Exogamy

Exogamy is the social norm of mating or marrying outside one's social group. The group defines the scope and extent of exogamy, and the rules and enforcement mechanisms that ensure its continuity. One form of exogamy is dual exogamy, in which two groups continually intermarry with each other. In social science, exogamy is viewed as a combination of two related aspects: biological and cultural. Biological exogamy is the marriage of people who are not blood relatives. This is regulated by incest taboos and laws against incest. Cultural exogamy is marrying outside a specific cultural group; the opposite being endogamy, marriage within a social group. Exogamy often results in two individuals that are not closely genetically related marrying each other; that is, outbreeding as opposed to inbreeding. In moderation, this benefits the offspring as it reduces the risk of the offspring inheriting two copies of a defective gene. Increasing the genetic diversity of the offspring improves the chances of offspring reproducing, up until the fourth-cousin level of relatedness; however, reproduction between individuals on the fourth-cousin level of relatedness decreases evolutionarily fitness. In native populations, exogamy might be detrimental if "the benefits of local adaptation are greater than the cost of inbreeding." However, non-native, "invasive" populations that have "not yet established a pattern of local adaptation" may derive some adaptive benefit from admixture. Nancy Wilmsen Thornhill states that the drive in humans to not reproduce or be attracted to one's immediate family is evolutionarily adaptive, as it reduces the risk of children having genetic defects caused by inbreeding, as a result of inheriting two copies of a deleterious recessive gene. In one Old Order Amish society, inbreeding increases the risk of "neonatal and postneonatal mortality." In French populations, people who reproduce with their first cousin develop cystinosis at a greater rate than the general population.