The Ashkenazi Jews—the Jews of Europe—began as a distinct community about 1,200 years ago along the Rhine.The word “Ashkenaz”was the Hebrew name for Germany,so the Ashkenazim are literally “German Jews,”although they later came to inhabit other areas,particularly Poland. Today the Ashkenazi Jews, some 11 million strong, live throughout the world,with the largest concentrations in Israel and the United States.There are many other Jewish communities— such as the Sephardic Jews who once lived in Spain,the Mizrahi Jews of the Middle East and North Africa,and the BeneIsrael of India—but the vast majority of the world’s Jews are Ashkenazi.
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They have had a surprisingly large influence on the world over the past couple of centuries,and they have played an outsized role in science,literature,and entertainment.Might they be smarter than other groups of people? Apparently so.Ashkenazi Jews have the highest IQ of any ethnic group known.They average around 112–115,well above the European norm of 100.This fact has social significance,because IQ (as measured by IQ tests and their equivalents,like the Graduate Record Exam [GRE] or the Scholastic Aptitude Test [SAT]) is the best available predictor of success in academic subjects and many jobs.1 Jews are just as successful in such jobs as their tested IQ would predict,and they are hugely overrepresented in those jobs and accomplishments with the highest cognitive demands. We’re not the first to notice this.Popular opinion has held that European Jews are smart for a long time.At the turn of the century in London, for example, Jews took a disproportionate share of prizes and awards in the school system.2 This was not the case in classical times:Surviving writings from the ancient Greeks and Romans offer no hint that the Jews were considered unusually smart. So why are the Ashkenazim especially intelligent? To solve this puzzle, it may be useful to look at what we know about the DNA of the Ashkenazi Jews, because it turns out that they have another interesting characteristic. Namely, they have an unusual set of serious genetic diseases, such as Tay-Sachs disease, Gaucher’s disease, familial dysautonomia, and two different forms of hereditary breast cancer (BRCA1 and BRCA2),and these diseases are up to 100 times more common in Ashkenazi Jews than in other European populations. For a
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long time,those disorders have posed another puzzle—why are they so common in this particular group? We believe that these two puzzles have a single explanation.We propose that the Ashkenazi Jews have a genetic advantage in intelligence that arose from natural selection for success in white-collar occupations during their sojourn in northern Europe. Strong selection for intelligence also produced some unpleasant side effects, in the form of alleles that boost IQ in carriers while causing harm to homozygotes. This kind of explanation is controversial,of course.It is true that many dismiss the idea that intelligence is measurable, is influenced by genes, or can vary from group to group. These criticisms and dismissals, interestingly, hardly ever come from scientists working in the area of cognitive testing and its outcomes: There is little or no controversy within the field. IQ tests work—they predict academic achievement and other life outcomes, and IQ scores are highly heritable. If genes influence intelligence, then, over time, a situation in which intelligence boosts fertility must result in higher intelligence. That simple logic is the very essence of the theory of evolution by natural selection:Genes that cause increased reproduction gradually become more and more common in a population.
ASHKENAZI INTELLECTUAL PROMINENCE
Jewish intellectual prominence is striking. As we have said, Ashkenazi Jews are vastly overrepresented in science. Their numbers among prominent scientists are roughly ten times greater than you’d expect from their share of the population in the United States and Europe. Over the past two generations
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they have won more than a quarter of all Nobel science prizes, although they make up less than one-six-hundredth of the world’s population.Although they represent less than 3 percent of the U.S.population,they won 27 percent of the U.S.Nobel Prizes in science during that period3 and 25 percent of the A. M. Turing Awards (given annually by the Association for Computing Machinery).4 Ashkenazi Jews account for half of twentieth-century world chess champions. American Jews are alsooverrepresented in other areas,such as business (wherethey account for about a fifth of CEOs5) and academia (where they make up about 22 percent of Ivy League students6). Although these statistics show intelligence in a broad range of disciplines,we emphasize measures of scientific and mathematical achievement in our present argument because we believe they are more objective measures than the others. Everyone agrees about what constitutes important discoveries in science and mathematics,whereas there are no comparable objective criteria to evaluate accomplishments in art and literature.Was Freudian theory, for example, a landmark achievement in psychology or the equivalent of the pet rock, a silly passing fad? We don’t know (although we do have a strong suspicion),and we have no objective way of finding the answer. The statistics about Ashkenazi accomplishment may seem pretty dry, but they’re referring to people like Albert Einstein, who developed the special theory of relativity.This theory unified mechanics and electromagnetism and led to atomic energy. We’re talking about John von Neumann,who was one of the developers of game theory and who played an important part in the Manhattan Project and in the development of the hydrogen bomb; and about Richard Feynman, Julian Schwinger, and
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Murray Gell-Mann, who developed many of the most important ideas in particle physics. The trend continues today,with scientists of Ashkenazi descent such as Ed Witten and Grigori Perelman.Witten,a professor of physics at the Institute for Advanced Study,has done important work in string theory and on emerging connections between mathematical physics and low-dimensional topology. He was the first physicist to win the Fields Medal (in 1990), the highest international award in mathematics, and won the Crafoord Prize, an international science award, in 2008. In 2002, Perelman, a Russian-Jewish mathematician, proved the Poincaré conjecture, the most famous unsolved problem in topology. For this work, he was offered—but refused—the Fields Medal (his refusal had to do with attempts by others to claim credit for his solution and his disappointment with the ethical standards of professional mathematics). None of this means that typical Ashkenazi Jews are especially intelligent.Their average IQ is around 112,about threequarters of a standard deviation above the European mean. However,a modest difference like this has a very strong impact on the number of individuals out at the far edge of the distribution because of the shape of the bell curve.It’s enough to greatly increase the fraction of individuals with high intelligence. This pattern among the Ashkenazim is far more interesting than most other kinds of human diversity.If a particular ethnic group had incredibly large ears, for example, we’d be amazed, but it wouldn’t have much impact on our lives.Ideas originated by Ashkenazi Jews, such as special relativity and game theory, however,affect our lives every day,whether we know it or not. Their intelligence has influenced the world in important ways,
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driving many of the most significant developments, advances, and creative works of our time. Ashkenazi intellectual prominence is also very recent, in evolutionary terms.This high level of intellectual achievement among the Ashkenazi Jews is less than two centuries old.
ASHKENAZI HISTORY UP TO 1800
The ancient Jewish population suffered remarkable vicissitudes— the Babylonian exile,the Hellenistic conquest and Hasmonean state,and the revolts against the Roman Empire,for example— but most of that history is irrelevant to our thesis,except to the extent that it helped create necessary cultural preconditions. That history is irrelevant because the Jews,in those days,were much like other people. Most Jews then were farmers, just like
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mostother people in settled populations,and they must have experienced evolutionary pressures similar to those experienced by other agricultural peoples. They were not intellectually prominent at that time. They made no contributions to the mathematics and protoscience of the classical era.A fair amount of classical commentary on the Jews has been preserved, and there is no sign that anyone then had the impression that Jews were unusually intelligent.By “no sign,”we mean that there is apparently no single statement to that effect anywhere in preserved classical literature.7 This is in strong contrast with the classical Greeks,whom everyone thought unusually clever. The key cultural precondition among the Jews—key,that is, to later events among the Ashkenazim—was a pattern of social organization that required literacy, that strongly discouraged intermarriage,and that could propagate itself over long periods of time with little change. That pattern (Rabbinical Judaism) had not always existed but gradually emerged in the centuries after Titus’s destruction of the Temple in the first revolt against the Roman Empire in AD 70.This happened first in Israel,then later in the Jewish community of Mesopotamia. It coincided with the development of the Talmud, a collection of writings about Jewish law,customs,and history.The Torah and the Talmud are the central documents of rabbinical Judaism. Literacy,which does not itself require high intelligence,was probably important to the Jews in their shift from a nation to an urban occupational caste during and following the Diaspora, acting as an entrée to many urban professions in which they at first had no special biological advantages.8 The prohibition against intermarriage mattered,because local selective pressures
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cannot change a population that freely mixes with neighbors. Intermarriage quickly dilutes the effect of beneficial alleles within a population,since the introduction of alleles from outside easily swamps the effects of selection within the group. Rabbinical Judaism’s long-term stability was also key,since natural selection takes many generations to effect large changes. In fact,pre-Diaspora Jewish genetics seems not to have been remarkable in any way.We make use of genetic markers indicating the amount of Middle Eastern ancestry among the Ashkenazim,but that is only important to our thesis insofar as it helps us estimate the extent of gene flow between the Ashkenazim and neighboring populations.In much the same way,the details of the Ashkenazi settlement of and migrations in Europe interest us because of their potential for creating genetic bottlenecks. After the Bar-Kochba revolt of AD 135, most Jews lived outside of Israel.They were concentrated in the Parthian (later Sassanid) Empire and in the eastern half of the Roman Empire. There was a substantial population of Roman Jews, and there were other western settlements,such as Cologne,though these are poorly documented.The Jewish Diaspora in classical times was largely urban, but those Jews were on average poor; they were artisans and laborers rather than moneylenders or managers.9There is a temptation to project recent cultural patterns, such as Jewish concentration in finance or Talmudic scholarship, back into the past, well before those patterns came into existence—but this is a mistake. After the Muslim conquests, the majority of Jews lived under Islamic rule. The Ashkenazim,the Jews living north of the Alps and the Pyrenees,appear in the historical record in the eighth and ninth centuries.Their origins are somewhat unclear.There are three
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different threads of history that may have led to the foundation of the Ashkenazi Jews in the centuries preceding,but the relative strengths of the theories are uncertain. The first possibility is that the Ashkenazim—or some fraction of them—had already lived in France and the Rhineland for a long time,perhaps going back to Roman times.We know that there were Jews in Cologne around AD 300,and that there were Jews living in France under the Merovingian monarchs in the fifth and sixth centuries.10 However,in 629 King Dagobert of the Franks ordered the Jews of his lands to convert,leave,or face execution. This conversion edict may have pushed them out of most of France. Certainly we hear little about French Jews for the next 150 years or so.The size and even the existence of this population is uncertain. The second thread involves Jewish merchants originating from the lands of Islam as distant as Palestine and Iraq. The Carolingian kings encouraged and protected these merchants, who brought luxury items such as silks and spices from the East, according to Agobard of Lyons.11 A few such traders served as interpreters on diplomatic missions;one brought Charlemagne an elephant from Haroun al-Rashid. The third thread, generally thought to be the best supported,is that most of the founding Ashkenazi population migrated from southern Europe, especially Italy. There are accounts of particular Jews—both individuals and families— moving from Italy to this area in the early Middle Ages. One was the Kalonymus family,which is said to have migrated from Lucca in Italy to Mainz in 917.12 When they first appear in the historical record,the Ashkenazim are long-distance merchants who trade with the Muslim
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world.This is the beginning of a unique occupational pattern; there were no other European groups—or other Jewish groups, for that matter—who were noted for this.The majority of Jews had already given up agriculture,but the Jews of Islam,although urban,mostly worked in various crafts.13 The Ashkenazim apparently seldom had such jobs.This pattern is detailed by one historian as follows:“Two entirely different patterns in the practice of crafts and their place in Jewish life and society are discernible throughout the Middle Ages. One characterizes the communities in countries around the Mediterranean,including in the south those in the continents of Asia and Africa,and in the north extending more or less to an imaginary demarcation line from the Pyrenees to the northern end of the Balkans. The other, in the Christian countries of Europe, was more or less north of the Pyrenees-Balkans line.”14 Furthermore,“North of the Pyrenees and in the Balkans crafts played a very small role as a Jewish occupation,from the inception of Jewish settlement there.” The Ashkenazi population,established in northern France by the early 900s,prospered and expanded.They settled in the Rhineland and then,after the Norman Conquest,in England. At first they were international merchants who acted as intermediaries with the Muslim world.As Muslims and Christians, especially Italians,increasingly found it possible to do business directly,Ashkenazi merchants moved more and more into local trade.When persecution became a serious problem and the security required for long-distance travel no longer existed, the Ashkenazim increasingly specialized in one occupation, finance,left open to them because of the Christian prohibition of usury.The majority of the Ashkenazim seem to have been
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moneylenders by 1100, and this pattern continued for several centuries.15 Such occupations (trade and finance) had high IQ demands, and we know of no other population that had such a large fraction of cognitively demanding jobs for an extended period. In some cases,we have fairly detailed records of Ashkenazi commercial activity.For example,concerning the Jews of Roussilon circa 1270: “The evidence is overwhelming that this rather substantial group of Jews supported itself by money lending,to the virtual exclusion of all other economic activities.Of the 228 adult male Jews mentioned in the registers, almost 80 percent appear as lenders to their Christian neighbors.Nor were loans by Jewish women (mostly widows) uncommon,and the capital of minors was often invested in a similar manner. Moreover, the Jews most active as moneylenders appear to have been the most respected members of the community.”16 The Jews in this period were prosperous.Historian H.BenSasson pointed out that “Western Europe suffered virtual famine for many years in the tenth and eleventh centuries,[but] there is no hint or echo of this in the Jewish sources of the region in this period.The city dweller lived at an aristocratic level, as befitted international merchants and honored local financiers.”17 Their standard of living was that of the lower nobility.18 The Ashkenazi Jews were thus spared malnutrition and occasional famine. This helped Jewish populations recover from their losses due to persecution; it may have affected selective pressures as well. And persecution was a very serious matter. Although the Jews of this region were prosperous,they were not safe.The first major crisis was the First Crusade of 1096,which resulted in the
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deaths of something like a quarter of the Jews in the Rhineland. Religious hostility, probably exacerbated by commercial rivalries,increased in Europe during this period,manifesting itself in the form of massacres and expulsions.This pattern of persecution kept the Ashkenazi Jews from overflowing their white-collar niche during the High Middle Ages, which otherwise would have happened fairly rapidly.It culminated in the expulsion of the Jews from most of Western Europe—from England in 1290, from France in 1394, and from various regions of Germany in the fifteenth century.The expulsions had greater effect on the demography of the region, in the long run, than massacres and persecutions. Jewish population growth rates were high due to both their prosperity and their belief system,as they favored large families,so their numbers tended to recover from attacks after a generation or two.But the potential for recovery decreased as Jews were excluded from more and more of Western Europe. Many of the Jews who were expelled moved east, first to Austria, Bohemia, and Moravia, and later to the PolishLithuanian Commonwealth.The Polish rulers welcomed Jewish immigrants who could help modernize and reconstruct the country,which had been devastated by Mongol raids.Jews were welcomed as urban developers,investors,and initiators of trade. Other skilled immigrants were also welcomed, but some of those groups brought political risks, particularly the Germans because of their connection with the Teutonic Knights. The Jews were politically neutral and therefore safe. As had been the case in Western Europe, the Jews of Poland had a very unusual occupational profile. None were farmers,and few were craftsmen,at least in the early centuries of that settlement. The very first to immigrate were mainly
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moneylenders,but that soon changed.They became tax-farmers (something like a freelance tax collector), toll-farmers, estate managers,and proprietors of mills and taverns.According to the historian B.D. Weinryb, in the middle of the fourteenth century “about 15 percent of the Jewish population were earners of wages,salaries and fees.The rest were independent owners of business enterprises.”19 They were the management class of the Polish-Lithuanian Commonwealth. Besides literacy, success in those specialized occupations depended upon skills similar to those of businessmen today,not least the ability to keep track of complex transactions and money flows. Eventually, as the Ashkenazi population of the PolishLithuanian Commonwealth increased,more and more Jews became craftsmen—there are, after all, only so many managerial and financial slots.Still,for 800 to 900 years,from roughly 800 to 1650 or 1700,the great majority of the Ashkenazi Jews had managerial and financial jobs,jobs of high complexity,and were neither farmers nor craftsmen. In this they differed from all other settled peoples of which we have knowledge. In fact, it would have been impossible (back then) for the majority of any territorial ethnic group to have such white-collar jobs,because agricultural productivity would have been too low.Ninety percent of the population had to farm in order to produce enough to feed themselves and a thin crust of rulers, scribes, soldiers, craftsmen,and merchants.Selection for success at white-collar tasks could only have occurred if those scribes and merchants could somehow become an ethnic group,one defined by occupation rather than location. Jews who were particularly good at these high-complexity jobs enjoyed increased reproductive success.As Weinryb noted: “More children survived to adulthood in affluent families than
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in less affluent ones.A number of genealogies of business leaders, prominent rabbis, community leaders, and the like—generally belonging to the more affluent classes—show that such people often had four,six,sometimes even eight or nine children who reached adulthood. . . . On the other hand,there are some indications that poorer families tended to be small ones. . . . It should also be added that overcrowding, which favors epidemics, was more prevalent among the poorer classes.”20 In short, Weinryb wrote, “the number of children surviving among Polish Jews seems to have varied considerably from one social level to another.”21 He also suggested that wealthier Jews were less crowded, as they lived in bigger houses; could keep their houses warmer; could afford wet-nurses; and had better access to rural refuges from epidemics.As an example,he cites a census of the town of Brody in 1764 showing that homeowner households had 1.2 children per adult member, while tenant households had 0.6.22 The occupational pattern of the Jews living in the Islamic world was different from that of the Ashkenazim.The Jews of Islam did not have a high concentration of white-collar occupations.Some had such jobs in some parts of the Islamic world,in some periods, but it seems it was never the case that most did in any given place and time. In part this was because other minority groups—Greek Christians, Armenians, and so on— competed successfully for these jobs,and in part it was because Muslims,valuing nonwarrior occupations more highly than medieval Christians did, took many of those jobs themselves. In addition,because there was less persecution overall,there were many more Jews in the Islamic world than in Europe—more Jews,in fact,than there were white-collar jobs.
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In fact,to a large extent,and especially during years of relative Muslim decline from the fourteenth to the twentieth centuries, the Jews of Islam tended to have “dirty” jobs.23 These included such tasks as cleaning cesspools and drying the contents for use as fuel—a common Jewish occupation in Morocco, Yemen, Iraq, Iran,and Central Asia.Jews were also found as tanners,butchers, and hangmen and in other disagreeable or despised occupations. Such jobs must have had low IQ elasticity:Brilliant tanners and hangmen almost certainly did not become rich.
EMERGENCE OF INTELLECTUAL DIFFERENCES
The selection process we are interested in may have been under way during the Middle Ages,but its results were not yet evident in 1800.Of course,there were no IQ tests then,but there were as yet no Ashkenazi discoveries in science or mathematics either. Perhaps if anti-Semitism had not prevented Jews from a wide range of career choices,this would not have been the case. Severe restrictions on Jewish occupations and participation in public life were just beginning to be lifted.But another reason they were not found among the early European scientists and mathematicians was that the Ashkenazim had been uninterested in natural philosophy for some centuries.Indeed,at times Ashkenazi leaders were positively hostile to such inquiries.Ever since the work of Maimonides in the twelfth century,the majority of Jewish religious leaders had tended to subordinate philosophy to the literal interpretation of the Torah (the first five books of the Hebrew Bible). A herem,or religious ban,had been placed on Maimonides’ philosophical work after his death in 1204,while Solomon ben
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Abraham Adret, a rabbi and scholar at the beginning of the fourteenth century,issued another herem on “any member of the community who, being under twenty-five years, shall study the works of the Greeks on natural science and metaphysics.”24 Although the Ashkenazim put a tremendous amount of intellectual energy into Talmudic analysis during this period, we do not believe that the resulting body of work provides evidence of unusual mental acuity. Despite its historical significance and its importance to the Jewish culture and religion,it is fair to say that it hasn’t drawn a great deal of interest outside of the Jewish community. Another factor is that the Ashkenazi Jews were in the wrong place:Science and technology were sprouting in Western Europe,not in the Polish-Lithuanian Commonwealth.In fact, the Jews had been welcomed because Eastern Europe was backward and therefore in need of their skills. All of these factors had begun to change by 1800.In 1791, France became the first country in Europe to grant legal equality to the Jews, and Napoleon’s conquests spread that policy over much of Europe.Even in countries without full civil equality, such as Great Britain, Jews generally enjoyed increased rights.Movements within Judaism began to support Enlightenment values and escape from the ghetto, and things began to happen.As the British historian Eric Hobsbawm has said,“It is as though the lid had been removed from a pressure cooker.”25 A trickle of Ashkenazi scientists and mathematicians began to appear in the first half of the nineteenth century—major talents like the eminent mathematicians Carl Jacobi and Leopold Kronecker.They originated in Germany, home to a comparatively small fraction of the Ashkenazim.Over time,more signif
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icant Jewish figures appeared in the arts and sciences. This might have happened more rapidly,except that the majority of Jews (after the partition of Poland) lived in Russia, which was slow to emancipate. Many Ashkenazim from Eastern Europe emigrated—some to Western Europe and some to the United States, Canada, Argentina, and South Africa. In those new lands they were much freer to exercise their talents,and by the beginning of the twentieth century,Ashkenazi Jews were playing a major role in all areas of science and mathematics.This trend strengthened in the second half of the twentieth century— even though the Nazis had murdered most of the Ashkenazim remaining in Europe—and it continues today.
A GENETICALLY DISTINCT GROUP
One standard counterargument to any thesis suggesting that the Ashkenazi Jews are in some way biologically different or special is that they are adherents of a religion rather than a race or ethnic group in the strictest sense, and that therefore they cannot be genetically distinct.Some have brought up conversion as a mixing mechanism,often mentioning Elizabeth Taylor or Sammy Davis Jr.,prominent converts to Judaism,as contemporary examples.Raphael and Jennifer Patai argued in The Myth of the Jewish Race that an inflow of genes from neighboring populations, via conversion, intermarriage, and illicit sex, kept Jewish populations from developing distinct genetic features.26 It’s true that Jews as a whole are not a single genetically distinct group;however,some subgroups are—in particular,the Ashkenazim.Strong evidence exists in the prevalence of genetic diseases like Tay-Sachs and others, but there is much more
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information these days as a result of new technologies for studying DNA.Take a look:SNPs don’t lie.27 The plot shown on page 205 makes use of those alleles that are considerably more common in one group than in others to determine group membership.Ashkenazi Jews (represented by circles and squares, the cluster in the upper-righthand corner) can easily be distinguished from the general European population (triangles). Irish, Scandinavians, Germans, and Brits occupy the upper left end of the archipelago, while Greeks and Italians are found at the lower left end.The Ashkenazi Jews are a distinct group;this is made especially clear by the cluster of dark squares,the group of Ashkenazi Jews with four Ashkenazi Jewish grandparents. For a very long time, Ashkenazi Jews (and most other Jewish groups as well) were endogamous, rarely marrying outside their faith or accepting converts. An endogamous group can remain genetically distinct, or become genetically different from neighboring peoples, if that social pattern persists. This is especially likely if some major fraction of the group’s ancestors came from somewhere else (in this case, the Middle East) or if the selective pressures they’ve experienced have been different from those present in neighboring peoples. There is reason to believe that a fair fraction (≈40 percent) of Ashkenazi ancestry is European,which we will discuss later, but it seems that for the most part those genes were added to the mix a long time ago,possibly back in the days of the Roman Empire. That notion is inherently plausible because many of the Jews in Rome arrived as enslaved prisoners of war,captured in the Great Revolt of AD 65–73 or in Bar-Kochba’s revolt of AD 132–135.Many of those slaves eventually became freemen,and
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it is likely that they were predominantly male.Many must have married local European women. There should therefore be a significant southern European component in the maternal ancestry of Roman Jews and,later,Ashkenazi Jews. Admixture has not kept the Ashkenazim from becoming genetically distinct.Even if a population starts out as a mixture of two peoples,as in this case,becoming endogamous (ending intermarriage) and staying so for a long time ensures that the population will become homogeneous.If the population’s ancestry is 60 percent Middle Eastern and 40 percent European,for example,a few dozen generations of endogamy will result in a population in which each individual’s ancestry is quite close to 60 percent Middle Eastern and 40 percent European.In other words, you eventually get a population that has a flavor all its own—even more so if it experiences special selective pressures.
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European genetic substructure analysis
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This means that if you look at the most informative parts of the genome,you can tell whether a certain individual is Ashkenazi (as opposed to, say, a non-Jewish Italian, Greek, or German) just about every time, particularly if all his or her recent ancestors are Jewish.In the plot,the circles represent Ashkenazi Jewish individuals,but the shaded circles represent individuals whose grandparents were all Ashkenazi Jews as well.That distinction matters,because Jews haven’t been nearly as endogamous over the past century as they were during the Middle Ages. Could these same methods distinguish the Ashkenazi from other Jewish groups, such as Moroccan Jews or Yemeni Jews? The answer is almost certainly yes. Although that particular measurement has not yet been made,it should be easy to make that distinction because the genetic distance between Ashkenazi Jews and Yemeni Jews is considerably larger than that between Ashkenazi Jews and Western Europeans. Members of the general public sometimes believe that individual genetic profiles do not necessarily reflect nationality. Somebody who is Swedish, for example, might be genetically closer to someone from Japan than to another Swede,according to this view of things.If this was true,it would apply to a group like the Ashkenazi Jews as well,even though they are not quite a “nationality.” However, that belief is false. In fact, a case where a person of one nationality is closer genetically to someone of a distant nationality than to his or her own compatriots never happens. If you’re Swedish, every Swede (not counting recent immigrants) is genetically closer to you than any person in Japan. It’s possible to be closer to someone in Japan if you consider only one gene: Both of you might have the same blood type
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while your next-door neighbor might not,for example.Nevertheless,it is somewhat more likely that your neighbor will have the same blood type as you,since the frequency of blood types varies according to nationality. If you look over the whole genome—about 20,000 genes—with a match with your neighbor being somewhat more likely for every single gene, the chance of the overall match with someone Japanese being closer than the match with your neighbor becomes vanishingly small. Think of it this way: When you make a bet at the casino, the odds favor the house, although not overwhelmingly so. You might win that first bet—it’s not that unlikely.But what is the chance that you’ll win most of the time over the course of a year—win the majority out of thousands of bets,with the odds against you on every one? The probability of that happening is vanishingly small, which is why the house always wins in the long run. As a practical matter, if you can distinguish between the members of two populations by looking at them,genetic analysis will be able to do so as well.And sometimes it will be able to make such distinctions when you can’t tell by looking at them. The question as to whether the Ashkenazim are genetically distinct is now settled:We know from the data that they are.But that by itself it not enough to prove our thesis that they are more intelligent than the rest of us,or even that they are significantly different in any other way—not by a long shot. Being measurably different is not necessarily the same as being significantlydifferent,and knowing that systematic genetic differences exist does not automatically tell us what their consequences are. However,this initial set of data could have disprovedour thesis. If the genetic evidence had indicated that the Ashkenazi
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Jews could not constitute a genetically distinct group—if there had been significant continuing gene flow—then we would have to concede that our proposed mechanism (natural selection) could not have occurred.But the initial dataset did not disprove our thesis.So what does the genetic evidence say about Ashkenazi intelligence?
NOTES ON IQ
IQ tests and scores are not in fact crucial to our thesis,but they are useful. Intellectual accomplishment is all that really matters: If people routinely won Nobel Prizes in physics with low IQ scores,or,for that matter,routinely aced calculus exams but flunked the IQ test, we’d junk the IQ tests. But that doesn’t happen:IQ is an imperfect but useful measure of intelligence. You’ll frequently hear that we don’t really know what intelligence is,that we don’t know how to measure it,that IQ tests are biased, and that IQ scores don’t predict anything, or that they don’t predict anything outside of school.Often these complaints are salted with personal anecdotes about some acquaintance that had a high IQ score but was lazy,crazy,or suffered from unforgivable personal hygiene.And in recent years,other forms of intelligence have become all the rage. Daniel Goleman has written of “emotional intelligence”and “social intelligence,” pointing out how they can help to predict job success and personal happiness. And other forms of intelligence have been proposed.In his 1993 book,Howard Gardner suggested that there are many types.28 But the data hardly support these attempts to complexify cognitive testing.The supposed special kinds of intelligence don’t predict anything useful or,when they
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do,predict only to the extent that they are correlated with general intelligence. Yet IQ tests work in the sense that they predict performance.They were originally developed in order to predict how well children would do in school,and they do an excellent job of that.They also have moderate to high predictive power on many other questions, such as job performance, health, risk of accidental death,income,and other characteristics that may be less obvious,such as susceptibility to Alzheimer’s disease.To make our position perfectly clear, we’d like to emphasize that saying IQ scores have some predictive power is not the same thing as saying that they determine everything. Of course,exceptions don’t make trends disappear.Muggsy Bogues may have played in the NBA while being 5 feet 3 inches tall, and there may be numerous individuals who are 6 feet 8 inches but were so clumsy on their high-school basketball teams that they sat on the bench the entire season. But in general, height still matters in basketball.It’s not the only thing that matters, it doesn’t absolutely determine success, but on average it makes a lot of difference.The same can be said of IQ:For most life events it’s not as important as height is in basketball,but it’s fairly important.Nor are IQ tests biased:They predict academic performance with the same accuracy in different ethnic groups.29 Moreover, IQ is highly heritable. What this means is that an individual’s IQ is partially determined by genetic factors,so that it tends to be more similar to that of his or her parents and siblings than a randomly chosen person’s IQ would be.Siblings with the same biological parents have similar IQs even when they are raised separately,whereas adopted siblings don’t,even when raised together.
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The same is true of height:Tall people tend to have tallerthan-average children.In fact,IQ in adulthood is about as heritable as height. IQ in childhood, on the other hand, is less heritable and more susceptible to environmental influences. These effects of environment on the measured IQs of children, which disappear at or after puberty,are the basis for claims that IQ can be improved by interventions like Head Start. Nongenetic factors also influence IQ,but for the most part, the ones that matter are not the ones people thought would matter. Prenatal care, breastfeeding, nutrition, access to early education, Mozart in the womb, and oat bran all have little or no effect.Surprisingly,the way in which a family raises children seems to have no effect on adult IQ.This argues against some popular environmental explanations for high intelligence among the Ashkenazi Jews—in particular, the notion that Jewish mothers have a special way of rearing children that boosts IQ.
ASHKENAZI PSYCHOMETRICS
As noted earlier,Ashkenazi Jews have the highest average IQ of any ethnic group for which there are reliable data. Just how much higher is it? Many studies have found that it is 0.75 to 1.0 standard deviations above the general European average,corresponding to an IQ of 112–115.30 Another, more recent study concluded that the advantage is slightly less,only half a standard deviation.31 While the difference between the Ashkenazim and other northern Europeans in average IQ may not seem large,it leads to a large difference in the proportion of the two populations with very high IQs.32 For example,if the mean northern European IQ is 100, the mean Ashkenazi IQ is 110, and the
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standard deviation in both populations is 15,then the number of northern Europeans with IQs greater than 140 should be 4 per 1,000, whereas 23 per 1,000 Ashkenazim should exceed the same threshold,about a sixfold difference.This is a general statistical effect,not something that happens only with IQ. The fact that Ashkenazi Jews have high IQs on average and corresponding high academic ability has long been known. In 1900 in London,Jews took a disproportionate number of academic prizes and scholarships in spite of their poverty.33 In the 1920s, a survey of IQ scores in three London schools with mixed Jewish and non-Jewish student bodies—one prosperous, one poor,and one very poor—showed that Jewish students,on average, had higher IQs than their schoolmates in each of the groups. The differences between Jews and non-Jews were all slightly less than one standard deviation, and the students at the poorest Jewish school in London had IQ scores equal to the overall city mean of non-Jewish children.34 That study,though conducted in 1928,is still important today because it contradicts a widely cited misrepresentation of a paper authored by researcher Henry Goddard in 1917.35 Goddard gave IQ tests to people suspected of being retarded and found that the tests identified retarded Jews as well as retarded people of other groups.The psychologist Leon Kamin reported in 1974 that Goddard had found that Jews had low IQ scores. Kamin’s reason for citing the study was to claim that Goddard and other IQ researchers of the 1920s had been biased against Jews and other minority groups.This erroneous analysis was picked up by many authors, including the well-known Harvard evolutionary biologist Stephen Jay Gould,who used it as evidence of the unreliability of IQ testing.36 Gould seems to
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have believed that a popular impression that Jews had low IQs contributed to the passage of the Immigration Act of 1924, which was aimed at restricting immigration from southern and eastern Europe.However,by 1922 Jews already made up more than a fifth of Harvard undergraduates,and the Ivy League was already instituting admissions policies aimed at limiting Jewish admissions (the infamous “Jewish quotas”), which involved placing less emphasis on academic merit.If some people in the 1920s had the impression that Jews had low IQs,that impression cannot have been widely shared. The 1928 study of IQ in three London schools shows that in fact there were already researchers in the West who were noticing that Jews seemed to have higher IQs,on average,than the members of other groups. But the achievements of Ashkenazi Jews are certainly not confined to IQ scores.They have an unusual ability profile when it comes to some other forms of testing as well.They have high verbal and mathematics scores on other types of standardized tests,though their visuospatial abilities—that is,their ability to rotate three-dimensional objects in their minds,for example— are typically somewhat lower,by about half a standard deviation, than the European average.The Ashkenazi pattern of success corresponds to this ability distribution—great success in mathematics and literature, more typical results in representational painting,sculpture,and architecture. It is noteworthy that non-Ashkenazi Jews do not have high average IQ scores. Nor are they overrepresented in cognitively demanding fields like medicine, law, and academics. In Israel, Ashkenazi Jews,on average,score 14 points higher than Oriental Jews, almost a full standard deviation, which is 15 or 16 points on most IQ tests.37That difference means that the aver
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age non-Ashkenazi Jew in Israel would have an IQ score that would be at the 20th percentile among the Ashkenazim.Academic accomplishment in the two groups seems to vary in the same way, even among those born and raised in Israel:Thirdgeneration Ashkenazi Jews in Israel are 2.5 to 3 times more likely to have graduated from college than third-generation Mizrahi Jews, for example (the ancestors of the Mizrahim moved to Israel from Asia and North Africa).38
THE ASHKENAZI MUTATIONS
The best-known of the genetic diseases disproportionately affecting Ashkenazi Jews are Tay-Sachs disease, Gaucher’s disease, and the breast-cancer mutations BRCA1 and BRCA2, but there are a number of others,such as Niemann-Pick disease, Canavan disease, and familial dysautonomia. Some of these cause neurological problems. And they’re unusually common among Ashkenazi Jews—so common that they constitute an enduring puzzle in human genetics. In principle,absent some special cause,genetic diseases like these should be rare. New mutations, some of which have bad effects,appear in every generation,but those that cause death or reduced fertility should be disappearing with every generation. Any particular harmful mutation should be rare;however,one in every twenty-five Ashkenazi Jews carries a copy of the TaySachs mutation, which kills homozygotes in early childhood. This is an alarming rate. The mutations that so frequently affect Ashkenazi Jews are mysterious in another way. Many of them fall into two categories or clusters involving particular metabolic pathways:They
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affect the same biological subsystem.Imagine a fat biochemistry textbook,where each page describes a different function or condition in human biochemistry:Most of the Ashkenazi diseases would be described on just two of those pages.The two most important genetic disease clusters among the Ashkenazim are the sphingolipid storage disorders (Tay-Sachs disease;Gaucher’s disease;Niemann-Pick disease;and mucolipidosis,type IV) and the disorders of DNA repair (BRCA1 and BRCA2; Fanconi anemia,type C;and Bloom syndrome). What is the explanation of this odd pattern? We know of only two mechanisms that can create high frequencies of dangerous, even lethal mutations: genetic drift in a bottleneck or natural selection.
THE BOTTLENECK HYPOTHESIS
Most medical geneticists believe that these common Ashkenazi genetic diseases are a product of population bottlenecks.A population bottleneck occurs when a population goes through a period in which it is quite small. This often happens in the founding of a population. In a bottleneck, gene frequencies change almost randomly; just as you can get unrepresentative results (different from 50–50) when you flip a coin just a few times,or when you poll 20 people rather than 1,000,in a population bottleneck you get random changes that can affect large portions of the population in question. When we say that a population is “small,” we generally mean a few hundred individuals at most.Europe,for example, did not go through a bottleneck following the Black Death in the Middle Ages.The plague may have killed off half the pop
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ulation of Europe,but 40 million survivors is not a small number.It left plenty of genetic variation intact. If a few lethal mutations became common in a bottleneck and a dramatic population expansion followed,we would see a large population with a surprising number of genetic diseases— diseases that were rare in most other populations.This has certainly happened in some cases. Amish communities that had very few original founders have seen this effect with their high incidence of several specific genetic disorders.It also happened in Pingelap, a Pacific island that was devastated by a typhoon around 1775, leaving about twenty survivors.Today almost 10 percent of the islanders suffer a form of severe color blindness. Genetic diseases made common in a bottleneck are the product of chance, however, so there is no particular tendency for them to fall into a few metabolic pathways:They’re scattered all over the biochemistry book,not concentrated on a few pages. Our knowledge of human genetics has expanded rapidly over the past few years,and we now have good estimates of the total number of human genes (about 22,000) and the number of genes in different functional categories—in particular,the number involved in sphingolipid metabolism (108). We looked at twenty-one genetic diseases among the Ashkenazi and calculated the probability of finding four that affect sphingolipid metabolism,assuming randomness,in a given population.That probability was very low, less than 1 in 100,000.That can’t be a coincidence. We can say some other things about a population that has recently passed through a tight bottleneck. There would be overall genetic changes:reduced genetic variety in nuclear genes, increased genetic linkage,and increased genetic differences from
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other populations. All of these properties are measurable, and none have occurred among the Ashkenazi Jews.39 Finally, a genetic bottleneck would not increase a population’s intelligence.If it was severe enough,it would almost certainly decrease intelligence as moderately deleterious genes became common. Therefore,although bottlenecks can explain high frequencies of genetic disease in some cases,the bottleneck hypothesis cannot possibly explain the genetic data and the spectrum of genetic disease observed among the Ashkenazi Jews.
NATURAL SELECTION
The alternative explanation is natural selection, the process by which some alleles cause increased reproduction in their bearers. Some gene variants have favorable effects in a given environment (in this case,the physical and social environment experienced by the Ashkenazi Jews during the Middle Ages),so that people with those variants have more children,on average,than others in that population. Those variants gradually become more common,ultimately leading to significant changes.In some cases,certain gene variants can have positive effects in individuals with one copy,and negative effects,such as disease,in individuals with two copies—the people with one copy have a “heterozygote advantage.” As we have discussed earlier, the most famous example is sickle-cell anemia, where a mutation causing a very dangerous form of anemia in those carrying two copies has risen to high frequency in some parts of the world. There are a number of other malaria defenses of this sort that are expensive in terms of human health.
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Clearly,natural selection can sustain quite high frequencies of serious,even lethal genetic diseases in some circumstances.Just as clearly,it can lead to a set of common mutations that cluster in a few metabolic pathways.This has happened with the malaria defenses: Many affect the hemoglobin molecule (sickle-cell, hemoglobin C, hemoglobin E, alpha- and beta-thalassemia), whereas others,such as G6PD deficiency or glycophorin C,affect other aspects of the red blood cell.Since the malaria parasite attacks red cells,this pattern is easy to understand. Heterozygote advantage isn’t confined to genetic defenses against malaria; it can also occur in other cases where certain traits are favored by selection.It seems that the key to such cases is that there has been strong selection (carriers have a big advantage) applied over a relatively short time period. Over longer periods,mutations with fewer side effects eventually occur and win out.The fact that heterozygote advantage can favor other traits is important,because we think that most of the characteristic Ashkenazi mutations are not defenses against infectious disease. One reason is that these mutations do not exist in neighboring populations—often literally people living across the street—that must have been exposed to very similar diseases. Instead, we think that the Ashkenazi mutations have something to do with Ashkenazi intelligence, and that they arose because of the unique natural-selection pressures the members of this group faced in their role as financiers in the European Middle Ages. We see a clear example of heterozygote advantage in a trait other than disease resistance in whippets,a breed of dog similar to a small greyhound.Some whippets carry a mutated version of myostatin, a gene that limits muscle development. Dogs with
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this mutation grow more muscle.Whippets with one copy are faster, on average, than other whippets and often win races.40 Those with two copies, called “bully whippets,” are extremely muscular,have muscle spasms,and are not competitive as racers. One copy gives an advantage in a particular ability;two copies actually have a negative effect on the same ability. Now,in order for natural selection to work in this way,the population has to be genetically isolated from its neighbors; otherwise it cannot become different.Admixture dilutes the effects of natural selection and can stop it in its tracks.You might compare it to boiling down soup while continually adding
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water—you won’t get anywhere that way. As it happens, the Ashkenazi Jews were genetically isolated during the Middle Ages:not because of the Pacific Ocean,as happened with Pingelap,but owing to social reasons—internal rules against intermarriage combined with external prejudice. For most of that period,both intermarriage with non-Jews and conversion to Judaism were very rare.That’s certainly what the historical record indicates,but we can check that record using genetics.If we look at alleles that are clearly from the Middle East, we see that they account for a substantial fraction of Ashkenazi ancestry today:at least 50 percent,according to our analysis.This shows strong limits on the rate of genetic admixture, since even 2 percent mixing per generation over the past 2,000 years would have caused the Ashkenazim to become almost completely (80 percent) European.A continuing process of admixture (as opposed to a lot of admixture in the early stages) interferes the most with ongoing natural selection, but even if we make that pessimistic assumption, it looks as if the admixture rate was under 1 percent per generation—low enough to allow for the sort of natural selection we’re suggesting. In fact, early European admixture might even have furthered the selective process,since even low levels of admixture can be an important source of beneficial alleles.More generally, the position of Israel at a natural crossroads,subject to invasion by Romans, Greeks, Persians, Babylonians, Assyrians, and Egyptians,may have resulted in unusually high genetic variety, which would also have furthered selection. Already it’s apparent that the Ashkenazi mutations were made common by natural selection, since there was sufficient genetic isolation for selection to occur,while nothing other than
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selection can explain the existence of common genetic diseases that are concentrated in a few metabolic pathways.You can also see why this kind of natural selection is usually confined to geographically isolated human populations,since such strict rules against intermarriage with neighboring groups are fairly unusual. But what kind of selection occurred? What traits were more valuable among the Ashkenazi than among their neighbors? Well,we have some strong hints.What trait is accentuated among the Ashkenazim today? Are they as large as Samoans,as tall as the Tutsi,as milk-tolerant as the Dutch? No:Their special trait is intelligence. The mutations themselves suggest this:Some of them look like IQ boosters,considering their effects on the development of the central nervous system.The sphingolipid mutations,in particular, have effects that could plausibly boost intelligence. In each,there is a buildup of some particular sphingolipid,a class of modified fat molecules that play a role in signal transmission and are especially common in neural tissues. Researchers have determined that elevated levels of those sphingolipids cause the growth of more connections among neurons, the basic cells of the central nervous system (see page 221). There is a similar effect in Tay-Sachs disease:increased levels of a characteristic storage compound (GM2 ganglioside), which causes a marked increase in the growth of dendrites,the fine branches that connect neurons.41 This increased dendritogenesis also occurs in Niemann-Pick type A cells and in animal models of Tay-Sachs disease and Niemann-Pick disease.These are the only known disease alleles that cause increased neural connections. We also have evidence—not definitive—that some of the mutations common among the Ashkenazim may boost intelli
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gence.We looked at the occupations of patients in Israel with Gaucher’s disease,essentially all of whom were being treated at the Shaare Zedek Medical Centre in Jerusalem.These patients are much more likely to be engineers or scientists than the average Israeli Ashkenazi Jew—about eleven times more likely, in fact.42 There are similar reports on torsion dystonia,another Ashkenazi genetic disease. Ever since it was first recognized, observers have commented on the unusual intelligence of the patients who suffer from it. In 1976,Roswell Eldridge described early literature on torsion dystonia:One patient showed “an intellectual development
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Axon growth in Gaucher’s disease.The top frame shows normal cultured rat neurons,the middle frame shows stunted axonal growth resulting from reduced levels of glucosylceramide,and the bottom frame shows increased growth and branching in an experimental model of Gaucher’s disease with increased levels of glucosylceramide.Glucosylceramide is the storage molecule involved in Gaucher’s disease.Elevated levels promote growth and branching in axons,the transmission lines of the nervous system.
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far exceeding his age,” and a second showed “extraordinary mental development for his age.”43 At least ten other reports in the literature have made similar comments. Eldridge studied fourteen Jewish torsion dystonia patients and found that their average IQ before the onset of symptoms was 121,compared to an average score of 111 in a control group of fourteen unrelated Jewish children matched for age, sex, and school district.44 There are also reports of individuals with higher-than-average intelligence who have nonclassic congenital adrenal hyperplasia (CAH), another common Ashkenazi genetic disease. CAH, which causes increased exposure of the developing brain in a fetus to androgens (male sex hormones),is relatively mild compared to diseases like Tay-Sachs. At least seven studies show high IQ in CAH patients, parents, and siblings, ranging from 107 to 113. The gene frequency of CAH among the Ashkenazim is almost 20 percent.45
HOW SELECTION HAPPENED
Our picture of how natural selection favored higher intelligence among European Jews in the Middle Ages relies upon three key observations. The first is that prosperous individuals had considerably more children,on average,than nonprosperous individuals in those days,as was then typical in most societies.46 Second,Ashkenazi jobs were cognitively demanding,since the members of this group were essentially restricted to entrepreneurial and managerial roles as financiers,estate managers,tax farmers,and merchants.These are jobs that people with an IQ below 100 essentially cannot perform. Even low-level clerical jobs require an IQ of something like 90.47 So,intelligence must
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have had greater rewards in those jobs than it does among farmers.This has to be true, really, since physical strength and endurance,which play a major part in success as a farmer,matter far less in finance and trade.If physical strength accounts for less of the variance,then cognitive and personality traits must account for more.Third,intelligence is significantly heritable.If the parents of the next generation are a little smarter than average,the next generation will be slightly smarter than the current one. We can construct a scenario using IQ scores that illustrates this principle.We’ll assume that parents of each generation averaged a single IQ point higher than the rest of the Ashkenazi adult population.In other words,let’s suppose there was a modest tendency (mediated through economic success) for intelligent parents to have more surviving children than average parents— a tendency that certainly would not have been noticed at the time. If we assume a heritability of 30 percent for IQ, a very conservative assumption,then the average IQ of the Ashkenazi population would have increased by about a third of a point (0.30 point) per generation. Over forty generations, roughly 1,000 years,Ashkenazi IQ would have increased by 12 points. If we assume that the Ashkenazim began with a typical European IQ of 100 in the year AD 600,they would have reached an average IQ of 112 by 1600,just about what we see in the Ashkenazim today.This picture is consistent with observations of high verbal and mathematical scores among Ashkenazi Jews,paired with average or lower-than-average visuospatial scores. Verbal and mathematical talent would have helped medieval businessmen succeed,whereas visuospatial abilities were irrelevant. There may well have been some selection for IQ among Europeans in general over this period. Christian merchants in
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London or Rotterdam may have experienced selective pressures similar to those of the Ashkenazi Jews,but there was an important difference between those merchants and the Jewish population: Christian merchant families intermarried. The mixing would have caused extensive gene flow with the general population,the majority of whom were farmers.It seems that if IQ increased in the general European population, there was a greater increase among the Ashkenazim. Our hypothesis also explains why certain things didn’t happen—in particular,why we don’t see high IQ scores and unusual intellectual achievement among other Jewish groups today. Although they, too, had very low rates of intermarriage, they never seem to have that high concentration of white-collar jobs that would have led to strong selection for verbal and mathematical intelligence.In part,this was because there were many more Jews in the Islamic world than in Christian Europe: With less persecution, there were more Jews than there were white-collar jobs.Our picture also explains why there’s no real sign of unusually high intelligence among the Jews back in the days of the Roman Empire:The required events simply hadn’t happened yet.