There are a couple of new papers out on how utterly fucked-up Neanderthals must have been, The Genetic Cost of Neanderthal Introgression by Kelley Harris and Rasmus Nielsen, and The Strength of Selection Against Neanderthal Introgression, by Ivan Juric, Simon Aeschbacher, and Graham Coop. The basic idea is the same in both: the effective population size of Neanderthals was considerably lower than that of anatomically modern humans (which makes sense, considering glaciation), and this had negative consequences. Theory says that selection is less effective in small populations: mutations with small negative effects (s < 1/2N) are not much noticed by selection and accumulate. Judging from the Neanderthal genomes we have sequenced, their effective (neutral) population size was about a tenth of modern humans. The first paper concludes that Neanderthals must have had much lower fitness than AMH: They say "Under an additive model, a recessive model, or anything in between, the severe reduction in fitness of Neanderthals would have doomed them to quick extinction if they had been competing for the same niche with humans under conditions of reproductive isolation. " Of course, one problem with this conclusion is that we know it is not correct. The projected Neanderthal excess genetic load would have accumulated over several hundred thousand years: it wouldn't have been much smaller 120,000 years ago (the Eemian) than it was ~50,000 years ago, when we displaced them. Modern humans in the Levant (Qafzeh/Shkul) ran into Neanderthals back in the Eemian – but they didn't win then : later Neanderthals seem to have reoccupied that area, and moderns don't seem to have developed the ability to displace Neanderthals until much later, some 50-60,000 years after their original contact with Neanderthals. Like they imply, if Neanderthals were that genetically screwed up, something like Florida panthers, we could have knocked them over with a feather – but it didn't happen. Presumably we would also see skeletal abnormalities, lots of them, in later Neanderthals – but I don't think we do.
Part of the problem is that the model is probably too simple. There is an argument, which makes sense to me, that suggest that small-N populations do better than you would think, because as the average population fitness gets farther from the optimum, strongly beneficial compensatory mutations become more and more possible. And the effective population size for generating those beneficial compensatory mutations is not the same as the neutral effective population size, which is dominated by the low points (probably glacial maxima) – it's much larger, closer to the average population size, instead of the harmonic mean.
In addition, I don't have much confidence in our models of the distribution of mutational effects, or our models of the way in which the effects of deleterious mutations add up.
Both papers talk about the likely genetic burden that Eurasians picked up from that Neanderthal admixture. Since East Asians have a somewhat higher level of Neanderthal admixture than people in Europe or the Middle East (~20% more) then they must have even more toxic Neanderthal genes, and Africans the least. This echoes earlier papers that have argued that population history (out-of-Africa bottleneck, Neanderthal admixture, etc) must have increased genetic load in Eurasians.
Evidently extra genetic load has anti-intuitive effects.
West Hunter 
“small-N populations do better than you would think, because as the average population fitness gets farther from the optimum, strongly beneficial compensatory mutations become more and more possible. And the effective population size for generating those beneficial compensatory mutations is not the same as the neutral effective population size, which is dominated by the low points (probably glacial maxima) – it’s much larger, closer to the average population size, instead of the harmonic mean.”
So, it would be possible for self-isolated Europeans like the Ashkenazim to flourish, though at the cost of some deleterious mutations.
It takes a long time, hundreds of thousands of years or more, for small-N ( ~1000) to have these negative effects, to the extent that it does. Drift is slow.
So even the Sentinelese may not be much affected by this accumulation of deleterious mutations?
I wouldn’t think so.
The Neanderthals who left population genetic traces in modern humans were very likely born into human tribes. Thus, some of the lack of fitness was a lack of fitness for a member of a human tribe that may not have been fitness reducing in a Neanderthal tribe (those tribes may have had modern human introgression, but since the entire tribes died out, those hybrids died with their tribes).
So, for example, if verbal communication aided by FOX2 was critical for fitness in a modern human tribe, but not in a Neanderthal tribe, then a hybrid individual lacking it would be less fit in the modern human tribe even though those genes would not impair fitness within a Neanderthal tribe.
Similarly, if, as seems to be the case, modern humans were generalist hunters taking on both large and small game and fish, while Neanderthals were predominantly big game hunters, then hyper-adaptation to traits useful only for big game hunting wouldn’t be as fitness enhancing in modern human tribes as in Neanderthal tribes, because big game hunting is a group effort and skill at doing so is only an edge if your peers want to hunt that kind of game.
With an extra 250,000 years to adapt, Neanderthals were probably more adapted to their particular niche than modern humans were to any niche. But, in the face of a wildly variable climate at the time of Neanderthal-modern human transition, lack of hyper-specific niche adaptations in favor of plasticistity may have given modern humans an edge.
“The Neanderthals who left population genetic traces in modern humans were very likely born into human tribes.”
I’m thinking the Neanderthals “who left population genetic traces in modern humans” very likely had children born into human tribes.
True. Of course.
Jews, including the Ashkenazim are less genetically isolated from local populations than conventional wisdom would suggest. All Jewish populations show substantial local admixture with local populations.
I think they are more genetically isolated than “conventional wisdom” would suggest. Of course it depends how you numerically define “conventional wisdom” and how long you believe it requires for a population to become genetically isolated.
How fast does selection work on these sorts of mutations? Has enough time passed for most of these harmful genes to disappear from the East Asian gene pool, given that they aren’t subject to the small Neanderthal population size?
“Evidently extra genetic load has anti-intuitive effects.” A wonderfully waspish remark.
Yup. 😀
I mentioned before on Razib’s blog that study of Neanderthals is going to be mired in ostrich effects since there’s such a clean line between Africans and non-Africans. (Though I guess with that recent paper about Eurasian admixture across Africa, the line has has become blurred…)
The first paper says Neanderthals must have had “at least a 40% lower fitness than the average human due to higher levels of inbreeding.”
There are so many flaws in this I don’t know where to begin. So I will make a list;
1)Evolution doesn’t work this way. A whole species doesn’t go down the de-evolution tubes when there is inbreeding unless that species has a population far lower than the Neanderthals had. Inbreeding would cause less infants to make it to adulthood which was part of the plan before humans got so full of themselves and modernized.
2) So were anatomically modern humans before they got frisky with Neanderthals. The first anatomically modern humans were far and few between just like the Neanderthals so we too would have been forced to settle for far more cousin love than was genetically healthy for us.
3) The effects of inbreeding aren’t species wide as this statement implies, it is expressed in some individuals but not all. Gimpy and Dumbshit the two inbred kids of Neanderthals never made it to adulthood. Gimpy never made it to age one and Dumbshit was useless tit as a hunter so his brothers ate him instead.
The ever present “ostrich effect” shall be lurking around each and every Neanderthal paper that comes out, unless of course the author desires to be employed in another profession.
@dave chamberlin: Correct! See Helen Dean King’s fascinating “Studies on Inbreeding” available for free on Google Books. The trick here is progeny maximization.
I should have said: “progeny maximization of the most fit phenotypes.”
Assuming that selection pressures were strong and that population size of the groups remained fairly constant for a long time, I would venture that it means that for these small groups they tended to practice strict monitoring of who gets to procreate (and, possibly, how much) and, very likely, infanticide of malformed offspring was widespread and, further, even fillicide — possibly, all the way to early teen years. I’d bet the elders always had to test children with special games and rites, in order to weed out those who couldn’t make the cut. All that, while still engaged in community protection and group hunting. You need a strong group cohesion and rule by the old & wise for that.
Dave, you’re destroying my mind. I can feel it. But seriously, their idea is not really crazy. A population really can get into trouble if Ne is too small: selection is less effective, and slightly deleterious mutations are not eliminated as efficiently as in a larger population. They accumulate, we think. Run this long enough and they become messed-up enough that Ne shrinks further, things get worse and worse: mutational meltdown, they call it, and it’s a concern in conserving threatened species. This is not the same thing as the problems of inbreeding.
I think it’s more complicated than the model they used, and I know that the Neanderthals didn’t fall right over the moment that they met anatomically modern humans, but I don’t think that the idea is crazy, and it may have some merit. It might play a role in archaic humans losing out to us, and maybe in homo erectus-types losing out to Neanderthals and Denisovans earlier.
I’ve always thought that those larger pop sizes in Africa favored evolutionary innovation – but selection pressures matter too. I don’t think we know much about how those pressures differed between sub-Saharan Africa and Eurasia, back in the day.
Their intent, in both papers, is to explain selection against most Neanderthal genes in humans as a consequence of Neanderthal genetic load rather than incipient speciation (incompatibilities), but I am more interested in other issues.
I think this kind of problems implicitly assumes that gene flow out of Africa is truly nonexistent for long periods: the Sahara was apparently a hell of a barrier in glacial periods.
I’m not sure if I’m stupefying, intoxicating, or you are Hal the computer in 2001 the movie. However genetically screwed up those Neanderthals were I don’t know, they sure could have been. I’ll betcha those inbred hairy guys stuck in the misery freezer for hundreds of thousands of years mutated some intelligence enhancing alleles that we should thank them for, but I can afford to say that because I am a man of leisure.
Razib Khan has just posted an article that helped me to understand Greg Cochran’s point here http://www.unz.com/gnxp/size-population-matters-for-inbreeding/
Isn’t that a standard observation in other species, that they speciate in the tropics and invade the poles?
I can understand why drift might, by chance, increase the frequency of certain slightly deleterious mutations in a small population — perhaps even to fixation, But I don’t understand why deleterious mutations across the board would be eliminated less efficiently in a small population than in a large one. Can you explain? Aren’t most deleterious mutations recessive? And doesn’t that mean that selection against them is driven by the probability of an individual getting two copies? And isn’t that probability much higher in a small population than in a large one? To me it sounds like selection ought to be more effective in a small population than a large one (even if it isn’t enhanced enough to outweigh drift, which would be enhanced to an even greater extent). Obviously I’m wrong, but I don’t see why.
My understanding is that the reason deleterious mutations tend to be recessive is not anything fundamental about deleterious mutations per se, but because that’s the best way they have to hang around – if they were dominant they’d quickly get purged.
But in a small population, they don’t get purged quite so easily – a bad gene can get passed along generation after generation and move around the group since there’s not enough competition to sort it out.
But in a small population, they don’t get purged quite so easily…
That’s the part I don’t get. My understanding is that recessive genes only get purged when an individual has two copies. Isn’t that much more likely to happen in a small population?
Deleterious mutations don’t have to be recessive – particularly in a small population.
Interesting, D-r Cochran. Yet neandertal stone tool kit diversified just before their demise as well as neanderthal technologies (composite axe where natural birch glue was used to stick the handle to the blade, which was beyond the capabilities of HS at this time). That kind of advancement does not go well with the notion of increased genetic load.
There is more – future eurasians were only a fraction of africa diversity by definition during the out of Africa event, hence, predisposed to increase their own load and to be at the pick of their fitness immediately after they left africa. Yet they backed off when first met neanderthals 120 000 years ago, but presumably submerged them 60 000 years later, with a genetic load that must have increased in the meantime. Still, there is something fishy about neanderthals – 400 000 years ago they were using spears as projectile weapons. 60 000 years ago they were not. Nor were they 120 000 years ago. Negatively changed anatomy?
Do we have any idea how many babies a neander woman was likely to bear? Makes a big difference if she had ten as opposed to 5. Lots more space on which selection can act.
“There is an argument, which makes sense to me, that suggest that small-N populations do better than you would think, because as the average population fitness gets farther from the optimum, strongly beneficial compensatory mutations become more and more possible.”
Very interesting and sensible. Dr. Cochran, if you have something like that readily available, could you refer to a paper that elucidates on this argument?
http://onlinelibrary.wiley.com/doi/10.1111/j.0014-3820.2000.tb00693.x/abstract
COMPENSATING FOR OUR LOAD OF MUTATIONS: FREEZING THE MELTDOWN OF SMALL POPULATIONS
Thank you.
the Sahara was apparently a hell of a barrier in glacial periods.
I had thought the Sahara was a savannah during glacial periods. Perhaps a savannah is a barrier? Could you clarify?
I don’t want to offend anyone but I think we need more skulls not more theories. I can’t remember a time when specialists weren’t arguing about Neanderthals. The direction of the argument never changes because someone has made a particularly persuasive point. It changes when new physical evidence is uncovered.
+1
talk is cheap
skulls are very scarce
better and better interpretation of ancestral DNA is what makes this area of study exciting
skulls are very scarce.
And you need a permit to have one(other than your own). I can only imagine what the application for such a permit might be like.
Earliest humans in Europe, or at least some of them, seem to have left little in modern Europeans. So much can be derived from the Oase 1 genome. While Ust-Ishim clings to anything but Oase 1 in D-stats, Oase 1 still doesn’t bother to choose [1], even as both are equally equidistant to East-Asians and old Europeans. Oase 1 had a lot of Neanderthal admixture as just a few generations ago one of his forefathers must have been a full blooded neanderthal. One would think that whatever caused the demise of the Neanderthals may have very well also cause the demise of these AMH’s mating with them.
It occurs to me that the admixture itself may actually be the culprit for the decline of both of them. I think I read there are signs that offspring of the both suffered from low fertility. Maybe at the last stage the Neanderthal mixted as much or more with humans, with a result of lowered fertility of their offspring.
[1] http://www.nature.com/nature/journal/v524/n7564/fig_tab/nature14558_ST2.html
The reduction in fertility is purely Haldane’s law at work and would apply only to the hybrid children who would have been a pretty small minority at first generation and would have decreasing impact in subsequent generations and would still be a minority until the Neanderthal admixture was so dilute that it had no more fertility impact. The overall tribe’s fertility rate impact should have been negligible.
Correct me if I’m wrong but this scenario would imply a one time admixture event. But the Oase 1 genome has a very recent Neanderthal admixture upon a already double load of Neanderthal genes that K14 or Ust’Ishim had. The latter who were considered to have had a single of very few admixture events. That points to continuous admixture, at least for a while.
“When we remove the seven longest segments, the estimate of Neanderthal ancestry in Oase 1 drops from 7.3% to 4.8%, which is still around twice the 2.0–2.9% estimated for the French, Han, Kostenki and Ust’-Ishim individuals in this remaining part of the genome. This additional Neanderthal ancestry could reflect an older Neanderthal admixture into the ancestors of Oase 1, or that we failed to find all segments of recent Neanderthal ancestry.”
So considering that they said it caused a burden to European populations could we call this ‘White Man’s Burden’?
Sperm and egg cells are selected too by the genes they cary. Sperm cells particularly so. Otherwise mammalian k-selection wouldn’t work because thermodynamics dictates that many more mutations are deleterious than are beneficial. Pregnancy and parental investment in losers would be too expensive otherwise. In this way genetic load is filtered out substantially. That’s a reason mammals don’t have a cloaca. Let’s call it pre-selection. Am I right Greg?
So is it possible that modern populations suffering from inbreeding, like some Muslim groups, are better off than we think?
Too-small effective population size is not the same thing as inbreeding.
How about this: The winner was the modern human + neanderthal hybrid. One could say that it displaced both — at least outside of sub saharan Africa.
No modern European looks anything like a SH african and none have zero Neanderthal genes. If the Ns were such dead end losers they would not even now be a big factor in the genetic brain wiring of every single person reading this blog.
Following on from this, if we share some 95+% of our DNA with Chimpanzees, and if non-Africans have 1-3% Neanderthal DNA, what fraction of the non-shared DNA is Neanderthal? It seems we’re quite hybrid-y, but I know you can’t just subtract these things.
Insignificant, that’s how much.
Great post! Learn something new again. It takes me two days to finally get it especially it is counter intuitive.
Appreciate it.
Basically, it is another form of increased selection pressure like those in enviroment. Like any selection pressure, it is risky business.
But natural selection itself is game of lottery that need both lucky combination for the right situation.
The meaning of life is russian routlette
The meaning of life is russian routlette
Quit watching “The Deer Hunter”, IC – it’s starting to warp your perspective…
.The first fossil discovery of an ancestor of a chimpanzee was discovered in 2005 and that fossil only survived because that chimp ancestor died in a non rain forest environment. If our closest ancestors did not leave any trace of their existence in a hot humid environment than neither should we. I am guessing (nobody knows) that southeast Asia was well populated with either Denisovans or other hominids that we can find very little trace of.
I am further guessing (nobody wants to know) that it wasn’t a lucky accident that hominid hybridization happened in the middle east, it is the intersection of Asia, Europe and Africa.
I am further guessing (nobody wants to know again) that evolution pushes harder on intelligence in the harsh conditions of the cold far north and evolution pushes harder on disease resistance in the tropics. Everybody gets it that disease resistance is more important where it is warmer but the later idea has serious ostrich effect ramifications for reasons that should be obvious. I don’t know why such an idea is hard to understand logically because it isn’t. If you screw up and get lost for a night outside when it is 20 degrees outside, you are dead. If you screw up and and get lost for a night when it is 80 degrees outside well you had a shitty night.
Now all this is just talk, just unproven ideas. But better and better interpretation of ancestral DNA is happening very quickly now and don’t be surprised if these dangerous ideas don’t start leaking through the dike. Last week, as per John Hawks, it appears our Neanderthal percentage was upped a half percent because of new information from ancient DNA. Who knows what the future will bring. We might be promoting those unknown inhabitants of southeast Asia to a more important status in our family tree.
Yeah, rainforests leave a pretty bad fossil record. Which is interesting when you consider all the places where rainforests once existed. Illinois, Queensland, even Antarctica. I wonder what we’re missing from those times?
GCochran: as the average population fitness gets farther from the optimum, strongly beneficial compensatory mutations become more and more possible
Something like this struck me as linked to the pattern of apparently greater numbers of sweeps of derived alleles with falling genetic diversity from Africa (The Role of Geography in Human Adaptation – http://journals.plos.org/plosgenetics/article?id=10.1371/journal.pgen.1000500).
As the fitness of populations initially falls further from the African bottleneck, the advantage of fitness boosting mutations increases, sweeps become more likely. (As well as sweeps involving many variants at standing variation becoming less possible with less variation to work with, favoring novel derived variants).
(The examples in the above paper are CEU, ASN and YRI, showing many more fixed differences between ASN-YRI, than CEU-YRI, but seems more particular to falling genetic variation, as over half of the ASN sweeps at much lower frequency in West Eurasians seem shared by Papuans).
Either that or Eurasia was somehow different from Africa. Which it was, in many ways. Some of those sweeps are consequences of agriculture, which happened later in Africa. For post-agricultural times, population was bigger in Eurasia than in Africa.
True. Though, for the signals where there is a good and strong frequency difference in the same direction for Negritos, Oceanians and East Asians to the exclusion of West Eurasians, which seems like a good part of the East Asian sweep*, the selection would seem like it would have to have happened in a tropical(?) Asian environment their ancestors all shared, or otherwise be pan-Asian. Likewise with the Out of Africa sweep patterns, if much of that is due to Africa vs non-Africa environmental difference in selection, there would need either shared elements in all Out of Africa environments, or to have happened at an early stage when all OoA shared the same environment.
*it would have been nice to have had a followup quantifying this between 2009 and the present.
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Greg, Out there they are already building synthetic genomes. Evolution is the study of the past. It is becoming irrelevant. The future will be something different. Why not focus on how will look a society applying those newfangled technologies? It’s time somebody updated A.Huxley.
Study what already happened can help to understand how living beining work/change. Knowlage from past then can be turned around to work on future. They are connected.
For example. Figuring out what extinct genes did from the past might even can reconstruct extinct genes for future use when thay are needed. Let nature evolution to re-envent the wheel to come back the extinct genes might be like game of lottery – super low odd.
Hope this is helpful. Forgive me if I am wrong
FYI
Studying past can help us understand what we have today and how it happened. In psychological research, introversion is associated with high g. With higher g, higher percentage of introverts. This is a fact/truth already. But how this happened? The follow is hypothesis I contructed from historical facts.
As a kid growing up in urban poor ghetto, my observation is that underclass kids are extremely verbal and social. They spent most their energy on social relationship with complicated gang or gang like structure. Using their words to manipulate each other is the core of their activities plus constant physical violence as part of game. They are less interested in figuring out natural truth/knowlage.
Poor people tend to live in close proximity due to reduced personal space (personal space reflecting size of property/wealth). Three generations living in the same room and single wall seperating neighbours are very much common in poor ghetto. Constant dealing with other people become necessary and unavoidable. Also poor people need supporting each other to survive. So dependency is sign of poverty. Extrovert personality is needed to be social and function in such enviroment. Introvert is disaster here. Self-actualizers style of isolation (few friends) and aloofness are truely luxury only avaible for the rich. So called top out of sight class.
Almost universally any gain in wealth often leads to increased personal space (your own private bedroom, more space in room, larger size of property, distance from neighbor ect). With higher g associated with more wealth over generations, characters associated with self-actualizer behavior might become genetically fixed. Especially during feudal time, the social status is inherited. After thousands years, the personality might be genetically fixed due to confortable fitting of such self-actualizer personality (enjoying being alone/intravert).This is more like devolution process to become less sociable. High class might also exhibit other devolution feature like depigmentation (becoming pale) since they were most indoor creatures like those animals forever living in caves. All they need is to have intelligence to gain and control large amount of wealth which can be used to control hired army and police to protect the rule or laws efforcement.
People develop their unique (genetic)skills to fit their enviroment. Extroversion is need to live as poor. Introversion and depigmentation are genetic products of devolution from high g upper feudal class.
To prove this hypothesis, research is need to carry out on relationship between g, personality, and ancestral class during feudal time. If this is confirmed, the hypothesis becomes truth.
Here is some personal pain as a kid living in poor neighbood due to Communist political reason.
From the very start, I just did not fit in every way. When stronger and older boys bullied me, I fought back or sought revenge later like throwing bricks at their home windows in the night. These dominant boys were really puzzled since weaker kids should show submission to their bully as normal. On the other hand, any boy would bully anyone else smaller and weaker than himself. I just did not have any instinct to bully anyone even they were smaller and weaker. I was really odd ball in the hood. Weaker kids had no submissive attitude toward me. Stronger boys hesitated bullying me. I just did not fit in the hierarchy in the hood. Also I am quite introvert. They just ignored me at end. Indeed, odd ball. But they all concluded that I was useless. They also considered pale boy as weak boy too. Ironically, the top kid in the whole hierarchy became my only friend. Really weird then. It was nice to have `alpha’ boy to be your only friend. I am not submissive to him at all. He treated me more like equal. This helped a lot. In the hindsight, this makes sense. This boy might be the smartest one in the whole hood. But at end, he was still not good enough to get into college. But he was the only friend who would help me any time.
Funny thing was my high school teachers also thought I was pretty dumb. You can imagine that teaching standard was quite low for such neigbhood and the course works were not challenging at all. All kids can pass such class works due to their low threshold and low ceiling. My poor handwritting did me great favor to make me looks dumb in the eyes of high school teachers. When finally Chinese SAT time come, I were scoring incredible high even for the whole province. The teachers and hood were all shocked and in disbelief. Well, this was the very moment even my best `alpha’ friend stop talking to me.
Well, life moves on.
I never threw bricks at windows – that would be vandalism! Only at the bullies themselves, and I missed. Fun watching them run for a change, though.
Sory, I was guilty of that, which made me partially fit in the hood:). But misdemeanor could get away easily there since there were much more severe stuff going. My best friend’s brother (who was couple of years senior to us) was excecuted for murder. Bunch of kids in hood went jail for manslaughter or other felonies. Certainly some of them were also killed in street fight. This hood before revolution was notoriously low class with all criminals and their crimes. But they also helped each other in need like true brothers. When I was in college got bullied by other ganster from different district, these home bullies all got excited and went on gang revenge on my behalf even I did not have direct contact with them. These guys could not tolerate some one from their hood got messed by others even they ignored me when was living there. They really love to fight.
I should mention that my best friend (top boy) was actually also top student in my high school. His high school records was way above mine. But he did not do well in gaokao (Chinese SAT).
Your first problem is probably “My observation”. On average, anyone you’re going to observe is going to be relatively more social, outgoing and talkative, on the whole.
It’s like the “Looking at my friends, it seems like most people are more extroverted than me” phenomenon. Well of course you’d think that, if you’re an average person, your friends tend to be the sort of people who make friends – http://nymag.com/scienceofus/2015/04/your-friends-are-more-extroverted-than-you-are.html#
Looking at the actual class structure, although this is just Britain, the actual Middle Classes tend to be the gregarious and sociable “Established Middle Class”, more socially engaged than the working class – http://www.independent.co.uk/news/uk/home-news/britain-now-has-7-social-classes-and-working-class-is-a-dwindling-breed-8557894.html and marginals.
There is a “technical middle class” of course, but on the whole the middle class and rich are better at socializing, have more contacts and do more of it. (Although China / Taiwan, whichever you’re from, may be different, as I hear your folk regard gravity, restraint, and sincerity as more “noble” (but also that PRC and Hong Kong Chinese can be quite loud and brash).
Interesting point. Do you have alternative hypothesis about g and introversion?
My personal experience is two extreme with social classes. However I do believe your infor regarding middle class as most sociable people since it make logical sense. People can easily associate with others with similar mental ability, which fasciliate sociable activities (social validation). People with mental ability are making sense with each others. Opposite example is that college professors and janitors rarelly have any common topics.
Looking at g Guassian (bell curve) distribution with population, the middle class is most likely at middle point of such distribution and must have numerous people at such point. More people you can make sense with, more sociable you can be.
For highly intelligent people at right end of distribution, you just do not have many people around you can make sense with. For example of IQ 150, you only get one out of
2,330 people possesses such intelligence. Sociable activity for such individual needs to pretend to be stupid and agree with those idiots (average people) around you. It is hard thing to do for any sane person. Thus sociability for super smarts is very restricted. If human intelligence were stable for thousands years, highly intelligent people might also lost their sociability tendency due to disuse atrophy (devolution) and become introverts.
However, this only adds another selection pressure to my previous isolated noble classes situation. So multiple selection factors favor introvert personality in high end of g and social class.
My ghetto friend (top boy)is the only one that was talking to me since he and I were the only kids interested in those `useless trivial knowlage” like science and other intellectual curiosity stuff. He would never talked about these kind of stuff with other kids in the hood.
His way clawing to the top dog position also reflects his intelligence. Fear is part of factors that determines hierachy in the hood. But killing some would put you in jail and end the game for power strugggle. He uses knife and homemade gun in the fight. But he told me his trick. He only stabbed people’s thigh or ass which offered the most safe place for deep penetration without vital organs in the region. Ony risk is femoral artery at inner side of thigh. Thus he got reputation for wlling to use knife. Most kids were terrified of him. No body knew his trade secret except me. He studied anatomy for his power struggle with many consideration in mind. Another kid who only use baseball bat for the fight caused death due to hemorraghe from ruptured spleen and ended up in jail for manslaughter. Other kids were just not so thoughtful as my friend.
Intelligenc is even important in ghetto struggle.
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