James Schipper: You are far too concerned about inbreeding. The problem with inbreeding is that it increases the probability of harmful recessive coming together. However, if those genes are very rare to begin with, then the negative consequences of inbreeding will be quite modest.
Some time ago, the German magazine Der Spiegel had an article about that.
Its conclusion was that the consequences of cousin marriages among Turks weren’t particularly severe. As an example, it mentioned a debilitating disease caused by a recessive gene.
This gene has the same frequency among Turks as among Germans, but 1/10,000 of Germans had the disease while 1/4,000 of Turks had it. So the frequency of the disease among Turks is 2.5 times higher than among Germans. However, 1/4,000 is not a huge problem.
Suppose that x is a recessive gene and that it has a frequency of 1%. If mating is purely random, then 1/10,000 of the population will be xx, (1x + 99X)(1x + 99X) = 1 xx + 198Xx + 9801XX. The more assortative mating becomes, the more it will approach the limiting case of 1xx + 99XX, in which case 1/100 of the population will be xx.
Contrary to widespread belief, inbreeding does not worsen the gene pool. It may lower the quality of population, yes, but that is something different. If anything, inbreeding may lead to an improvement of the gene pool. This can happen if people with xx are sterile or so undesirable that nobody will mate with them. In such a case, the x gene will gradually disappear as more and more people who are xx will be born.
All indications are that the predominantly Muslim populations of Northern Africa, West Asia, and South Asia are on average not as intelligent as Europeans. However, neither are the Hindus.
Ashkenazim Jews, on the other hand, are on average smarter than Gentile Europeans, but consanguineous marriages were common among them as they were among blue-blooded Europeans. European kings or aristocrats who married a cousin were legion. Queen Victoria and William III, for instance, were married to a cousin.
The problem is that something called inbreding depression has been well-documented for IQ. The more inbreeding, the lower the IQ goes on average. You can plot it on a graph. However, if this is true then why is Ashkenazi Jewish IQ so high? And they do have quite a few genetic diseases too.
The high rate of genetic diseases among certain Arab Muslim populations, especially in the Gulf, has been well-documented. And the effect of inbreeding depression on the IQ’s of Muslim groups who practice this has also been documented. I think what James’ comment leaves out is the well-documented inbreeding depression effect that cousin marriage has on the IQ of offspring. He leaves that out of his critique.