41 Comments

i've told you guys this, the outbreeding issues are not genetic (though they can be sociological). people have looked for several decades (from the late henry harpending to pontus skoglund) at evidence of purification of outgroup alleles like you see with neanderthals in modern human backgrounds by comparing african hunter-gatherers to outgroups (eg san bushmen who have eurasian DNA from nilotic and afrikaner). there is no evidence of this.

also, i figured the last decade of ancient DNA would clue you guys in on this; outbreeding is how most modern lineages emerged. the pairwise Fst btwn WHG and EFF is 0.10, same value as btwn han chinese and n europeans.

a word of constructive criticism: aporia should focus on open controversies and take my judgement on things that i know about. i've been in this game longer than most of you :)

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more concrete, outbreeding depression on the genetic level with incompatibilities seems to be something that arises in humans btwn 200K and 700K BP. the probability that it is occurring btwn eurasians who diverged at most 50K and usually 40K is trivial, but we don't see it with africans/nonafricans either

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Jan 10·edited Jan 10

How do you explain the Icelandic study?

I've read a high-powered study on inbreeding that finds, through statistical methods, decreases in fertility, height, and IQ out to 5th cousin crosses, and certainly no sweet spot within that range, so those scandi studies make no sense to me.

Some artifact of scandi societies? Some sociological effect, such as cousin marriage families have stronger family ties and networks whose benefits trump the genetic hit?

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"Some artifact of scandi societies? Some sociological effect, such as cousin marriage families have stronger family ties and networks whose benefits trump the genetic hit?"

the icleanders are not even genetically scandinavian. they're about 30-40% british (irish). but, they are very panmixed and homgeneous.

it's the same explanation for why so many societies engage in cousin marriage: less cultural distance and marital breakdown.

but whatever the reasons are, they are not genetically in a generalizable way. ppl have looked for several decades with better and better genomic methods. no evidence of purifying selection. period.

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Hi Razib,

Sorry, I put my reply in the wrong place. Please scroll down.

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Thank you for your insights. I would have expected outbreeding issues to arise in contemporary humans, but at much greater distance than what is considered optimal here.

My uninformed opinion is that cephalo-pelvic disproportion would cause problems : a mother with narrower birth canal giving birth to a baby inheriting his morphology from a 'wider' father. Because it seems to me that birth canals vary quite a bit in dimensions and shape and have evolved until very recently. But that would be more of a 'Ethiopian woman giving birth to the baby of an Inuit father' problem than a 6th degree cousin marriage problem. However, it could account for the disproportion of birthing problems for African Americans if White Supremacy wasn't so readily available.

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east asian women tend to have gestational diabetes issues with non-east asian. the reverse is not a problem.

east asian women are not known for having wide pelvises to my knowledge. they never have issues so i doubt that's a major consideration.

a lot of issues re: birth are due to the mother vs. fetus dynamic, and there is genetic work on this. father seems to matter a lot less. eg the mother determines baby birth weight; it's heritable. father not important

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Glad this comment is here because I was going to write the same thing after reading this ... my observation, growing up, was that the "mutts", i.e. people who were mixed-race or mixed-ethnicity, seemed to be, on average, a little smarter etc. (To note bias here, I'm a mutt, my husband is a mutt, and we're having mutt children). I would have guessed that lower rates of mixed people having children would have more to do with higher mobility, less family support, less family-orientation in general, less of a "cultural" identity, people in cities (where people are more likely to be mixed) having fewer children etc.

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Razib,

The negative effects of outbreeding should disappear through purifying selection after several generations. Consequently, remote Neandertal admixture isn't an issue. Nor is Eurasian DNA in Khoisan populations. Nor is East Asian admixture in Central Asia. That's why we need to examine first-generation effects, particularly effects on fertility. There may be other "canaries in the coal mine" but I'm not aware of any.

Yes, a lot of people have mused on this issue, like Ernst Mayr. But there have been very few controlled studies. In fact, there have been only two: the Icelandic one and the Danish one. I strongly suspect that the rise of male infertility is due to the increase in outbreeding, but that possible cause has to be disentangled from the possible cause of environmental estrogens. In my humble opinion, the outbreeding explanation is a better match for the epidemiology, but I could be wrong. This is an ongoing debate, and, incidentally, I'm not the academic who began it.

I realize this is a sensitive issue, and in some cases a deeply personal one. Of course. But it's not my role to tell people what they want to hear. Charles Darwin married his first cousin, yet that didn't stop him from investigating the deleterious effects of inbreeding.

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Peter, you did not respond directly to Razib's comment and so he will not be alerted to your response.

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"The negative effects of outbreeding should disappear through purifying selection after several generations."

no, this is wrong. purifying selection is the outbreeding effect that leaves a mark, and that is what you see from neanderthal genomes that admixed 50,000 years ago. there is no evidence of purifying selection of eurasian heritage in khoisan. therefore, no genetic issues (the incompatibilities you are talking about).

"that's why we need to examine first-generation effects, particularly effects on fertility."

harpending did a lit review, found nothing that he found genetically persuasive.

"But there have been very few controlled studies. In fact, there have been only two: the Icelandic one and the Danish one."

these are very genetically homogeneous populations. it's very strange to think you would notice a massive outbreeding effect in this group and not the widespread admixed groups around the world. it is also really strange that admixed populations are the origin of almost all modern populations but there is some massive outbreeding effect.

this just isn't a real thing. and you don't have to explain to me about setting aside political correctness.

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In the case of Neandertal and Denisovan admixture, we do have evidence of reduced fertility. Sankaramana et al. (2016) found that “the proportion of archaic ancestry in modern humans is significantly reduced on chromosome X compared to the autosomes. This is suggestive of reduced male fertility as loci contributing to this phenotype are concentrated on chromosome X in hybrids of other species.” In addition, “there is a reduction of archaic ancestry in genes that are disproportionately expressed in testes, a known characteristic of male hybrid fertility.”

Most academics accept that Neandertals and humans are separate species, but some still argue that they are subspecies. This supports my point: the distinction between “subspecies” and “species” is an arbitrary one. These are categories that we impose on a continuum of increasing genetic differentiation. So it’s unrealistic to believe that fertility is reduced only in crossings above the species level. "Species" is a human construct.

You mention that Henry Harpending did a literature review on this subject and found nothing. Are you referring to his paper with Alan Rogers in 2000? If so, that paper predates the Icelandic study (2008) and the Danish one (2008). In my conversations with Henry, I remember he wished to show that Neandertal genes had entered the human gene pool despite the possibility of reduced fertility. That’s not the same thing, however, as arguing against the existence of reduced fertility.

The Icelandic and Danish studies are important precisely because those populations are relatively homogenous, both genetically and socially. It is thus easier to disentangle the effects of outbreeding from socioeconomic confounds. The situation is different in multi-ethnic contexts, as with Davenport’s study in Jamaica. Jamaica, at least in his time, had a social hierarchy based on European admixture. The same is still true, to some extent, among African Americans. Yes, there is evidence that biracial individuals show higher incidences of certain health concerns, but it’s impossible to disentangle those concerns from socioeconomic confounds. Such disentanglement is far from “easy.”

If you wish, we could pursue a long and boring discussion about the advantages and disadvantages of ethnic mixture in the course of human history. Unfortunately, such a discussion would be worthless, for several reasons:

1. Intermixture between human populations usually occurs under conditions of social and economic inequality. There are thus too many confounds to disentangle.

2. In most cases, we lack good data. Evidence is often anecdotal in nature and subject to bias. “Victors write history.”

3. With aDNA, we are starting to get good data on genetic change, but this kind of research is still in its infancy.

On a final note, I thought long and hard before submitting that article. Some people will feel offended, not so much for “political” reasons as for personal ones. I understand all of that, but nothing will change if we refuse to talk about this subject. Reality doesn't care about our feelings.

References

Harpending, H., & Rogers, A. (2000). Genetic perspectives on human origins and differentiation. Annual review of genomics and human genetics, 1(1), 361-385. https://doi.org/10.1146/annurev.genom.1.1.361

Helgason, A., Pálsson, S., Guðbjartsson, D.F., Kristjánsson, þ., and Stefánsson, K. (2008). An association between the kinship and fertility of human couples. Science 319(5864): 813-816. https://doi.org/10.1126/science.1150232

Labouriau, R., and Amorim, A. (2008). Comment on "An Association Between the Kinship and Fertility of Human Couples." Science 322(5908): 1634. https://doi.org/10.1126/science.1161907

Sankararaman, S., S. Mallick, N. Patterson, D. Reich, S. Djebali, H. Tilgner, G. Guernec, D. Martin, A. Merkel, D.G. Knowles, et al. (2016). The combined landscape of Denisovan and Neanderthal ancestry in present-day humans. Current Biology 26(9): 1241-1247. https://doi.org/10.1016/j.cub.2016.03.037

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"3. With aDNA, we are starting to get good data on genetic change, but this kind of research is still in its infancy."

no, there are thousands of genetic samples from ancient DNA. it's not in its infancy, though it is certainly still expanding. the latest issue of nature shows how robust this field is:

https://www.nature.com/nature/volumes/625/issues/7994#:~:text=Elevated%20genetic%20risk%20for%20multiple%20sclerosis%20emerged%20in%20steppe%20pastoralist,has%20shaped%20modern%20immune%20responses.

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"In the case of Neandertal and Denisovan admixture, we do have evidence of reduced fertility. "

and we have no similar evidence in crosses of modern humans. none. people have checked.

" Yes, there is evidence that biracial individuals show higher incidences of certain health concerns, but it’s impossible to disentangle those concerns from socioeconomic confounds. Such disentanglement is far from “easy.”"

purifying selection shows up. you said that yourself above! but now you are saying it's hard. ok.

(for the nongeneticists out there, if you want to you could calculate these statistics now too; it's not that hard if you have technical skills. the data is out there to doublecheck with your own eyes what i' saying)

"On a final note, I thought long and hard before submitting that article. Some people will feel offended, not"

the only thing i'm offended by is you ignoring me on the fact that the genetic statistics that would show what you are arguing exist but you refuse to acknowledge they don't support your point. i hope that doesn't offend you!

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"But there have been very few controlled studies. In fact, there have been only two: the Icelandic one and the Danish one."

let me be clear: if there is a genetic effect to fertility, what peter is arguing above in part, there is no way this would not be very easy to detect btwn individuals who way way way more genetically distinct than icelanders are from each other.

what's being detected is almost certainly a sociological phenomenon, not a genetic one

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https://academic.oup.com/mbe/article/39/1/msab304/6400258

1) the evidence of purifying selection is very strong here in many papers

2) this study claims that most of it happened in the first 10 generations. that means we should EASILY detect it in african americans. we don't.

to reiterate: there is no genomic evidence of widespread incompatibilities at this evolutionary scale. the statistic is not hard (eg compare selection of outgroup admixture on X chromosome vs. autosome or intergenic vs. genic)

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[Me] “it’s impossible to disentangle those concerns from socioeconomic confounds. Such disentanglement is far from “easy.”" [Razib] purifying selection shows up. you said that yourself above! but now you are saying it's hard. "

There seems to be a communication breakdown here. I was talking about the difficulties of disentangling outbreeding effects from socioeconomic effects. The difficulties of purifying selection are something else altogether.

Purifying selection will, over successive generations, remove intrusive alleles that interact less efficiently, or not at all, with the rest of the genome. How many generations? That depends. I could build a cute model that predicts removal after X generations, but that model would be no better than its assumptions, and each assumption would have a high degree of uncertainty.

Take the case of Neandertal admixture in modern humans. How long did it take for purifying selection to do its job? There is no clear answer because the time of admixture remains uncertain. Perhaps it happened when modern humans entered Europe. Or perhaps it happened much earlier, within an intermediate population that already existed in the Middle East (Kramer et al., 2001). You cite the estimate of 10 generations. Perhaps that estimate is correct, and it wouldn’t bother me if it were. But it’s just an estimate based on a model.

What about purifying selection in a modern population? How many generations would it require? Even more. We have greatly reduced purifying selection by reducing infantile mortality, and now we’re starting to reduce fetal mortality.

“we have no similar evidence in crosses of modern humans. none. people have checked”

We have three studies where “people have checked:” the Icelandic study (Helgason et al., 2008); the Danish study (Labouriau and Amorim, 2008); and the Jamaican study (Davenport and Steggerda, 1928). You may disagree with the evidence, but it is a falsehood to say that the authors of those studies looked for evidence and failed to find any. Keep in mind that the issue here is not crossing between “races” (however you may define that term) but rather crossing as a function of genetic relatedness.

“there are thousands of genetic samples from ancient DNA. it's not in its infancy”

We have thousands of samples overall, but the number is small for any one research project … and often insufficient. This was the case with a study of cognitive evolution in ancient Greece. It looks like mean cognitive ability increased rapidly during the early period and then fell off a cliff at some point in classical times. Unfortunately, we need more data. Believe me, I love this area of research and follow it with passion. But I recognize its limitations.

“the only thing i'm offended by is you ignoring me on the fact that the genetic statistics that would show what you are arguing exist but you refuse to acknowledge they don't support your point.”

I’m not ignoring you, and I’m sorry if you feel that way. I’ve tried to address each of your criticisms, and not simply out of politeness. I’m interested in your point of view. If I’ve overlooked any of your criticisms, could you please indicate them?

Reference

Kramer, A., Crummett, T. L., & Wolpoff, M. H. (2001). Out of Africa and into the Levant: replacement or admixture in Western Asia? Quaternary International, 75(1), 51-63. https://doi.org/10.1016/S1040-6182(00)00077-X

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you are asking your readers here to believe that genetic outbreeding depression happens in iceland, but it is undetectable btwn khoisan and other human lineages.

fwiw, i know ppl who worked at decode in iceland on these projects. most of them seem skeptical of your interpretation no matter what the original authors might posit in the discussion.

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"Take the case of Neandertal admixture in modern humans. How long did it take for purifying selection to do its job? There is no clear answer because the time of admixture remains uncertain"

i already posted the paper above. the best answer is ~10 generations probably. and the time is not uncertain at all; the admixture into modern eurasians is 50,000 years ago(there were other events, but those events did not lead to the segments in modern humans). we know this from segment-based methods on modern genomes + ancient DNA (longer segments in individuals 45 K BP).

anyone who wants to read about the latest on neanderthal genomes... https://www.razibkhan.com/p/what-neanderthals-meant-to-you-what?utm_source=%2Fsearch%2Fneanderthal&utm_medium=reader2

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[Razib] “you are asking your readers here to believe that genetic outbreeding depression happens in iceland, but it is undetectable btwn khoisan and other human lineages.”

Unfortunately, we cannot study those cases with the methodology of the Icelandic study. The Icelandic genealogical database is almost unique in providing researchers with complete information on an entire population across many generations. This point is made by the study's authors:

“The advantage of using the Icelandic data set lies in this population being small and one of the most socioeconomically and culturally homogeneous societies in the world, with little variation in family size, use of contraceptives, and marriage practices, in contrast with most previously studied populations” (Helgason et al., 2008)

I know of only one other genealogical database of comparable worth: the BALSAC database right here in French Canada. It has not yet been used to replicate the findings of the Icelandic study.

There are alternative ways to estimate the reduction of fertility due to outbreeding but they are a lot less sensitive, particularly for admixture events that happen deep in the past.

[Razib] “the best answer is ~10 generations probably. and the time is not uncertain at all; the admixture into modern eurasians is 50,000 years ago(there were other events, but those events did not lead to the segments in modern humans).”

I don’t have a dog in this fight, and the estimate of 10 generations may indeed be true. Let me point out, however, that Neandertals may have intermixed with ancestral Eurasians much earlier. According to one model, they may have intermixed with Skhul-Qafzeh hominins in the Middle East between 80,000 and 120,000 years ago. Later, that admixture may have been absorbed by modern humans as they pushed into the Middle East some 60,000 years ago and then into Europe.

Again, I’m not bothered by the estimate of 10 generations, and I don’t understand why I should be. Why do you think that bothers me?

[Razib] “i know ppl who worked at decode in iceland on these projects. most of them seem skeptical of your interpretation no matter what the original authors might posit in the discussion.”

An unnamed source is of no value. You have to provide a verifiable source, like a paper in a journal or an article in a newspaper. Otherwise, there is no evidence to back what you say.

This is a recurring problem in your style of argumentation: you present yourself as an expert with inside knowledge, and you demand that we believe you — as an expert with inside knowledge. And if we refuse to believe, we become bad people — because we refuse to believe.

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" And if we refuse to believe, we become bad people — because we refuse to believe."

bad is not the word i would use :)

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"The Icelandic genealogical database is almost unique in providing researchers with complete information on an entire population across many generations. This point is made by the study's authors:"

the authors were writing in 2008. it's not as unique now fwiw. finland and estonia are replicating it for one

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How do these studies square with the concept of “hybrid robustness”, as discussed in “The Beak of the Finch”? The concept maintains that mating between difference species (the example often used is dogs + wolves) yield offspring with superior characteristics to either species.

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Mating between species is high-variance. In the long run yes, new innovations will often have emerged from hybrids. In the short run no, most of them will be weak. There's no contradiction here.

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birds have more hybrid robustness than mammals because they are not live-bearing (the major issue in mammals early on is miscarriage)

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many ppl here fancy themselves politically incorrect and fearsome, but i've been in this game a long time. look at question #5 in 2005 https://www.gnxp.com/blog/2005/12/10-questions-for-armand-m-leroi.php

i've been looking into this for a long time. there is no persuasive genetic evidence. ppl can be edgy all they want (mostly anonymously from what i can see in these comments), but if the evidence does not fit, you must acquit. just move on. you can oppose mass immigration for cultural reasons. adding explanations that do not hold water just make ppl think you are cranks

(this is the second time this issue has come up on aporia, and this will be the last time i address it, i have better things to do)

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Jan 10·edited Jan 10

I have a feeling that the people who are incline to do IVF + embryo selection to get high IQ children are not people looking only to marry and have kids with their 3rd or 4th cousins. Starting with myself.

If the criteria is to get the highest IQ and lowest disease risk, then the polygenic scores will tell us exactly that, regardless of who the person is. I wouldn't start this process by looking at my 3rd cousins. It will be from people I went to university or at work where I can evaluate their health and IQ easily. And verify that later with actual polygenic scores.

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The Flynn effect occurred because people got better at test-taking. During the 20th century, students were prepped to take the SAT and ACT tests, so this made them more prepared to take the IQ test and were less nervous during the test. This led to an increase in registered IQ during the 20th century because the subjects were more comfortable taking tests, concealing an actual cognitive decline.

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I've wondered how Morman inbreeding could produce such attractive and intelligent peoples (minus the temple garments).

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Mormon inbreeding is extremely rare, as they have an abundant inflow, and have for years, of new people, with many that travel and meet each other at BYU, etc.

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Jan 10·edited Jan 10

I think that the main difference lands on Outbreeding + Weak Selection (With modernity, vacines, etecetera) are able to create and accumulate genetic problems including fertility problems.

Is highly probable that with a Strong selection + Outbreeding generates some differential genetic combinations that can manifest in small groups and be more dominant as time goes without any effect on health or fertility of the population.

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miscarriage rates probably have gone up to compensate

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I wonder if in part, somewhat, if you are breeding with people dissimilar it tells the female body that there has been a war and you are breeding with the arrivals. Ie, like you are being raped by a foreign tribe and maybe your genes are programmed to die out more readily in that scenario. As your genes caused the loss of the males in the environment. Interesting brain trip I went down with this. Excellent article. Thoroughly enjoyed.

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Fascinating- thank you. Do you think hybridization in humans - coupled with severe evolutionary pressures - could accelerate fitness for those that are positioned at the geographical crossroads of different human populations?

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Thank you - armchair geneticist here

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If you compare a global map of exogamous countries with a global map of the average IQ of nations, a pattern will emerge. The more exogamous societies have a higher average/median IQ.

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Jan 11·edited Jan 11

Is it possible that until recently western men married girls from their village (probably related though not too closely) while more recently they are marrying girls from the other side of the world? I.e. could it be the case that we once had close to ideal genetic similarity with our spouses but in modern times it has become ever further from ideal? The average IQ of such a population would still be relatively high compared to many other places in the world while the fertility rate declined. That all seems quite plausible to me.

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There is an imaginary line between Eastern and Western Europe known as Hajnal line.

Marriage practices East and West of the line differ considerably (mainly, people on the West are exogamous due to following Church's edicts since 6th century CE).

Consequently, countries on the west of Hajnal have considerably higher average IQ and the reason most probably is exogamy.

This pattern recurs in most of the world's populations. I have written a book about it that was recently published.

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I see. Sounds like you know a lot more about it than me! Considering you have written a book about it you did well to summarize it so succinctly. I would have been tempted to go into too much detail. Anyway, good luck with your book.

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I want to see Peter's reply but can't see it.

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The study results were exciting but not what I expected.

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