Editing Incest taboo (section)

====Birth defects and inbreeding====
The increase in frequency of birth defects often attributed to inbreeding results directly from an increase in the frequency of [[homozygous]] alleles inherited by the offspring of inbred couples.<ref>{{cite journal | last1 = Livingstone | first1 = Frank B | year = 1969 | title = Genetics, Ecology, and the Origins of Incest and Exogamy | journal = Current Anthropology | volume = 10 | pages = 45–62 | doi=10.1086/201009| s2cid = 84009643 }}</ref> This leads to an increase in homozygous allele frequency within a population, and results in diverging effects. Should a child inherit the version of homozygous alleles responsible for a birth defect from its parents, the birth defect will be expressed; on the other hand, should the child inherit the version of homozygous alleles not responsible for a birth defect, it would actually decrease the ratio of the allele version responsible for the birth defect in that population. The overall consequences of these diverging effects depends in part on the size of the population.

In small populations, as long as children born with inheritable birth defects die (or are killed) before they reproduce, the ultimate effect of inbreeding will be to ''decrease'' the frequency of defective genes in the population; over time, the gene pool will be healthier. However, in larger populations, it is more likely that large numbers of carriers will survive and mate, leading to more constant rates of birth defects.<ref>Thornhill, Nancy, ed. 1993 ''The Natural History of Inbreeding and Outbreeding''. Chicago: UNiversity of Chicago Press</ref> Besides recessive genes, there are also other reasons why inbreeding may be harmful, such as a narrow range of certain [[immune system]]s genes in a population increasing vulnerability to infectious diseases (see [[Major histocompatibility complex and sexual selection]]). The biological costs of incest also depend largely on the degree of genetic proximity between the two relatives engaging in incest. This fact may explain why the cultural taboo generally includes prohibitions against sex between close relatives but less often includes prohibitions against sex between more distal relatives.<ref>{{cite journal |last1=Antfolk |first1=Jan |last2=Lieberman |first2=Debra |last3=Santtila |first3=Pekka |title=Fitness Costs Predict Inbreeding Aversion Irrespective of Self-Involvement: Support for Hypotheses Derived from Evolutionary Theory |journal=PLOS ONE |date=2012 |volume=7 |issue=11|doi=10.1371/journal.pone.0050613 |pages=e50613 |pmid=23209792 |pmc=3509093|bibcode=2012PLoSO...750613A |doi-access=free }}</ref> Children born of close relatives have decreased survival.<ref name="doi10.1038/ng1094-117"/><ref name="moral"/> Many mammal species, including humanity's closest [[primate]] relatives, avoid incest.<ref name="WolfDurham2005"/>