The Psychological Diversity of Mankind

The dom­i­nant be­lief on this site seems to be in the “psy­cholog­i­cal unity of mankind”. In other words, all of hu­man­ity shares the same un­der­ly­ing psy­cholog­i­cal ma­chin­ery. Fur­ther­more, that ma­chin­ery has not had the time to sig­nifi­cantly change in the 50,000 or so years that have passed af­ter we started mov­ing out of our an­ces­tral en­vi­ron­ment.

In The 10,000 Year Ex­plo­sion, Gre­gory Cochran and Henry Harpend­ing dis­pute part of this claim. While they freely ad­mit that we have prob­a­bly not had enough time to de­velop new com­plex adap­ta­tions, they em­pha­size the speed at which minor adap­ta­tions can spread through­out pop­u­la­tions and have pow­er­ful effects. Their ba­sic the­sis is that the no­tion of a psy­cholog­i­cal unity is most likely false. Differ­ent hu­man pop­u­la­tions are likely for biolog­i­cal rea­sons to have slightly differ­ent minds, shaped by se­lec­tion pres­sures in the spe­cific re­gions the pop­u­la­tions hap­pened to live in. They build sup­port for their claim by:

  • Dis­cussing known cases where se­lec­tion has led to rapid phys­iolog­i­cal and psy­cholog­i­cal changes among animals

  • Dis­cussing known cases where se­lec­tion has led to phys­iolog­i­cal changes among hu­mans in the last few thou­sand years, as well as pre­sent­ing some less cer­tain hy­pothe­ses of this.

  • Pos­tu­lat­ing se­lec­tion pres­sures that would have led to some cog­ni­tive abil­ities to be fa­vored among hu­mans.

In what fol­lows, I will pre­sent their case by briefly sum­ma­riz­ing the con­tents of the book. Do note that I’ve picked the points that I found the most in­ter­est­ing, leav­ing a lot out.

They first chap­ter be­gins by dis­cussing a num­ber of in­ter­est­ing ex­am­ples:

  • Dogs were do­mes­ti­cated from wolves around 15,000 years ago: by now, there ex­ists a huge va­ri­ety of differ­ent dog breeds. Dogs are good at read­ing hu­man voice and ges­tures, while wolves can’t un­der­stand us at all. Male wolves pair-bond with fe­males and put a lot of effort into helping raise their pups, but male dogs gen­er­ally do not. Most of the dog breeds we know to­day are no more than a cou­ple of cen­turies old. There is con­sid­er­able psy­cholog­i­cal var­i­ance be­tween dog breeds: in 1982-2006, there were 1,110 dog at­tacks in the US that were at­tributable to pit bull ter­ri­ers, but only one at­tributable to Border col­lies. Border col­lies, on av­er­age, learn a new com­mand af­ter 5 rep­e­ti­tions and re­spond cor­rectly 95 per­cent of the time, while a bas­set hound needs 80-100 rep­e­ti­tions for a 25 per­cent ac­cu­racy rate.

  • A Rus­sian sci­en­tist needed only forty years to suc­cess­fully breed a do­mes­ti­cated fox. His foxes were friendly and en­joyed hu­man con­tact, very un­like wild foxes. Their coat color also light­ened, their skulls be­came rounder, and some of them were born with floppy ears.

  • While 50,000 years may not be enough for new com­plex adap­ta­tions to de­velop, it is enough time for them to dis­ap­pear. A use­less but costly adap­ta­tion will van­ish in a quick pe­riod: fish in lightless caves lose their sight over a few thou­sand years at most.

  • An of­ten-re­peated claim is that there’s much more within-group hu­man ge­netic vari­a­tion than be­tween-group (85 and 15 per­cent, to be ex­act). While this is true, the fre­quently drawn con­clu­sion, that phe­no­type differ­ences be­tween in­di­vi­d­u­als would be larger than the av­er­age differ­ence be­tween groups, does not fol­low. Most (70 per­cent) of dog ge­netic vari­a­tion is also within-breed. One im­por­tant point is that the di­rec­tion of the ge­netic differ­ences tends to be cor­re­lated: a par­tic­u­lar Great Dane may have a low-growth ver­sion of a cer­tain gene while a par­tic­u­lar Chihuahua has a high-growth ver­sion, but on the whole the Great Dane will still have more high-growth ver­sions. Also, not all mu­ta­tions have the same im­pact: some have prac­ti­cally no effect, while oth­ers have a huge one. Since the com­mon an­ces­try of hu­mans (or dogs) is so short, ob­serv­able differ­ences be­tween pop­u­la­tions must have evolved rapidly, which is only pos­si­ble if the mu­ta­tions had a strong se­lec­tive ad­van­tage.

  • There are gene var­i­ants caus­ing ob­serv­able differ­ences in ap­pear­ance be­tween hu­man pop­u­la­tions, such as the ones caus­ing light skin color or blue eyes. For such sys­tem­atic differ­ences to ap­pear, there must have been big effects on fit­ness, any­thing up from a 2 or 3 per­cent in­crease. From the rate at which new alle­les have spread, this must be the case at least for genes that de­ter­mine skin color, eye color, lac­tose tol­er­ance, and dry ear­wax.

  • Molec­u­lar ge­net­ics has found hun­dreds of cases of mu­ta­tions that in­di­cate re­cent se­lec­tion. Many of them are very re­cent. A sig­nifi­cant num­ber of Euro­peans and Chi­nese bear mu­ta­tions that origi­nated at about 5,500 years ago. The rate at which new mu­ta­tions have been pop­ping up and spread­ing over the past few thou­sand years is on the or­der of 100 times greater than the long-term rate over the past few mil­lion years.

The sec­ond chap­ter of the book is de­voted to a dis­cus­sion about the “big bang” in cul­tural evolu­tion that oc­cured about 30,000 to 40,000 years ago. Dur­ing that time, peo­ple be­gan com­ing up with tech­nolog­i­cal and so­cial in­no­va­tions at an un­prece­dented rate. Cave paint­ings, sculp­ture and jew­elry start­ing show­ing up. Tools made dur­ing this pe­riod were man­u­fac­tured us­ing ma­te­ri­als hun­dreds of miles away, when pre­vi­ously they had been man­u­fac­tured with lo­cal ma­te­ri­als—im­ply­ing that some sort of trade or ex­change de­vel­oped. Hu­mans are claimed to have been maybe 100 times as in­ven­tive than in ear­lier times.

The au­thors ar­gue that this was caused by a biolog­i­cal change: that ge­netic changes al­lowed for a cul­tural de­vel­op­ment in 40,000 BC that hadn’t been pos­si­ble in 100,000 BC. More speci­fi­cally, they sug­gest that this could have been caused by in­ter­breed­ing be­tween “mod­ern” hu­mans and Ne­an­derthals. Even though Ne­an­derthals are viewed as cog­ni­tively less de­vel­oped than mod­ern hu­mans, arche­olog­i­cal ev­i­dence sug­gests that at least up to 100,000 years ago, they weren’t se­ri­ously be­hind the mod­ern hu­mans of the time. Ne­an­derthals also had a differ­ent way of life, be­ing high-risk, highly co­op­er­a­tive hunters while the anatom­i­cally mod­ern hu­mans prob­a­bly had a mixed diet and were more like mod­ern hunter-gath­er­ers. It is known that on­go­ing nat­u­ral se­lec­tion in two pop­u­la­tions can al­low for si­mul­tae­nous ex­plo­ra­tion of di­ver­gent de­vel­op­ment paths. It would have been en­tirely pos­si­ble that the anatom­i­cally mod­ern hu­mans in­ter­bred with Ne­an­derthals to some de­gree, the Ne­an­derthals be­ing a source of ad­di­tional ge­netic var­i­ance that the mod­ern hu­mans could have benefited from.

How would this have hap­pened? In effect, the mod­ern hu­mans would have had their own highly benefi­cial alle­les, in ad­di­tion to which they’d have picked up the best alle­les the Ne­an­derthals had. Out of some 20,000 Ne­an­derthal genes, it’s highly likely that at least some of them were worth hav­ing. There wasn’t much in­ter­breed­ing, so Ne­an­derthal genes with a neu­tral or nega­tive effect would have dis­ap­peared from the mod­ern hu­man pop­u­la­tion pretty quickly. On the other hand, a benefi­cial gene’s chance of spread­ing in the pop­u­la­tion is two times its fit­ness ad­van­tage. If benefi­cial genes are ev­ery now and then in­jected to the mod­ern hu­man pop­u­la­tion, chances are that even­tu­ally they will end up spread­ing to fix­a­tion. And in­deed, both skele­tal and ge­netic ev­i­dence shows signs of Ne­an­derthal genes. There are at least two genes, one reg­u­lat­ing brain size that ap­peared about 37,000 years ago and one play­ing role in speech that ap­peared about 42,000 years ago, that could plau­si­bly have con­tributed to the cul­tural ex­plo­sion and which may have come from the Ne­an­derthals.

The third chap­ter dis­cusses the effect of agri­cul­ture, which first ap­peared 10,000 or so years ago. 60,000 years ago, there were some­thing like a quar­ter of a mil­lion mod­ern hu­mans. 3,000 years ago, thanks to the higher food yields al­lowed by agri­cul­ture, there were 60 mil­lion hu­mans. A larger pop­u­la­tion means there’s more ge­netic var­i­ance: mu­ta­tions that had pre­vi­ously oc­curred ev­ery 10,000 years or so were now show­ing up ev­ery 400 years. The changed liv­ing con­di­tions also be­gan to se­lect for differ­ent genes. A “gene sweep” is a pro­cess where benefi­cial alle­les in­crease in fre­quency, “sweep­ing through” the pop­u­la­tion un­til ev­ery­one has them. Hun­dreds of these are still on­go­ing to­day. For Euro­pean and Chi­nese sam­ples, the sweeps’ rate of origi­na­tion peaked at about 5,000 years ago and at 8,500 years ago for one Afri­can sam­ple. While the full func­tions of these alle­les are still not known, it is known that most in­volve changes in metabolism and di­ges­tion, defenses against in­fec­tious dis­ease, re­pro­duc­tion, DNA re­pair, or in the cen­tral ner­vous sys­tem.

The de­vel­op­ment of agri­cul­ture led, among other things, to a differ­ent mix of foods, fre­quently less healthy than the one en­joyed by hunter-gath­er­ers. For in­stance, vi­tamin D was poorly available in the new diet. How­ever, it is also cre­ated by ul­tra­vi­o­let ra­di­a­tion from the sun in­ter­act­ing with our skin. After the de­vel­op­ment of agri­cul­ture, sev­eral new mu­ta­tions showed up that led to peo­ple in the ar­eas more dis­tant from the equa­tor hav­ing lighter skins. There is also ev­i­dence of genes that re­duce the nega­tive effects as­so­ci­ated with e.g. car­bo­hy­drates and al­co­hol. To­day, peo­ple de­scend­ing from pop­u­la­tions that haven’t farmed as long, like Aus­tralian Abo­rigines and many Amerindi­ans, have a dis­tinc­tive track record of health prob­lems when ex­posed to Western diets. DNA re­trieved from skele­tons in­di­cates that 7,000 to 8,000 years ago, no-one in cen­tral and north­ern Europe had the gene for lac­tose tol­er­ance. 3,000 years, about 25 per­cent of peo­ple in cen­tral Europe had it. To­day, about 80 per­cent of the cen­tral and north­ern Euro­pean pop­u­la­tion car­ries the gene.

The fourth chap­ter con­tinues to dis­cuss mu­ta­tions that have spread dur­ing the last 10,000 or so years. Peo­ple in cer­tain ar­eas have more mu­ta­tions giv­ing them a re­sis­tance to malaria than peo­ple in oth­ers. The hu­man skele­ton has be­come more lightly built, more so in some pop­u­la­tions. Skull vol­ume has de­creased ap­par­ently in all pop­u­la­tions: in Euro­peans it is down 10 per­cent from the hight point about 20,000 years ago. For some rea­son, Euro­peans also have a lot of va­ri­ety in eye and hair color, whereas most of the rest of the world has dark eyes and dark hair, im­ply­ing some Europe-spe­cific se­lec­tive pres­sure that hap­pened to also af­fect those.

As for cog­ni­tive changes: there are new ver­sions of neu­ro­trans­mit­ter re­cep­tors and trans­porters. Sev­eral of the alle­les have effects on sero­tonin. There are new, mostly re­gional, ver­sions of genes that af­fect brain de­vel­op­ment: axon growth, synapse for­ma­tion, for­ma­tion of the lay­ers of the cere­bral cor­tex, and over­all brain growth. Ev­i­dence from genes af­fect­ing both brain de­vel­op­ment and mus­cu­lar strength, as well as our knowl­edge of the fact that hu­mans in 100,000 BC had stronger mus­cles than we do have to­day, sug­gests that we may have traded off mus­cle strength for higher in­tel­li­gence. There are also new ver­sions of genes af­fect­ing the in­ner ear, im­ply­ing that our hear­ing may still be adapt­ing to the de­vel­op­ment of lan­guage—or that spe­cific hu­man pop­u­la­tions might even be adapt­ing to char­ac­ter­is­tics of their lo­cal lan­guages or lan­guage fam­i­lies.

Rul­ing elites have been known to have far more offspring than those of the lower classes, im­ply­ing se­lec­tive pres­sures may also have been work there. 8 per­cent of Ire­land’s male pop­u­la­tion car­ries an Y chro­mo­some de­scend­ing from Niall of the Nine Hostages, a high king of Ire­land around AD 400. 16 mil­lion men in cen­tral Asia are di­rect de­scen­dants of Genghis Khan. Most in­ter­est­ingly, peo­ple de­scended from farm­ers and the lower classes may be less ag­gres­sive and more sub­mis­sive than oth­ers. Peo­ple in agri­cul­tural so­cieties, fre­quently en­coun­ter­ing lots of peo­ple, are likely to suffer a lot more from be­ing overly ag­gres­sive than peo­ple in hunter-gath­erer so­cieties. Rulers have also always been quick to elimi­nate those break­ing laws or oth­er­wise op­pos­ing the cur­rent rule, se­lect­ing for sub­mis­sive­ness.

The fifth chap­ter dis­cusses var­i­ous ways (trade, war­fare, etc.) by which differ­ent genes have spread through the hu­man pop­u­la­tion through­out time. The sixth chap­ter dis­cusses var­i­ous his­tor­i­cal en­coun­ters be­tween hu­mans of differ­ent groups. Amerindi­ans were dec­i­mated by the dis­eases Euro­peans brought with them, but the Euro­peans were not like­wise dec­i­mated by Amer­i­can dis­eases. Many Amerindi­ans have a very low di­ver­sity of genes reg­u­lat­ing their im­mune sys­tem, while even small pop­u­la­tions of Old Wor­lders have highly di­verse ver­sions of these genes. On the other hand, Euro­peans had for a long time difficulty pen­e­trat­ing into Africa, where the lo­cal in­hab­itants had highly evolved ge­netic re­sis­tances to the lo­cal dis­eases. Also, Indo-Euro­pean lan­guages might have spread so widely in part be­cause an an­ces­tor pro­tolan­guge was spo­ken by lac­tose tol­er­ant herders. The abil­ity to keep cat­tle for their milk and not just their flesh al­lowed the herders to sup­port larger amounts of pop­u­la­tion per acre, there­fore dis­plac­ing peo­ple with­out lac­tose tol­er­ance.

The sev­enth chap­ter dis­cusses Ashke­nazi Jews, whose av­er­age IQ is around 112-115 and who are vastly over­rep­re­sented among suc­cess­ful sci­en­tists, among other things. How­ever, no sin­gle state­ment of Jews be­ing un­usu­ally in­tel­li­gent is found any­where in pre­served clas­si­cal liter­a­ture. In con­trast, ev­ery­one thought that clas­si­cal Greeks were un­usu­ally clever. The rise in Ashke­nazi in­tel­li­gence seems to be a com­bi­na­tion of in­ter­breed­ing and a his­tory of be­ing pri­mar­ily in cog­ni­tively challeng­ing oc­cu­pa­tions. The ma­jor­ity of Ashke­nazi jews were moneylen­ders by 1100, and the pat­tern con­tinued for sev­eral cen­turies. Other Jewish pop­u­la­tions, like the ones the liv­ing in the Is­lamic coun­tries, were en­gaged in a va­ri­ety of oc­cu­pa­tions and do not seem to have an above-av­er­age in­tel­li­gence.