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October 11[edit]

Incest depression in humans[edit]

Is there any robust historical evidence for the occurrence of incest depression in a human population? 2A02:8071:60A0:92E0:CCFA:494F:469C:B47C (talk) 20:18, 11 October 2023 (UTC)[reply]

I assume you mean inbreeding depression? 'Incest' is a legal/cultural concept, rather than a biological one. AndyTheGrump (talk) 20:40, 11 October 2023 (UTC)[reply]

I thought he meant what Luke felt when he found out he was in love with his sister. --Trovatore (talk) 20:43, 11 October 2023 (UTC) [reply]

I'm not quite sure how you are defining robust or historical in this context. But is this, Associations of autozygosity with a broad range of human phenotypes [1], what you are looking for? Skullcinema (talk) 21:07, 11 October 2023 (UTC)[reply]

There's a lot of confusion regarding inbreeding depression in humans. Actually, the effect is worst in the first generation of inbreeding. In humans, first cousin marriage produces offspring that take the rate of detectable birth defects (including very minor ones) from 5% to 8%, that is, a 60% increase. But in ethnic groups that practice a lot of cousin marriages, the rates are about the same, 5%. So there aren't cumulative effects as alluded to in the movie Deliverance, quite the opposite. This is because deleterious alleles are purged more rapidly if they are exposed to natural selection by homozygosity. Abductive (reasoning) 06:03, 12 October 2023 (UTC)[reply]

All very interesting, but I think the OP was looking for references (evidence). Skullcinema (talk) 15:47, 12 October 2023 (UTC)[reply]

Incest in humans is often covert so no evidence. Abductive (reasoning) 18:45, 12 October 2023 (UTC)[reply]

Inbreeding, however, is a well-documented practice in some cultures. --Lambiam 05:32, 13 October 2023 (UTC)[reply]

You would have to be omnipotent to do a proper study. For instance, if you were looking at DNA, you would need the family tree going back generations—but it would have to be the true family tree, and you can't use DNA to obtain it or check it. People lie about parentage at a shockingly high rate, babies are switched at birth, etc. About all we know I have already stated above. Humans are expected to show the effects of inbreeding just like any other mammal, and there is no reason to not use what we know in animals, where we have done controlled experiments. Abductive (reasoning) 17:45, 13 October 2023 (UTC)[reply]

You gave us data on the rate of detectable birth defects among the offspring of first-cousin marriages. Where did you get the data from? From omniscient researchers, or from improper studies? --Lambiam 18:17, 13 October 2023 (UTC)[reply]

Alas, it was a long time ago that I read it, probably in a textbook. If one wanted to know more, they could just look at the animal studies and substitute the word "human". Abductive (reasoning) 18:37, 13 October 2023 (UTC)[reply]

Recent work on inbreeding depression in humans has not required family trees. Rather it relies on databanks of whole-genome sequences that reveal homozygosity. For instance, here is a study that calculates that if parents are first cousins, height tends to be 3 cm smaller, but with much variation in the effect between populations. This study uses the UK Biobank to identify 125 offspring (0.03%) of parents that were 1st- or 2nd-degree relatives, and draws conclusions about their reduction in health and about the incidence of genetic variants that are lethal when homozygous. For a recent example of the traditional approach using pedigrees, see this study in Swiss villages: closely related parents did not have fewer children, but their daughters had fewer. JMCHutchinson (talk) 11:23, 14 October 2023 (UTC)[reply]

Those studies did use or obtain parentage information, and have a lot of, let's say, leveraging of small sample sizes to find that there is indeed inbreeding depression in humans. The Biobank study looked at closer than first cousin mating. Moreover, by defining highly inbred people by their homozygosity levels, that study was bound to find the worst-affected people. As I mention above, those studies show that the effects are not as severe as popularly believed, and in ethnic groups that practice more marriage between cousins, the negative effects has been largely bred out of them. Abductive (reasoning) 11:52, 14 October 2023 (UTC)[reply]

No, the first two studies that I cited did not use or obtain parentage information; they just used homozygosity. And the Biobank study was likely biased in the other direction: people badly affected by genetic diseases are less likely to recruit themselves to become part of the database. JMCHutchinson (talk) 20:59, 15 October 2023 (UTC)[reply]

I think we are back to asking for citations to back up your assertions. Skullcinema (talk) 15:46, 15 October 2023 (UTC)[reply]

Read whatever Google Scholar throws at you, and you will see that my assertions are correct. Do you want an article that says doing experiments in humans is fraught with difficulties? An article saying humans can't self like plants, have internal fertilization, and average levels of genetic variation? Abductive (reasoning) 18:21, 15 October 2023 (UTC)[reply]