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Hmm aside from inbreeding, which combines a lot of similar alleles which then increases the probability of inheriting defects. (For example if you have like 11 alleles of disease A and you marry a person not related to you, he probably won't carry that allele so your children won't have disease A. However, if you marry a related person, whom you share a large fraction of alleles, then your offspring will have a larger percent of that allele than both parents, and will then create a higher probability of having disease A. That is what cause defects in incestuous coupling.
However, long before they discovered that incest can cause a higher probability for defects and early death, it is already considered a taboo by most cultures. There are even stories in Greek Mythology where the gods punish mortals for committing incest so deficiency is not the highest priority but morality
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