Couplings between close cousins, brothers and sisters, parents and children, children with uncles and aunts, and siblings with the same spouse occur frequently in the great works of literature, from the biblical tales of times past to the sizzling Game of Thrones that people are binge watching today. Sometimes the mating is consensual, and other times not. Sometimes the outcome is happy, and other times not so happy. When it comes to incest, our society doesn’t approve, and that’s consistently, whether one is attached to an organized religion or lives a completely atheistic life. But, on the other hand, all humans are cousins. Track your DNA sequences and you’ll find that you’re related to everybody. Very few humans on Earth will be anything less than your ninth or tenth cousin, because if you look far back enough on your pedigree any given ancestor starts showing up more than once. She may be a great-great-great-great grandmother, for instance, and also a great-great-great aunt and a 5th cousin once removed. And so, biologically speaking, what is incest, actually? What is inbreeding, but not incest, and what is simply mating between distant cousins?
If you ask a genetic counselors when it’s okay to marry a close-cousin and have children, you’ll be advised strongly against it, and this attitude is partly reflected in the laws throughout North America and most of the rest of the developed world. First cousins will not be issued marriage licenses in 25 US states, while some other states will allow the marriage, but with restrictions, such as requiring that the couple does not reproduce.
One interesting restriction is in the state of Maine, which requires you to consult a genetic counselor, but then you are allowed to marry your fist cousin. Finally, there is North Carolina, which allows marriage between first cousins, so long as they are not your first cousin, so long as you’re not double first cousins, meaning the result of two siblings of one family marrying two siblings of another family. If your husband’s sister is married to your brother, for example, you are both a mother to your children and an aunt by married. You hold two positions in the pedigrees of your children, but this combination puts that generation at no particular risk for a genetic disease. The problem comes on, however, if your kids get together with your nephews and nieces. In such cases, genetically, your grandchildren would have even more in common than children of other first cousin marriages. In cases of first cousin marriages, the risk for what’s called a recessive genetic disease –such as Tay-Sachs disease in Ashkenazi Jews, sickle cell disease in African Americans, or phenylketonuria in northern Europeans– would skyrocket. That’s because the combination amplifies the chances that gene for a recessive condition present in one great grandparent will be duplicated into the same family. In the same of double first cousins, the chances of producing a child with a recessive disease skyrockets even more, so there is actually a certain kind of logic to the North Carolina law.
State laws notwithstanding, any genetic counselor worth her salt will advise strongly against marrying your first cousin, but what happens with higher order cousins? When does inbreeding just become plain old breeding? Looking at a typical community, not in most US cities, where there has been an abundance of immigration in recent times, but in older countries, often the chances of marrying somebody in your city who is any more than a fifth or sixth cousin are very slim. Often, in some communities, the typical marriage is between third, fourth, or maybe fifth cousins, but in this case the relation is so distant that biologically it is not considered inbreeding, and the risks of negative consequences are strongly reduced compared with incest, or with first cousin marriages.
But what about possible advantages? To explore this issue, we need to consider that wasps interbreed all the time and it helps them. I’m referring to fig wasps, not the cultural, semi-ethnic group known as White Anglo-Saxon Protestants. Male wasps actually don’t have dominant and recessive genes, because the presence of just one copy of a gene for a given trait determines how the trait looks. When there is a gene for something bad, something lethal, a male wasp mating with his sister can expose that lethal gene to the force of natural selection, which then can eliminate it from the population. What’s fascinating is that geneticists have hypothesized that similar benefits can be operating on the human population, such that mixing with people a little bit close you genetically can have advantages. This must be balanced with the risks of negative outcomes resulting from inbreeding, and when you do the math, a genetic sweet spot shows up around the level of third cousins getting together.
But just because third cousin marriages might be good for the population, it doesn’t mean that you will be attracted to somebody in your family, even of you share only one great-great-grandparent Research published last year in Scientific Reports involving couples that were compared based on the types of proteins on their cell surfaces that immune system uses to distinguish “self” from foreign entities suggests that people tend to be attracted to those who are substantially different genetically from themselves. It contradicts the hypothesis that we seek people who look, sound, or smell like ourselves. An important caveat, though is that people do tend to get along better with those whose behavior is mostly in line with their own, so if you interpret this research as evidence for the old saying that “opposites attract”, it would apply only to physical opposites.