It has long been known that inbreeding is bad for you. A new paper in Nature (Directional dominance on stature and cognition in diverse human populations) finally gives us a good quantitative estimate of just how bad it is. They find that the offspring of first cousins suffer an average reduction of 1.2 cm in height and 0.3 sd in g ( ~4.5 IQ points) . They directly measured runs of homozygosity – more accurate than estimating from genealogy, and better in other ways as well. Children of first cousin marriages also suffer an elevated incidence of significant genetic disease, roughly 1.5-2 times the non-inbred risk.
That’s not all that far from my previous horseback guess. The field was full of old and messy studies, so horseback guesses were the best you could do, until now.
The article mentions that Darwin recognized negative effects of inbreeding (a bit too late): they also say that he was among the first to do so, which is not true. Al-Jahiz talked about it back in the Abbasid caliphate (around 800 AD) and assumed that everybody knew about it: “as the reader knows, the same is the case with horses, camels, asses and pigeons when they are inbred. ” I suspect that people noticed this soon after farm animals were domesticated, thousands of years ago.
In some parts of the world, mainly the Middle East and North Africa, cousin marriage is very common, with tens of percent of the population practicing it. Repeated marriages among close relatives can sometimes push the homozygosity up considerably higher than the first-cousin level, with even worse results.
Some retards (British papers) have been spinning this as saying that there are big benefits to mixed-race marriage. Untrue: to avoid lots of ROH (runs of homozygosity), just marry someone who isn’t from the same isolated population as you. We’re talking outside the valley or across the river : intercontinental travel is not necessary. Now there might be a degree of hybrid vigor in some distant crosses (currently unclear) – but likely not enough to compensate for someone coming from a group that has low trait values. Marry a Pygmy and your kids are going to be short. Marry someone from a population whose average IQ is below 90 (much of the world) and your kids will on average be less smart.
Naturally, enlightened opinion increasingly supports legalization of first-cousin marriage, due to its usual ignorance, perversity, and nihilism.