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Inbreeding leads to an increase in homozygosity, that is, the same allele at the same locus on both members of a chromosome pair.
This occurs because close relatives are much more likely to share the same alleles than unrelated individuals.
This is especially important for recessive alleles that happen to be deleterious, which are harmless and inactive in a heterozygous pairing,
but when homozygous can cause serious developmental defects. Such offspring have a much higher chance of death before reaching the
age of reproduction, leading to what biologists call inbreeding depression, a measurable decrease in fitness due to inbreeding among
populations with deleterious recessives. Recessive genes which can contain various genetic problems have a tendency of
showing up more often if joined by someone who has the same gene. If a son who has hemophilia becomes intimate with
his sister who may have the same gene for hemophilia, and they have a child, the odds are in favor that the child will have hemophilia as well.

Some anthropologists are critical of including biology in the study of the incest taboo, and have argued that there can be no
biological basis for inbreeding aversion because inbreeding may in fact be a good thing.
Leavitt (1990) is a good representative of this point of view, writing that "small inbreeding populations,
while initially increasing their chances for harmful homozygotic recessive pairings on a locus,
will quickly eliminate such genes from their breeding pools, thus reducing their genetic loads" (Leavitt 1990, p.974)

Other specialists claim that this notion betrays a misunderstanding of basic genetics and natural selection.
They argue that, while technically possible, the proposed positive long-term effects of inbreeding are almost
always unrealized because the short-term fitness depression is enough for selection to discourage inbreeding.
Such a scenario has only occurred under extremely unusual circumstances, either in major population bottlenecks,
or forced artificial selection by animal husbandry. In order for such a "purification" to work,
the offspring of close mate pairings must only be homozygous dominant (free of bad genes)
and recessive (will die before reproducing). If there are heterozygous offspring, they will be able to transmit the defective genes
without themselves feeling any effects. What's more, this model does not account for multiple deleterious recessives
(most people have more than one), or multi-locus gene linkages. The introduction of mutations negates the weeding out of bad genes,
and evidence exists that homozygous individuals are often more at risk to pathogenic predation. Because of these complications,
it is extremely difficult to overcome the initial "hump" of fitness penalties incurred by inbreeding. (see Moore 1992, Uhlmann 1992)

Therefore, it is not surprising that inbreeding is uncommon in nature, and most sexually-reproducing species have mechanisms
built in by natural selection to avoid mating with close kin. Pusey & Worf (1996) and Penn & Potts (1999)
both have found evidence that some species possess evolved psychological aversions to inbreeding,
via kin-recognition heuristics.

Given such overwhelming evidence of inbreeding depression as being an important force in sexual reproduction,
evolutionary psychologists have argued that humans should possess similar psychological heuristics against incest.
The Westermarck effect is one strong piece of evidence in favor of this, indicating that children who are raised together in
the same family find each other sexually uninteresting, even when there is strong social pressure for them to mate.
In what is now a key study of the Westermarck's hypothesis, the anthropologist Melford E. Spiro
demonstrated that inbreeding aversion between siblings is predicatably linked to co-residency.
In a cohort study of children raised as communal, that is to say, fictive, siblings in the Kiryat Yedidim kibbutz in the 1950s,
Spiro found practically no intermarriage between his subjects as adults, despite positive pressure from parents and community.
The social experience of having grown up as brothers and sisters created an incest aversion,
even though genetically speaking the children were not related.

Further studies have backed up the hypothesis that some psychological mechanisms are in play that "turn off"
children who grow up together. Spiro's study is corroborated by Fox (1962), who found similar results in Israeli kibbutzum.
Likewise, Wolf and Huang (1980) report similar aversions in Taiwanese "child" marriages, where the future wife was brought
into the family and raised together with her fiancee. Such marriages were notoriously difficult to consummate, and for unknown
reasons actually led to decreased fertility in the women. Lieberman et. al (2003) found that childhood co-residency with an
opposite-sex individual strongly predicts moral sentiments regarding third-party sibling incest, further supporting the Westermark hypothesis.

While the exact nature of kin-recognition psychology is still waiting to be defined, and to what degree it can be overcome by
cultural forces is as yet poorly understood, an overwhelming body of research now shows that evolutionary biology and
evolved human psychology plays a central role in human aversion to incest.