It is generally understood that inbreeding has some negative biological consequences for complex animals. Recessive diseases are the most straightforward. The rarer a recessive disease is the higher and higher fraction of sufferers of that disease will be products of pairings between relatives (the reason for this is straightforward, as extremely rare alleles which express in a deleterious fashion in homozygotes will be unlikely to come together in unrelated individuals). But when it comes to traits associated with inbred individuals recessive diseases are not what comes to mind for most, the boy from the film Deliverance is usually the more gripping image (contrary to what some of the actors claimed the young boy did not have any condition).
Some are curious about the consequences of inbreeding for a trait such as intelligence. The scientific literature here is somewhat muddled. But it seems likely that all things equal if two people of average intelligence pair up and are first cousins the I.Q. of their offspring will be expected to be 0-5 points lower than would otherwise be the case. By this, I mean that the studies you can find in the literature suggest when correcting for other variables that the inbreeding depression on the phenotypic level is greater than 0 (there is an effect) but less than 5 (it is not that large, less than 1/3 of a standard deviation of the trait value). Presumably for higher levels of inbreeding the consequences are going to be more dire.
In the links below I alluded to a controversy over the “Neurodiversity movement”. The basic issue is that people with Asperger syndrome and high functioning autism are being accused of putting their concerns above and beyond those of the large number of mentally disabled autistic individuals (some of whom are non-verbal, and exhibit severe cognitive deficits) in the grab for “rights.” Rights here understood as the rights which black Americans, women, and gays have claimed, to be recognized as equal before the law and endowed with the same value in the eyes of society. As a deep philosophical matter I’m skeptical of Rights in a fundamental sense. As a conservative I’m skeptical of the push for a huge array of rights by a plethora identity groups. Socially recognized rights are valuable, and are cheapened and debased by dispensing them too liberally.
My post below on atheism and autism caused some confusion. I want to quickly clear up some issues in regards to the model which I had in mind implicitly. In short I’m convinced by the work of cognitive scientists of religion (see Religion Explained and In Gods We Trust) that belief in gods and spirits is intuitively plausible to most people. It does not follow from this that when you have an intuitive belief that that belief is unshakable. This explains the variation in levels of atheism across societies as well as shifts of views across one’s lifetime. But, it also explains why in pre-modern societies acceptance of supernatural entities is the null or default position, if not necessarily universal.
But what’s the basis for the idea that belief in gods is intuitive? To reduce a lot of results down to a few sentences, humans live in a universe of other actors, agents, which we preoccupy over greatly. Additionally, we can conceive of agents which aren’t present before us. In other words, the plausibility of supernatural narratives derives from our orientation toward populating the universe with social beings and agency. There’s a lot of evolutionary psychological models for why this phenotype is adaptive, but that’s not relevant to us here. The point is that religious beliefs and systems use these intuitions and impulses as atoms with which they can build up more complex cultural ideas.