20th Dec 2015

“Do you think incest and polygamy will be legalized?” – Part 3

I came across a poll on the subject of the future of full marriage equality:

I was watching a TV show recently and surprisingly, three characters (two men and a woman) got together in a sexual AND romantic relationship – as in, it wasn’t just a kink, they were serious about it, they even made their relationship official to their family/friends. And it got me thinking, do you guys think polygamy will ever be legalized? And incest? And should they be, and if not, why? Will our society ever see it as acceptable/normal like with gay relationships?

Science!!

Death, on 26 Apr 2014 – 6:30 PM, said:

Your statistic is also incredibly high, and smacks of stereotypes and not science. Of all the reasonably well done studies I’ve seen, the most pessimistic estimate for genetic problems for 1st degree relatives is 30%. [The more realistic estimate I’ve seen is around 11%, 1.5% lower than for middle aged women. (That’s for a single generation, of course.)]

http://www.lotscave….il 4, 2002).pdf 😀
^Meta-analysis of genetic defects due to incest
Incest is not like other recessive characteristics, the risk of serious birth defects and premature death is about 42% of births with another 14% having mild retardation making over half of children born to first degree relatives have some kind of genetic defect:

Ah, yes. I have seen pieces of this one around. (I appreciate the source, by the way.)

The section on 2nd and 1st degree consanguineous unions is taken from Bittles’ chapter (2nd chapter) in “Inbreeding, Incest, and the Incest Taboo”. One of the points to remember is that the “excess” rate of defect is ~30%, which is important because the control for Bittles’ meta-analysis showed a defect rate of ~8%, which is higher than would be expected. I’ve seen other studies (some are sighted in the cousin studies of that very paper) with defect rates of 1-4% in the general population.

Also, they quote Bittles himself from that same meta-analysis saying that the data for anything closer than 1st cousins is highly dubious, since finding unbiased samples is nearly impossible given the current legal consequences in most places, the social stigma almost universally, and the general lack of academic motivation on the subject. He later on says that, if only those consanguineous couples are considered who had no mental or physical defects themselves,their children don’t have a significantly higher rate of defect than the control group.

The samples in those studies have an elevated proportion of people who were themselves mentally or physically disabled (many, even most of those children are the result of abuse and rape, not consensual unions). The children were also frequently found out because of mental or physical problems which tipped off doctors. Just in general, loving, consenting adult relatives (closer than 3rd degree), who chose to have their baby or chose to carry it to term without abortion, and who are not themselves mentally or physically disabled or disturbed, are the hardest group to get in studies. They’re almost impossible to find, and they gain nothing from taking part.

I’ll just point out that in the cousin studies in that paper, which have much better samples, the rate of defect above the control group was only a few percentage points. Some of those in those studies were even [double 1st-cousins], and so count as 2nd degree relatives. Despite that, we’re asked to believe that when the degree of relation is one degree closer, the probability of defect explodes. I find that questionable, and the studies on the subject are acknowledged (sometimes by those doing the studies) to have little use for generalization.

None of that sounds like a sound basis for abusive, draconian punishments, blind social stigma, or marital discrimination. My other point also still stands, that a population wide statistic doesn’t tell you jack about any individual couple’s chances, given further knowledge of their genomes, lifestyles, and family history. Even if the probability actually was that high, it still would place them, as a population, at similar or lower risk than those with known dominant harmful genes. (I know of a few 1st degree couples with several healthy children, not to mention that a purely genetic view of “inbreeding depression” violates current scientific knowledge.)

Besides, I would question single studies with small, biased samples that show such alarmingly high rates over a single generation, given all the historical cases known of where several generations of 1st degree inbreeding had little to no effect. That’s another thing mentioned in “Inbreeding, Incest, and the Incest Taboo”: we have the mummies of Tutankhamen, his relatives, some ancestors, and other Pharaohs who were known offspring of siblings – several generations of siblings even – and no-one has found a regular pattern of deformity. The results of consanguineous marriages over many generations are heavily dependent on what kind of gene-pool the original people were coming from.

At this point, Tutankhamen is known to have had disabilities, but he was the end of a long line of arranged consanguineous unions, so that’s not surprising.