In the US, only 24 of the 50 states flatly prohibit marriage between first cousins. I don't know if that applies to "removed" ones, as well, as in "first cousin, twice removed."
Since another famous Austrian scientist, Gregor Mendel, wasn't born until 1822 - Mendel was the "the first person to lay the mathematical foundation of the science of genetics (Britannica Online) - it's safe to safe that for most of 315,000 years that homo sapiens have been around, they didn't have much sense of how genes work. That doesn't mean that they didn't figure out things about heredity.
And, as Andrade carefully spells out, there are genetic risks associated with siblings pairing and reproducing, a risk that is significantly less in "consanguinity," sexual pairings between relatives not as close as siblings.
But is prohibition on incest physically, biologically based in some way? Or is it primarily a social institution?
I'll introject here for people who haven't much concerned themselves with the work of Freud or Sir Edward Burnett Tylor, the "incest taboo can be counted as one of the truly universal human institutions," as Andrade notes. Even if the commonly assumed genetic dangers of it may not be as well founded as most people think, it's still a foundational principle of human society and morality. So, don't boink your siblings, people!
As Andrade writes, Freud looked at incest as a social institution, not literally biologically based. As Freud himself posed the question, if people had some strong physical, biological aversion to incest, how do we explain the elaborate customs and practices in all human societies to avoid and prohibit it?
Skeptic, along with the roughly similar Skeptical Inquirer in the US, has generally tended to have a strong positivist leaning, in my experience. Positivists tend to view Freudian psychoanalysis as a pseudoscience, because it's claims about causation cannot be empirically tested according to Karl Popper's falsifiability criterion. But not all scientific evidence can be directed subjected to a falsifiability test, human evolution being one of them. So it's not exactly feasible to reconstitute a new set of original homo sapiens and do repeated experiments where we observe their activity and development over 315 millennia and do a statistical analysis of the results. So less pristine Popperian methods have to suffice. (Both Skeptic and Skeptical Inquirer are very committed to the theory of evolution, just to be clear.)
And it's also not as though falsifiable hypotheses have no place in studying human evolution. For years, one of the significant controversies among paleoanthropologists over whether Neanderthals (Homo sapiens neanderthalensis) and modern humans interbred successfully. Until fairly recently, those studies had to rely on comparisons of morphology and evidence of the close physical coexistence of the two types of humans.
But advances in DNA analysis provided away to formulate a falsifiable hypothesis like this: Modern humans show distinct traces of Neanderthal DNA. If that hypothesis is true and distinctively identifiable and analyzable samples of Neanderthal DNA are available, then scientists should be able to identify Neandenthal DNA traces in modern humans. And, in fact, that has been done and it seems to be generally accepted now - since 2010 or so - that Neanderthals and homo sapiens did successfully interbreed. (See: Maya Wei-Haas, You may have more Neanderthal DNA than you think National Geographic 01/30/2020
Andrade makes an evolutionary argument that because there is a biological benefit to humans to minimizing incest within immediately families, it must somehow have developed in some degree as a biological evolutionary trait, invoking the Westermarck Effect:
Yet, one particular theoretician was ignored for a long time. Edward Westermarck posited that we have biological drives, not towards sexually pairing with our closest relatives, but rather to feel sexual repulsion towards them. Westermarck realized that incest is indeed dangerous (although he did not have a full genetic understanding of why this might be the case), and that evolution selected for a mechanism that ensured that we would never feel sexual attraction for those that have been raised with us since infancy.But what he is describing in those two examples are children growing up in societies that observe the incest taboo, just like all others. In fact, the Taiwanese example could be argued to show an opposite tendency from the kibbitz one. In the kibbitz, as he notes, the children were not all from the same two parents, but growing up as children in that society, they developed something like an incest taboo toward their generational fellows in the kibbitz. In the Taiwanese example, the kids raised in close proximity did get married, even though some of the marriages turned out to "have far lower fertility rates and far higher divorce rates" than others Taiwanese families. Some of those pairings presumably worked well.
Westermarck was ultimately proven right. For example, genetically unrelated children who are raised together in Israeli kibbutzim very rarely marry or even have sexual relations with each other later on as adults. In Taiwan, there is a cultural custom in which very young girls are raised together with their future husbands; in turns out that, compared to the rest of the population, these marriages have far lower fertility rates and far higher divorce rates.
In addition, both examples give some indication that the social conditioning of growing in close proximity to siblings does (probably) create a kind of incest taboo that has some physical, evolutionary advantage if we're looking at many generations. But the fact that a social practice provides an evolutionary advantage does not mean that the practice becomes genetically hardwired into future generations. In fact, that would seem to require something like a Lamarckian theory of evolution rather than a Darwinian one, which really would be a radical claim that would require very extensive proof.
Ironically, one of Freud's personal quirks was that he argued at least to a limited extent for the Lamarckian theory of acquired characteristics, although I don't believe he applied it in the case of the incest taboo in the way Adrade's argument might suggest.
Going to another historical Austrian reference, Adrade writes:
On the other hand, if the founders had deleterious genes, then inbreeding will magnify their expression. This was the case with another famous but not-so-successful family, the Hapsburgs. These royals practiced inbreeding for many generations and as a result many family members developed the prominent “Hapsburg jaw.” One of them, Charles II of Spain, was born with serious genetic defects, thus ending Hapsburg rule in Spain in the late 17th century. The fate of the Hapsburgs (at least regarding the jaw) was probably set by the founder effect of Maximilian I, an emperor in the 15th century with the prototypical Hapsburg jaw, when the family had still not begun to practice cousin marriage extensively.I've heard the Hapsburg example mentioned often. I can't say I've tried to research it. But I don't know how well this has actually been researched or even could be. Anecdotal evidence is just that, anecdotal.
Likewise, the risks of cousin marriage have a cumulative effect. The Hapsburgs soon developed their prognathic jaw, but the effects of inbreeding became more pronounced (by the time of Charles II) when various generations practicing inbreeding accumulated the deleterious recessive genes. Thus, two first cousins whose families have not consistently inbred in the past, are relatively free of risk should they decide to marry.
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