1960s, found practically no intermarriage between his subjects as adults despite pressure from parents and community. The immunoselective theory, however, does not account for the converse phenomenon (dubbed genetic sexual attraction) in which close genetic relatives growing up separately in adoptive arrangements have been observed to become strongly attracted to each other upon reunion.

Related Topics:
Adaptive immunity - Antibody - Westermarck effect - Anthropologist - Melford E. Spiro - Cohort - Communal - Kiryat Yedidim - Kibbutz - 1960s - Genetic sexual attraction - Adoptive

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