The standard argument for why there is aversion to incest among humans as matter of innate disposition is the Westermarck effect, which is a model where aversion to mating emerges if you are raised with an individual of the opposite sex. Some basic illustrations are sketched out in The Blank Slate: The Modern Denial of Human Nature. But some comments below make me wonder if there is are alternative explanations. Robin Fox has made the claim, repeated in many places, that cousin marriage was ubiquitous in the human past:
Beaudet wrote in the letter that “clinicians uncovering a likely incestuous relationship may be legally required to report it to child protection services and, potentially, law enforcement officials” since the pregnancy might have occurred “in the setting of sexual abuse.”
The letter was prompted by a Baylor laboratory’s discoveries that developmental disorders in a number of pediatric patients were caused by incestuous relations not previously disclosed to doctors.
The testing is done to find the disorder’s genetic basis, typically involving mutations, deletions or duplications. But large blocks of identical DNA are evidence the child’s parentage involved first-degree relatives.
If you’re curious if you are the product of incest, David Pike’s runs of homozygosity (ROH) detector (Mozilla only) might be useful if you have a genotype file. Sometimes I wonder if mass technology is going to come to fruition far earlier than it takes to write up editorials and publish them.
Of course even people who are not the product of first-degree incest can have very long ROH. Look at Zack (though is in part a legacy of the consanguineous marriage practices common in his parents’ community). And there will be many people who are going to get their siblings typed (as I did), so amongst the intelligent set cuckoldry, rare as it is, will be exposed.
The map above shows the distribution of consanguineous marriages. As you can see there’s a fair amount of cross-cultural variation. In the United States there’s a stereotype of cousin marriage being the practice of backward hillbillies or royalty. For typical middle class folk it’s relatively taboo, with different legal regimes by state. The history of cousin marriage in the West has been one of ups & downs. Marriage between close relatives was not unknown in antiquity. The pagan emperor Claudius married his niece Agrippina the Younger, while the Christian emperor Heraclius married his niece Martina. Marriage between cousins were presumably more common. With the rise in the West of the Roman Catholic Church marriages between cousins were officially more constrained. Adam Bellow argues in In Praise of Nepotism: A Natural History that there’s a material explanation for this: the Roman church used its power over the sacrament of marriage to control the aristocracy. Though the church required dispensations for marriages between cousins of even distant degrees of separation, they were routinely given, as was obviously the case among Roman Catholic royal families like the Hapsburgs. But once given the dispensation could be revoked, rendering the marriage null and void. A highly convenient power politically.