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Closing in on the intelligence genes—at least some Jewish ones.
The most sensitive and explosive battle within academia, education, and public policy is whether intelligence is innate or genetic—and if it is primarily innate between individuals is it innate as the average intelligence difference between races? This is the primary fear that a host of academic and public employee advocates has with regards to mental ability. If it is innate, or primarily genetic, then numerous programs from Head Start, higher teacher salaries, along with the civil rights movement, are dead. Blacks are not discriminated against but rather they are behind other races on numerous factors of well being because they have on average low intelligence. It is simply their lot in life to be where they are.
The general pecking order of average intelligence goes something like this: Ashkenazim Jews 115; East Asians 105; Whites 100; Amerindians and most Hispanics 90; African Americans 85; and sub-Saharan Africans 70. By the same order, accumulation of wealth, the level of income, achieved academic success, etc., also follows closely with the average IQs of these six racial groupings. So far, the left has claimed that White racism has made Hispanics and Blacks look less intelligent than Whites and the outstanding family discipline and educational encouragement has boosted the intelligence of East Asians and Jews. The problem is, no one has been able to show exactly how these environmental differences alter the expressed intelligence of individuals or races. Genetics is merely denied as the primary difference, and anyone who says differently is a racist.
Now for the first time that I am aware of, a study has been published, AND REPORTED ON, that some of the high intelligence of Ashkenazi Jews is due to genes that are unique to them alone. And if Ashkenazim Jews are more intelligent than Whites by 15 IQ points because of genes, it is just as likely that Whites are 15 IQ points above Blacks because of genes also.
From the New York Times: "A team of scientists at the University of Utah has proposed that the unusual pattern of genetic diseases seen among Jews of central or northern European origin, or Ashkenazim, is the result of natural selection for enhanced intellectual ability. The selective force was the restriction of Ashkenazim in medieval Europe to occupations that required more than usual mental agility, the researchers say in a paper that has been accepted by the Journal of Biosocial Science, published by Cambridge University Press in England." (The complete article along with two others are available here.)
Other researchers are contending that the cause of Jewish high intelligence is due to other factors than just these recessive genes, or the selection was caused by persecution, religious practices selecting for scholarship, a founders effect, etc. But none deny that Jews are intelligent because of some sort of selection process and it is genetic, not cultural. The New York Times article openly attributes Jewish success in such areas as the number of Nobel prizes won by Jews, as where a second article on this research published in The Economist states that "Ashkenazim generally do well in IQ tests, scoring 12-15 points above the mean value of 100, and have contributed disproportionately to the intellectual and cultural life of the West, as the careers of Freud, Einstein and Mahler, pictured above, affirm. They also suffer more often than most people from a number of nasty genetic diseases, such as Tay-Sachs and breast cancer. These facts, however, have previously been thought unrelated. The former has been put down to social effects, such as a strong tradition of valuing education. The latter was seen as a consequence of genetic isolation. Even now, Ashkenazim tend to marry among themselves. In the past they did so almost exclusively."
This is the first direct evidence that relates specific genes to intelligence. The reason that this type of discovery is possible is because the genes for recessive diseases are relatively easy to find—unlike other genes that are far more complicated in how they interact with development leading to high or low general intelligence. The Economist states, "Most of the dozen or so disease genes that are common in them [Jews] belong to one of two types: they are involved either in the storage in nerve cells of special fats called sphingolipids, which form part of the insulating outer sheaths that allow nerve cells to transmit electrical signals, or in DNA repair. The former genes cause neurological diseases, such as Tay-Sachs, Gaucher's and Niemann-Pick. The latter cause cancer."
Not mentioned in these articles is the importance of fats that surround axons, called myelination, which contributes to high intelligence. This fatty tissue is laid down as a person reaches adulthood, and it is thought that this fatty tissue insulates signaling from interference and/or increases the speed of nerve conduction—both of which are important for intelligence. It also explains why heritability of intelligence goes from about 40% in childhood to 80% in adulthood. Myelination, or the amount of fatty tissue variance between individuals (and races) is estimated at about 90% heritability, and does not develop fully until adulthood.
In fact in one case—Gaucher's disease—The Economist noted that a clinic that specializes in the disease has far more patients that are in high IQ occupations than would normally be expected. This could be just one of those so-called smart-genes. It also shows that intelligence genes can come in many varieties and forms in different races. That is, the genes that contribute to Jewish high intelligence may not be the same genes that contribute to high intelligence in Whites.
Jewish intelligence is also unique in its loading on verbal IQ versus performance IQ. Though Jews have an average intelligence of 115, they have verbal skills closer to an average 125.6, with a more normal performance IQ closer to 100. This is highly unusual as most races usually have about equal verbal and performance IQs. This fact seems to indicate that the "Jews are a self-selected minority group, and their high intelligence is the result of self selection, not natural selection," according to Howard Metzenberg.
In summary then, eugenics is and has been a very useful tool for improving human intelligence, resulting in the Jewish race that is the most successful people on a host of social indicators like income, wealth, political power, etc., according to Jewish scholars. There is also no indication that this success and high intelligence has anything to do with Jewish culture because there is no monolithic Jewish culture. Jews as a race span a great deal of different cultures and life histories, and they have found themselves in varying ecological niches and cultures. From the Marxist secularist to the religiously orthodox, as a people they have on average an extremely high IQ. No one has been able to duplicate a "culture" that can boost IQ by a standard deviation generation after generation. Intelligence is in the genes. (Matt Nuenke--6/2005)
New York Times, June 3, 2005
Researchers Say Intelligence and Diseases May Be Linked in Ashkenazic Genes By NICHOLAS WADE
A team of scientists at the University of Utah has proposed that the unusual pattern of genetic diseases seen among Jews of central or northern European origin, or Ashkenazim, is the result of natural selection for enhanced intellectual ability.
The selective force was the restriction of Ashkenazim in medieval Europe to occupations that required more than usual mental agility, the researchers say in a paper that has been accepted by the Journal of Biosocial Science, published by Cambridge University Press in England.
The hypothesis advanced by the Utah researchers has drawn a mixed reaction among scientists, some of whom dismissed it as extremely implausible, while others said they had made an interesting case, although one liable to raise many hackles.
"It would be hard to overstate how politically incorrect this paper is," said Steven Pinker, a cognitive scientist at Harvard, noting that it argues for an inherited difference in intelligence between groups. Still, he said, "it's certainly a thorough and well-argued paper, not one that can easily be dismissed outright."
"Absolutely anything in human biology that is interesting is going to be controversial," said one of the report's authors, Dr. Henry Harpending, an anthropologist and a member of the National Academy of Sciences.
He and two colleagues at the University of Utah, Gregory Cochran and Jason Hardy, see the pattern of genetic disease among the Ashkenazi Jewish population as reminiscent of blood disorders like sickle cell anemia that occur in populations exposed to malaria, a disease that is only 5,000 years old.
In both cases, the Utah researchers argue, evolution has had to counter a sudden threat by favoring any mutation that protected against it, whatever the side effects. Ashkenazic diseases like Tay-Sachs, they say, are a side effect of genes that promote intelligence.
The explanation that the Ashkenazic disease genes must have some hidden value has long been accepted by other researchers, but no one could find a convincing infectious disease or other threat to which the Ashkenazic genetic ailments might confer protection.
A second suggestion, wrote Dr. Jared Diamond of the University of California, Los Angeles, in a 1994 article, "is selection in Jews for the intelligence putatively required to survive recurrent persecution, and also to make a living by commerce, because Jews were barred from the agricultural jobs available to the non-Jewish population."
The Utah researchers have built on this idea, arguing that for some 900 years Jews in Europe were restricted to managerial occupations, which were intellectually demanding, that those who were more successful also left more offspring, and that there was time in this period for the intelligence of the Ashkenazi population as a whole to become appreciably enhanced.
But the Utah researchers' analysis comes at a time when some geneticists have suggested natural selection is not the reason for the Ashkenazic diseases after all. Two years ago, Dr. Neil Risch, a geneticist now at the University of California, San Francisco, proposed a different genetic mechanism known as a founder effect, which occurs when a population is reduced for a time.
He found that all the Ashkenazic diseases had similar properties, including having arisen within the last 1,100 years. Therefore they had all arisen through the same cause, he argued, which must be founder effects, because it was unlikely that all could be due to natural selection. Last year, Dr. Montgomery Slatkin of the University of California, Berkeley, came to much the same conclusion for different reasons.
The Utah team agrees with Dr. Risch that the diseases all arose in historical times from the same cause but say natural selection is more likely because none of the non-disease Ashkenazic genes they tested showed any sign of a founder effect. They say the clustering of four of the diseases in the same biochemical pathway could only have arisen under the influence of natural selection, and calculate that the odds of a founder effect producing such a cluster are vanishingly low.
The four diseases, all of which are caused by mutations that affect the cell's management of chemicals known as sphingolipids, are Tay-Sachs, Niemann-Pick, Gaucher, and mucolipidosis type IV. A second cluster of diseases affects repair of DNA.
Turning to the possibility that some infection was the cause of the selective effect, the Utah researchers noted that Ashkenazim and Europeans lived together in the same cities and were exposed to the same microbes. If disease were the agent of selection, the Utah team argues, the European population would have developed a similar genetic response.
Ashkenazi Jews occupied a different social niche from their European hosts, and that is where any selective effect must have operated, the Utah researchers say. From A.D. 800, when the Ashkenazi presence in Europe is first recorded, to about 1700, Ashkenazi Jews held a restricted range of occupations, which required considerable intellectual acumen. In France, most were moneylenders by A.D. 1100. Expelled from France in 1394, and from parts of Germany in the 15th century, they moved eastward and were employed by Polish rulers first as moneylenders and then as agents who paid a large tax to a noble and then tried to collect the amount, at a profit, from the peasantry. After 1700, the occupational restrictions on Jews were eased.
As to how the disease mutations might affect intelligence, the Utah researchers cite evidence that the sphingolipid disorders promote the growth and interconnection of brain cells. Mutations in the DNA repair genes, involved in second cluster of Ashkenazic diseases, may also unleash growth of neurons.
In describing what they see as the result of the Ashkenazic mutations, the researchers cite the fact that Ashkenazi Jews make up 3 percent of the American population but won 27 percent of its Nobel prizes, and account for more than half of world chess champions. They say that the reason for this unusual record may be that differences in Ashkenazic and northern European I.Q. are not large at the average, where most people fall, but become more noticeable at the extremes; for people with an I.Q. over 140, the proportion is 4 per 1,000 among northern Europeans but 23 per 1,000 with Ashkenazim.
The Utah researchers describe their proposal as a hypothesis. Unlike many speculations, it makes a testable prediction: that people who carry one of the sphingolipid or other Ashkenazic disease mutations should do better than average on I.Q. tests.
The researchers have identified two reasonably well accepted issues, the puzzling pattern of diseases inherited by the Ashkenazi population and the population's general intellectual achievement. But in trying to draw a link between them they have crossed some fiercely disputed academic territories, including whether I.Q. scores are a true measure of intelligence and the extent to which intelligence can be inherited.
The authors "make pretty much all of the classic mistakes in interpreting heritability," said Dr. Andrew Clark, a population geneticist at Cornell University, and the argument that the sphingolipid gene variants are associated with intelligence, he said, is "far-fetched."
In addition, the genetic issue of natural selection versus founder effects is far from settled. Dr. Risch, whose research supports founder effects, said he was not persuaded by the Utah team's arguments. Dr. David Goldstein, a geneticist at Duke University who was not connected with either Dr. Risch's or the Utah study, was more open on the issue, saying Dr. Risch had made "quite a strong case" that founder effects could be the cause, but had not ruled out the possibility of selection.
Dr. Slatkin, though favoring a founder effect over all, said he agreed with the Utah team that this would not account for the cluster of sphingolipid diseases.
As for the Utah researchers' interpretation of Jewish medieval history, Paul Rose, professor of Jewish studies at Pennsylvania State University, said, "I think that some of their conclusions may be right though they still need a lot of work to be persuasive to historians and others."
Dr. Gregory Cochran, the first author on the Utah team's paper and a physicist who took up biology, said he became interested in the subject upon learning that patients with a particular Ashkenazic disease known as torsion dystonia were told by their physicians that "the positive thing is that this makes you smart."
"When you're in a hurry and have strong selection, you have a lot of genes with bad side effects," he said. The Ashkenazi Jewish population seemed to fit this pattern, he said, since they married only inside the community, making selection possible, and they had an urgent need for greater intelligence. Evolution had therefore selected every possible mutation that worked in this direction, despite their harmful side effects when inherited from both parents. "In a sense, I consider this a very boring paper since it raises no new principles of genetics," Dr. Cochran said.
The evolution of intelligence: Natural genius?
Jun 2nd 2005—The Economist
The high intelligence of Ashkenazi Jews may be a result of their persecuted past
THE idea that some ethnic groups may, on average, be more intelligent than others is one of those hypotheses that dare not speak its name. But Gregory Cochran, a noted scientific iconoclast, is prepared to say it anyway. He is that rare bird, a scientist who works independently of any institution. He helped popularize the idea that some diseases not previously thought to have a bacterial cause were actually infections, which ruffled many scientific feathers when it was first suggested. And more controversially still, he has suggested that homosexuality is caused by an infection.
Even he, however, might tremble at the thought of what he is about to do. Together with Jason Hardy and Henry Harpending, of the University of Utah, he is publishing, in a forthcoming edition of the Journal of Biosocial Science, a paper which not only suggests that one group of humanity is more intelligent than the others, but explains the process that has brought this about. The group in question are Ashkenazi Jews. The process is natural selection.
History before science
Ashkenazim generally do well in IQ tests, scoring 12-15 points above the mean value of 100, and have contributed disproportionately to the intellectual and cultural life of the West, as the careers of Freud, Einstein and Mahler, pictured above, affirm. They also suffer more often than most people from a number of nasty genetic diseases, such as Tay-Sachs and breast cancer. These facts, however, have previously been thought unrelated. The former has been put down to social effects, such as a strong tradition of valuing education. The latter was seen as a consequence of genetic isolation. Even now, Ashkenazim tend to marry among themselves. In the past they did so almost exclusively.
Dr Cochran, however, suspects that the intelligence and the diseases are intimately linked. His argument is that the unusual history of the Ashkenazim has subjected them to unique evolutionary pressures that have resulted in this paradoxical state of affairs. Ashkenazi history begins with the Jewish rebellion against Roman rule in the first century AD. When this was crushed, Jewish refugees fled in all directions. The descendants of those who fled to Europe became known as Ashkenazim.
In the Middle Ages, European Jews were subjected to legal discrimination, one effect of which was to drive them into money-related professions such as banking and tax farming which were often disdained by, or forbidden to, Christians. This, along with the low level of intermarriage with their gentile neighbors (which modern genetic analysis confirms was the case), is Dr Cochran's starting point.
He argues that the professions occupied by European Jews were all ones that put a premium on intelligence. Of course, it is hard to prove that this intelligence premium existed in the Middle Ages, but it is certainly true that it exists in the modern versions of those occupations. Several studies have shown that intelligence, as measured by IQ tests, is highly correlated with income in jobs such as banking.
What can, however, be shown from the historical records is that European Jews at the top of their professions in the Middle Ages raised more children to adulthood than those at the bottom. Of course, that was true of successful gentiles as well. But in the Middle Ages, success in Christian society tended to be violently aristocratic (warfare and land), rather than peacefully meritocratic (banking and trade).
Put these two things together—a correlation of intelligence and success, and a correlation of success and fecundity—and you have circumstances that favor the spread of genes that enhance intelligence. The questions are, do such genes exist, and what are they if they do? Dr Cochran thinks they do exist, and that they are exactly the genes that cause the inherited diseases which afflict Ashkenazi society.
That small, reproductively isolated groups of people are susceptible to genetic disease is well known. Constant mating with even distant relatives reduces genetic diversity, and some disease genes will thus, randomly, become more common. But the very randomness of this process means there should be no discernible pattern about which disease genes increase in frequency. In the case of Ashkenazim, Dr Cochran argues, this is not the case. Most of the dozen or so disease genes that are common in them belong to one of two types: they are involved either in the storage in nerve cells of special fats called sphingolipids, which form part of the insulating outer sheaths that allow nerve cells to transmit electrical signals, or in DNA repair. The former genes cause neurological diseases, such as Tay-Sachs, Gaucher's and Niemann-Pick. The latter cause cancer.
That does not look random. And what is even less random is that in several cases the genes for particular diseases come in different varieties, each the result of an independent original mutation. This really does suggest the mutated genes are being preserved by natural selection. But it does not answer the question of how evolution can favor genetic diseases. However, in certain circumstances, evolution can.
West Africans, and people of West African descent, are susceptible to a disease called sickle-cell anemia that is virtually unknown elsewhere. The anemia develops in those whose red blood cells contain a particular type of hemoglobin, the protein that carries oxygen. But the disease occurs only in those who have two copies of the gene for the disease-causing hemoglobin (one copy from each parent). Those who have only one copy have no symptoms. They are, however, protected against malaria, one of the biggest killers in that part of the world. Thus, the theory goes, the pressure to keep the sickle-cell gene in the population because of its malaria-protective effects balances the pressure to drive it out because of its anemia-causing effects. It therefore persists without becoming ubiquitous.
Dr Cochran argues that something similar happened to the Ashkenazim. Genes that promote intelligence in an individual when present as a single copy create disease when present as a double copy. His thesis is not as strong as the sickle-cell/malaria theory, because he has not proved that any of his disease genes do actually affect intelligence. But the area of operation of some of them suggests that they might.
The sphingolipid-storage diseases, Tay-Sachs, Gaucher's and Niemann-Pick, all involve extra growth and branching of the protuberances that connect nerve cells together. Too much of this (as caused in those with double copies) is clearly pathological. But it may be that those with single copies experience a more limited, but still enhanced, protuberance growth. That would yield better linkage between brain cells, and might thus lead to increased intelligence. Indeed, in the case of Gaucher's disease, the only one of the three in which people routinely live to adulthood, there is evidence that those with full symptoms are more intelligent than the average. An Israeli clinic devoted to treating people with Gaucher's has vastly more engineers, scientists, accountants and lawyers on its books than would be expected by chance.
Why a failure of the DNA-repair system should boost intelligence is unclear—and is, perhaps, the weakest part of the thesis, although evidence is emerging that one of the genes in question is involved in regulating the early growth of the brain. But the thesis also has a strong point: it makes a clear and testable prediction. This is that people with a single copy of the gene for Tay-Sachs, or that for Gaucher's, or that for Niemann-Pick should be more intelligent than average. Dr Cochran and his colleagues predict they will be so by about five IQ points. If that turns out to be the case, it will strengthen the idea that, albeit unwillingly, Ashkenazi Jews have been part of an accidental experiment in eugenics. It has brought them some advantages. But, like the deliberate eugenics experiments of the 20th century, it has also exacted a terrible price.
An Unnatural History of Jewish Population Genetics
June 7, 2005
This article is a review of "The Natural History of Ashkenazi Intelligence," by Gregory Cochran, Jason Hardy, and Henry Harpending.
Cochran, Hardy, and Harpending have presented a new paper, recently accepted for publication in the Journal of Biosocial Sciences, which argues that high Ashkenazi intelligence is the result of recent natural selection that occurred between 800 CE and 1700 CE, and is associated with a characteristic cluster of genetic disorders. The authors propose that these genetic disorders, which are autosomal and recessive, promote high intelligence when a single allele is present. Thus, their high frequency in an Ashkenazi population that concentrated in intellectual occupations during the Middle Ages is the result of natural selection, and reflects a dynamic equilibrium similar to the presence of sickle cell anemia in populations from malarial regions.
In this commentary, I will show that other factors, more cultural than biological, are probably responsible for the high measured IQs of European Jews. Although the Cochran-Harpending paper presents a hypothesis that is coherent, it reflects a Merchant of Venice view of Jewish history, in which the bulk of Jews were involved in usury and financial occupations. The actual occupations of Jews, especially before 1700, are poorly documented.
My commentary focuses not on the scientific plausibility of their argument, but on the concordance of their theory with the historical record. I will make the following points:
1. Jews are a self-selected minority group, and their high intelligence is the result of self selection, not natural selection.
2. Jewish intellectual achievement was already well established by 800 CE. Above average Jewish intelligence can be traced to the origins of rabbinical Judaism itself around the time of the destruction of the Temple in Jerusalem.
3. Self-selection and group selection for religious scholarship created a distinct Jewish population with a higher than average IQ.
4. Above average intelligence is characteristic not only of Ashkenazi Jews, but also of large portions of the Sephardic and Mizrahi (oriental) Jewish populations. Furthermore, the Jews living in urban centers of North Africa and the Middle East were concentrated in intellectual occupations just as the European Jews were.
Most Jewish history before the 19th century is intellectual history, and focuses on the development and interplay of ideas. Jewish scholarship is concerned primarily with Jewish law, not with the population dynamics or economic survival of Jews as a people. My interpretation of the transition between ancient and rabbinical Judaism is based on my study of other fields of history, and is not a mainstream view of Jewish history. In particular, my characterization of Jews as a "self-selected minority group" might be controversial to many Jews, although I don't think it will be hurtful to anyone.
Jews are a self-selected minority group, and their high intelligence is the result of self selection, not natural selection.
The authors view the Jewish population as a relatively closed population in which high intelligence evolved endogenously through natural selection. But the actual mechanism that created a genetically distinct population with high intelligence was cultural, not biological. Jews have had many opportunities throughout history to not be Jewish.
It is often remarked that the Jewish people are a nation or tribe as well as a religion. The modern Jewish population is actually a heavily self-selected group, the result of several defining population events. The first of these occurred in the later years of the Roman Empire, when the bulk of the Jewish population in the Roman Empire was probably converted to Christianity. Those who remained Jewish were principally the Pharisees, a movement of activists and scholars. Thus, the development of a distinct Jewish population with a higher average IQ dates not to the Middle Ages, but to the Talmudic period many centuries earlier.
In the ancient world of Greece and Rome, Judaism was a proselytizing religion and took in many converts. It has been estimated that in the 1st and 2nd centuries, about 10% of the population of the Roman Empire was Jewish, and as many as 25% in the Greek speaking cities of the Eastern Mediterranean. It was for this largely Greek speaking population that the Septuagint (a Greek translation of Hebrew scriptures) was produced. And it was amongst this Greek speaking Jewish population that Paul of Tarsus traveled, proselytizing Christianity.
Cochran, Hardy, and Harpending remark that no commentary survives that would suggest that ancient Jewish populations were set off from other peoples by ability or intelligence. Of course, no such documentation exists for any other group, for there were no social scientists and modern concepts of intelligence did not exist in ancient times.
Even so, it is likely that Jews already had a distinctive occupational structure in the Roman Empire. Historical sources suggest that male literacy was already universal among Jews in Roman times, although it is hard to say what level of literacy was attained. Diaspora communities had been established for more than 500 years, and the dispersion of the Jewish people around the Roman and Parthian (Persian) empires, as well as universal literacy among them, facilitated long distance trade.
The peak of Jewish population as a share of world population probably dates to the Roman era. So what happened to all those Jews?
Partly, the answer is that the population of Europe and the Mediterranean region declined precipitously at the end of the Roman era. The Romans had presided over an era of prosperity, in which Western Europe became progressively Romanized, speaking a common language, with a network of roads and other infrastructure to promote economic growth and trade. With the decline of the Roman Empire, which actually began long before the sacking of Rome in 410 CE, populations collapsed throughout Western Europe. Invasions of so- called "barbarians" were a symptom as much as a cause of this decline. In the Greek-speaking east, the decline was postponed for a century or more, but was ultimately almost as severe. In the reign of the Emperor Justinian during the 6th Century, the entire empire was decimated by plague.
Thus, the collapse of Roman authority was an economic catastrophe for the ancient world. In many regions, literacy came to an end and the written record of civilization vanishes for hundreds of years. Jewish populations, like urban populations in general, declined to a tiny remnant. How many of the Jews died in plagues or famines, and how many converted to Christianity and later Islam, is undocumented. Those that remained, a self-conscious and self-selected minority, created a unique intellectual culture, and it is that culture that survives today as modern Judaism.
Jewish intellectual achievement was already well established by 800 CE. Above average Jewish intelligence can be traced to the origins of rabbinical Judaism itself around the time of the destruction of the Temple in Jerusalem.
High measured Jewish intelligence results not from the unusual occupational structure of Ashkenazi Jews in the Middle Ages, but from the origins of rabbinical Judaism hundreds of years earlier. At the beginning of the Christian Era, Judaism was deeply divided and in ferment between several competing religious factions, notably the Pharisees and the Sadducees. Both movements were elites. Although the Pharisees are best known outside of Judaism from Jesus' rebuke in the Gospel of Matthew, there are other historical sources, from Josephus' History of the Jews to the Mishna, a compilation of Jewish law that was initiated by the Pharisees after the fall of Jerusalem.
Since they are history's losers, we know about the Sadducees mostly from the negative viewpoint of the Pharisees, who survived the destruction of the Temple and went on to compile their oral tradition as the Talmud. The Sadducees had been the local ruling elite in Jerusalem and other urban centers prior to the destruction of the Temple. They favored the power and privileges of Judaism's aristocracy and hereditary priesthood, and they accommodated rather than resisted the pressures of Hellenization (assimilation into the dominant Greek and Roman culture).
By contrast, the Pharisees were a democratizing movement that sought to redefine Jewish observance, promoting talent over birthright, and extending the rituals and laws of purity from the priesthood of the Temple to the general population. The consummate Pharisee was Hillel, a kind of "hero-scholar" who traveled to Palestine from Babylon and lived a simple and humble life in order to dedicate himself to the study of Torah. It was the Pharisees that maintained the study of ancient text in Hebrew, no longer the vernacular of the Jewish people, and promoted the ideal of scholarship.
Thus, the Jewish population that survived the destruction of the Second Temple in 70 CE, the plunder of Mesopotamia under Trajan, and the suppression of the Bar Kochba revolts (132-135 CE) was a self- selected remnant population of intellectuals. Of the religious movements that existed in the time of Jesus, it is the Pharisees that formed the basis of modern rabbinical Judaism.
Followers of the Sadducees, and the great mass of people that were not such partisan believers, probably assimilated in large numbers into Christian and Greek cultures and disappeared from Jewish history. But the Pharisees, conscious of themselves as scholars in exile, redoubled their efforts, compiling the works that form the basis of modern Jewish law. The Pharisees certainly had intellectual and verbal skills far beyond the level of the general population, and they are the progenitors in disproportionate numbers of modern Jewish populations.
Self-selection and group selection for religious scholarship created a distinct Jewish population with a higher than average IQ.
Judaism's survival after the fall of Jerusalem reflects the intellectual achievement of the surviving Pharisees, whom I will henceforth refer to as "rabbis" with a lowercase "r". In an intellectual effort that lasted hundreds of years, they compiled Jewish oral tradition into a series of tractates or volumes of encyclopedic scope, known collectively as the Talmud. It is through this written record that we know so much about ancient Judaism, at least about the Judaism of the Second Temple period.
Compilation of the Talmud began shortly after the fall of the Temple, with the founding of an academy at Yavneh in what is today modern Israel and the compilation of the Mishna or Palestinian Talmud. After the Bar Kochba rebellion (132-135 CE), such intellectual activity became impossible within the Roman Empire, and the remaining rabbis and their intellectual descendants relocated in the Parthian (Persian) empire, near modern Baghdad. Compilation of Talmudic learning and later commentary continues after conditions began to deteriorate in the Middle East, moving first to Spain and later to Northern and Eastern Europe in the Middle Ages.
In the Judaism of the Second Temple period, Jews had supported a professional class of priests, who by tradition are descendants of Aaron, the brother of Moses. Modern Judaism retains a few ceremonial roles for the descendants of priests, known as Kohanim, but it is unlikely that membership in the Jewish priesthood was a "sexually selected marker" after the fall of Jerusalem. To paraphrase the authors, rich families preferred to marry their daughters to men who excelled as scholars and intellectuals.
Cochran, Hardy, and Harpending find it implausible that Jewish scholarship as an occupational choice has contributed to high Jewish IQs. The authors seem to have concluded that this is implausible by taking the modern concentration of professional rabbis and Jewish scholars and projecting it backwards in time. They write, "Another theory suggests that there was selective breeding for Talmudic scholarship. This seems unlikely to have been an important selective factor, since there weren't very many professional rabbis, certainly less than one percent of the population. A selective force that only affects a tiny fraction of the population can never be strong enough to cause important evolutionary change in tens of generations."
Because they didn't understand the structure of Medieval Jewish societies, which were in place until the 18th or 19th century throughout much of Europe and the Middle East, the authors dismiss the effect of Jewish scholarship on Jewish intellectual achievement. Until the Enlightenment and the grant of citizenship which followed the French Revolution, Jews lived in separate, self-governing communities and paid taxes to the ruling Christian aristocracy as a community rather than as individuals.
Cochran, Hardy, and Harpending assert, with absolutely no evidence, that less than one percent of the Jewish population was part of a professional class of rabbis and scholars. They seem to be imagining that Jewish rabbis have always functioned like Christian ministers, as professionals leading congregations, although the modern Jewish congregation was a product of the 19th and 20th centuries, and a reaction to the organization of Protestant Christianity.
Although no occupational or census information exists, religious scholarship was a vast enterprise within the Jewish community. The best evidence for this is that such a large body of scholarship survives, even from a period when there was no printing or reproduction technology.
This scholarship, and the class of scholars who produced it, preserved not only the written record itself, but also the two ancient languages in which most of it was written: Hebrew and Aramaic. Although real fluency and literacy in these languages was probably limited in Medieval Europe, ordinary Jews were required to learn them, not only for ritual in the synagogue, but also for ceremonies in the home.
Rabbinical scholarship did not just determine what direction to face while praying, or in what order to light the Chanukah candles. If a neighbor's donkey damaged your shop, Jewish law determined his obligations to pay you damages. And if a tradesman died owing money, rabbinical law determined the obligations of his widow.
Jewish scholarship involved far more than the one percent of adult males suggested by Cochran, Hardy, and Harpending. Indeed, every male in a traditional Jewish community is considered to have an obligation to study and learn Jewish law. Jewish scholarship was concerned not just with abstract, spiritual, and ceremonial manners, but also with civil law, administration, government, commerce, tax collection, social welfare, and the regulation of everyday life, including marriage and child rearing.
The rabbis whose discussions are preserved in the Babylonian Talmud, known as the Amoraim, were not professional rabbis. The academies of Pumedita and Sura where the Talmud was compiled, located near Babylon in modern Iraq, met during the off-season, and closed during the growing season so that their attendees could tend their crops.
Scholars and commentators of subsequent generations were also not professional rabbis. The two greatest scholars of the Middle Ages were Rashi and Maimonides. Rashi, whose 11th century commentary is traditionally printed near the spine on every page of Talmud, tended a vineyard in France. Maimonides, who might be considered the consummate Sephardic Jewish scholar, was a physician.
Of course, Jewish scholarship and literacy contributed to Jewish survival during the Middle Ages. Hebrew script came to be used not only for sacred writing, but for everyday spoken language as well, including the Yiddish spoken by Ashkenazi Jews and the Judeo-Spanish or Ladino spoken by Sephardic Jews. This ability to write and record information facilitated the development of international trading and financial networks through which Jews supported themselves as they moved into Europe. At a time when most Europeans were peasant farmers and literacy was confined primarily to religious orders, the Jewish community was an alternate source of commercial talent, capable of conducting long-distance financial transactions.
In both Medieval Europe and in the Caliphates and Sultanates of the Middle East, small numbers of Jewish intellectuals were hired as "court Jews," offering their skills in finance, administration, and government as retainers for the Christian and Islamic royal elites. The success of this small and visible minority brought protection and prestige to the larger Jewish community, and reflects the survival of scholarship within that community.
Perhaps the ultimate testimony to the importance of religious scholarship among Jews is that Hebrew, an ancient language used only for religious purposes for nearly 2500 years, has been reborn as a modern language.
Above average intelligence is characteristic not only of Ashkenazi Jews, but also of large portions of the Sephardic and Mizrahi (oriental) Jewish populations. Furthermore, the Jews living in urban centers of North Africa and the Middle East were concentrated in intellectual occupations just as the European Jews were.
Cochran, Hardy, and Harpending cite several comparative studies of intelligence, achievement, and educational attainment by Ashkenazi versus Sephardic and Mizrahi Jews. Of course, these studies and the conclusions they reach are very controversial in Israel. This final point is central to their thesis, and my critique of it, because the authors claim that only Ashkenazi Jews have a higher mean IQ, and that only Ashkenazi Jews had the unique occupational structure which lead to natural selection for high IQ.
Because I wanted to respond very quickly to their paper, I am going to partially duck this issue. I have not personally read the studies these authors cite, though I do intend to when I can find them. (I'm confident that my many high IQ Sephardi and Mizrahi friends will move quickly to support me here!)
All of these sources were published before the Internet was available, and they are not available to me as abstracts. I do not have a university library at my disposal. I am admitting my prejudice here, but I believe even without having read these papers (the most recent of which was published in 1990) that further analysis will show that they are based on old data and old measurement concepts that are invalid, or that they simply don't measure and support what Cochran, Hardy, and Harpending claim they measure.
The truth is that any evaluation of these papers today is at best an intellectual exercise, because the communities that they purport to measure no longer exist, and may never really have existed. The ethnic fabric of modern Israel is a complex tapestry of overlapping groups, some difficult to define. There would be no way to repeat the studies today with modern testing procedures because the groups themselves have started to assimilate into one another.
Two years ago, I was the guest of an Iraqi Jewish family at a Passover Seder in Israel. "Look at my beautiful family," said the grandfather, pointing to his daughters and his grandchildren. "This one has married a Jew from Poland, that one a Jew from India, that one a Jew from Libya. I have the whole Diaspora in my family!"
Was it really an Iraqi Seder? His wife was born in Morocco, so the cuisine was actually Moroccan. But the Moroccan Jewish community itself is a mixture of Sephardim from Spain and the original Magrebi Jews who lived there before the Spanish Inquisition. In some parts of Morocco the two groups assimilated into each other, and in others they did not. It would never have occurred to me at the time to ask her which Moroccan community she came from.
Israel is the only place where Sephardic, Mizrahi, and Ashkenazi Jews have lived side-by-side. When the state of Israel was formed, about 900,000 Jews of Arab and North African countries were forced to flee. Most of them settled in Israel, but a disproportionate number from the professional elites chose the United States, Canada, Australia, the United Kingdom, or France, especially those who were fluent in English or French. Thus, the refugee population that came to Israel after 1948 was also self selected, with a bias towards those with the fewest skills.
In the 1950s and 60s, Ashkenazi Jews were the mainstream culture of Israel, part of a Hebrew-speaking community that had been established since the 1920s or earlier. At that time, Mizrahi Jews were still newcomers, not fluent in the language, living in great poverty in tents or temporary housing. Israel was a poor country, where eggs, cheese, and milk were rationed. Its not surprising that non-Ashkenazi Jews scored poorly on the IQ tests of the time. Disadvantaged groups have always fared poorly on IQ tests. Since the 1960s, Israeli society has attained a standard of living comparable to European countries, and the gaps between Ashkenazi, Sephardi, and Mizrahi Jews have narrowed.
Israeli data is based on national origin, not on a true ethnographic definition of peoples as being Sephardi, Ashkenazi, and so forth. The Mizrahi Jewish population is actually the most complex, for there were many small communities in the Middle East, North Africa, and Central Asia that survived intact to modern times, perhaps as genetic isolates.
It may be that these groups, which probably have remained in isolation since Roman times, did not experience the self-selection effects that characterized the birth of rabbinical Judaism.
One of the reasons that the authors could find no evidence of a population bottleneck in the Ashkenazi Jewish population is that the Ashkenazi population has never been as distinct or isolated as they imagine. The original Ashkenazi population probably descends from Italian Jews who migrated north in the early Middle Ages. Genetic testing of Y-Chromosome phenotypes supports the theory that Ashkenzi Jewish males are closely related to the remnant community of Romaniote Jews, who by tradition are the descendants of Judeans brought to Italy and Greece as captives after the many religious wars and rebellions in the Middle East. Throughout the Middle Ages, Jews of Europe were engaged in trade with the Middle East, and it was through those trade routes that Jewish scholarship from the Middle East entered Europe. Later on, there was considerable gene flow from Sephardic populations escaping the Inquisition into the Ashkenazi community, and this is reflected in language and surnames.
In fact, a considerable portion of the Jews that migrated to Israel from western European countries such as England, France, and Holland were of Sephardic origin. As evidence of high Ashkenazi intelligence, Cochran, Hardy, and Harpending cite IQ data from London schools in the early 20th century. But the wealthiest and most established Jews in London at that time were descendants of Sephardim who came to England in the 18th century, not of more recent Ashkenazi immigrants.
My point is really that there is no way to measure Ashkenazi versus Sephardic and Mizrahi intelligence today, and there probably never was. The original Sephardic and Mizrahi communities were dissolved before IQ tests could be administered, and there is no way to reassemble their inhabitants. Sephardic communities such as Sarajevo and Thessalonica were destroyed in the Holocaust, while Mizrahi and Sephardic communities in North Africa and the Middle East were forced to flee en masse after the founding of Israel in 1948.
One source of confusion in any discussion of the relative intellectual performance of different Jewish groups, is that the label "Sephardic" is sometimes attached to all non-Ashkenazi Jews, although some are more accurately labeled Mizrahi, and others such as the Ethiopians, are none of the above. There is actually a religious reason for this, because there are slight differences in religious practice between the two groups. For a religious Jew living in Israel (where most Jews are relatively secular), it is a matter of which rabbinical court has jurisdiction. An Ashkenazi Jew would go to an Ashkenazi religious court to settle a divorce matter, while a non-Ashkenazi Jew would go to a Sephardic court.
The best evidence is that Jews of the urban Sephardic and Mizrahi communities in countries such as Egypt, Iraq, and Iran were concentrated in intellectual occupations just as the European Jews were. If those countries had gone through an intellectual transformation like the Enlightenment in Europe, then their native Jewish populations would have flourished just as European Jews did before the Holocaust.
There is a considerable literature in Spanish, written in the late 19th and early 20th centuries, about the Sephardic communities of the Balkans and the Ottoman Empire. After the repeal of the Inquisition, philosemetic Spaniards rediscovered these communities in the late 1800s, and some advocated repatriating them and restoring their Spanish citizenship. They generally noted the high level of professional and occupational achievement of Sephardic Jews in their host societies. Unfortunately, the Sephardic communities of Greece and the Balkans were destroyed in the 1940s, before modern psychometric measurement techniques could be tried.
The Jews of Babylon and Persia, descendants of the rabbis who compiled the Talmud, were active throughout the Middle East as traders, often trading with India and China, where they established small communities that survived to modern times. Before the founding of Israel, when the Iraqi Jewish community was dissolved and 130,000 Jews were forced to flee, Jews formed much of the urban middle class in Baghdad and other Iraqi cities.
Cochran, Hardy, and Harpending suggest that the Jews in Islamic countries were involved more in "dirty" occupations. In fact, they have chosen their evidence about Jewish occupations in these countries very selectively, while exaggerating the involvement of European Jews in finance and money lending. They might be surprised that the Jews of Europe also had to be their own butchers and bakers, because of Jewish dietary laws. And there may have been an even greater need for Jews to serve as money lenders in early Islamic societies, because Islamic law prohibits charging interest.
Although I disagree with the authors' hypothesis that [Ashkenazi] Jewish intellectual achievement is the result of natural selection in a people that were restricted to a limited range of occupations, I welcome their paper. Even though their hypothesis is wrong, it is at least testable! It can be accepted or rejected on its own merits.
While I take for granted that there are innate differences between communities and ethnic groups, and that these may reflect biological as well as cultural differences, I personally fall more on the "nurture" side of the "nature versus nurture" debate when it comes to IQ and intelligence. Because the authors relied too heavily on biological explanations and mechanisms, they ignored cultural ones, and they failed to grasp that Jewish populations are actually open and partially self-selected. I have not even touched on any of the cultural or environmental reasons why Jews are high achievers.
I have characterized the Cochran-Harpending theory as being based on a Merchant of Venice view of Jewish history. If anyone reads this paper, that is surely the sound-bite they will take away, but I don't mean this perjoratively. I don't want anyone who comments on my paper to say that I accused these authors of using stereotypes or caricatures of Jews.
This is my first entry in the "Blogosphere." I am not writing this commentary to extract a pound of flesh from the authors. Although I don't agree with them, I do applaud them for tackling a difficult subject that crosses many fields, and for daring to talk of issues that mainstream academia has ignored because of "political correctness."
This paper is in the public domain. It can be reproduced anywhere without cost or special permission. I encourage people to copy it rather than linking to it here, if that suits them. Please cite me as a courtesy, and spell my name correctly.
Howard Metzenberg; metzenberg[nospam]@yahoo.com
From the book by Kevin MacDonald, 1994, "A People That Shall Dwell Alone."
Data reviewed in Chapter 5 indicate a general Jewish overrepresentation in a wide range of fields in the modern world, including business, science, social science, literature, and the arts. At the pinnacle of achievement, Jewish overrepresentation is particularly striking. Patai and Patai (1989, 159) show that Jews received a highly disproportionate number of Nobel prizes in all categories from 1901 to 1985, including 11 percent for literature, 12.7 percent for chemistry, 20.2 percent for physics, 35.2 percent for physiology and medicine, and 26.1 percent for economics. Moreover, the extent of overrepresentation has increased since World War II, since Jews were awarded twice the number of prizes in the years 1943-1972 compared to 1901-1930. In Germany, Jews received 10 of 32 Nobel prizes awarded to German citizens between 1905 and 1931 despite constituting less than 1 percent of the population during this period (Gordon 1984, 14).
Studies of gifted children are of particular interest because IQs in the gifted range are unlikely to result from environmental influences acting on individuals whose genetic potential is near the population mean. Terman's (1926) classic study found twice as many Jewish gifted children as expected on the basis of their representation in the population, although the true representation of Jews in this group may have been higher because some may have concealed their Jewish identity. These subjects had IQs ranging from 135 to 200 with a mean of 151. One of Terman's Jewish subjects had an IQ of 184 when tested at age seven. His close relatives included a chief rabbi from Moscow, a prominent lawyer, a self-made millionaire, a concert pianist, a writer, and a prominent Polish scientist. His maternal great-grandfather was a rabbi famous for his compilation of a Jewish calendar spanning over 400 years, and the rabbi's descendants (the boy's cousins) had IQs of 156, 150, 130, and 122.
Research suggests an average IQ of Ashkenazi Jewish children in the range of 117. In two studies of representative samples of Jewish children, Bachman (1970) and Vincent (1966) found an average IQ of 117 and 117.8, respectively, although Vincent's results are said to be an underestimate because they excluded a large percentage of an elite group of Jewish children attending fee-paying schools.
There is good evidence that Jewish children's Verbal IQ is considerably higher than their Performance IQ. Brown (1944) found several sub-test differences compatible with the hypothesis that Jewish children are higher on verbal abilities, while Scandinavian children are higher on visuo-spatial abilities. Lesser, Fifer, and Clark (1965) found large differences favoring Jewish children over Chinese-American children on verbal ability, but insignificant differences in favor of Chinese-American children on visuo-spatial abilities. And Backman (1972) found that Jewish subjects were significantly higher than non-Jewish Caucasians on a measure of verbal knowledge but were significantly lower on visuo-spatial reasoning.
Large verbal/performance IQ differences have been found within Jewish populations. Levinson (1958) studied a representative sample of yeshiva students and found an average Verbal IQ of 125.6, an average Performance IQ of 105.3, and an average Full Scale IQ of 117.86, although he suggests that there may have been a ceiling effect for some students on the verbal portion. Whereas in the general population there was a correlation of 0.77 between Verbal and Performance IQs, among Jewish children it was only 0.31. Finally, Levinson (1960b) found that a sample of Jewish boys (age 10-13) with an average Verbal IQ of 117 had a Performance IQ of 98, while Irish and Italian samples matched for Full Scale IQ had Verbal/Performance differences of only approximately 5 points (approximately 110-105). Levinson (1959) provides evidence that the Verbal/Performance difference for Jewish children increases from pre-school to young adulthood. When children were matched on the basis of full-scale Wechsler IQ, pre-school children showed a small (3-point) difference between Performance and Verbal IQ, while elementary school-age and college student subjects showed a difference of approximately 20 points.
Taken together, the data suggest a mean IQ in the 117 range for Ashkenazi Jewish children, with a Verbal IQ in the range of 125 and a Performance IQ in the average range. These results, if correct, would indicate a difference of almost two standard deviations from the Caucasian mean in Verbal IQ exactly the type of intellectual ability that has been the focus of Jewish education and eugenic practices. While precise numerical estimates remain somewhat doubtful, there can be no doubt about the general superiority of the Ashkenazi Jewish children on measures of verbal intelligence (see also Patai & Patai 1989, 149).
There are important implications of the finding of higher verbal intelligence among Jews. Lynn (1992) notes that higher socio-economic status groups tend to have high verbal intelligence, but these groups are not particularly high on Visuo-spatial abilities. This indicates that verbal intelligence is more important for upward mobility and success in contemporary societies, and this was undoubtedly the case in traditional stratified societies as well: Wilken (1983) notes that education in rhetoric was the key to upward mobility in the Greco-Roman world of antiquity.
In this regard, it is worth mentioning that economic historians have noticed that Jewish economic activity has tended not to be characterized by technological innovation related to mechanical abilities tapped by tests of visuo-spatial abilities (i.e., Performance IQ). Thus, Mosse (1987, 166) suggests that the distinguishing features of Jewish economic activity in 19th-century Germany are to be found "less in outright innovation or invention than in a special aptitude for economic 'mediation' in the forms of the export of German goods, of 'secondary innovation', technology transfer through the introduction into Germany of processes and methods observed abroad, and new techniques for the stimulation of demand."
This is a difficult area because a theme of anti-Semitic writing in Germany was that Jews were not innovators, but only appropriated the inventions of others (Mosse 1987, 166, 404). Anti-Semites emphasized that inventors of new technology such as Rudolf Diesel and Werner von Siemons were predominantly gentile, while several Jewish fortunes in technical areas, such as those of Ludwig Loewe and Emil Rathenau, were made by importing technology that originated elsewhere and were dependent on capitalization provided by Jewish private banks. While among Jews ownership was divorced from technical competence, the prototypical gentile entrepreneur was the "inventor-artisan" whose technical competence was crucial to the success of the company. Whereas technical competence and inventiveness were crucial to the success of the prototypical gentile firms, among Jews success was related to having access to capital or to having "commercial flair and the ability to inspire confidence" (p. 312).
Enhance Intelligence of Jews?, James Owen, for National Geographic News, July
Albert Einstein, Sigmund Freud, Leonard Bernstein, Saul Bellow, to name a few, all shared European Jewish ancestry.
Known as Ashkenazim, this ethnic group is blessed with more than its fair share of talented minds.
But they are also prone to a number of serious genetic diseases.
Researchers now suggest that intelligence is closely linked to such illnesses in Ashkenazi Jews, and that the diseases are the result of natural selection.
The Ashkenazim are descended from the Jewish communities of Germany, Austria, Poland, and Eastern Europe that date back to the 10th century. Today they make up around 80 percent of the world's Jews.
Ashkenazim have the highest average IQ of any ethnic group, scoring 12 to 15 points above the European average. They are also strongly represented in fields and occupations requiring high cognitive ability. For instance, European-origin Jews account for 27 percent of U.S. Nobel science prize winners but make up only about 3 percent of the U.S. population.
But the group is also associated with neurological disorders, including Tay-Sachs, Gaucher's, and Niemann-Pick. Tay-Sachs is a fatal inherited disease of the central nervous system. Sufferers lack an enzyme needed to break down fatty substances in the brain and nerve cells. Gauchers and Niemann-Pick are similar, often fatal diseases.
Researchers at the University of Utah's anthropology department investigated a possible link between these genetic illnesses and above-average intelligence in Ashkenazi Jews. They suggest both are the result of natural selection for enhanced brainpower.
Because Jews were discriminated against in medieval Europe, they were often driven into professions such as moneylending and banking which were looked down upon or forbidden to Christians.
Writing last month in the Journal of Biosocial Science, the researchers said, "For the most part they had jobs in which increased IQ strongly favored economic success, in contrast with other populations, who were mostly peasant farmers. They lived in circumstances in which economic success led to increased reproductive success."
Historians suggest Jews with lucrative jobs often had four, six, or sometimes eight or nine children. Poorer families, meanwhile, tended to be smaller, possibly because they lived in overcrowded areas of town and children were more prone to disease.
As a result, the researchers say, over hundreds of years European Jews became more intelligent than their gentile countrymen.
But increased intelligence may have come at a cost, with genetic diseases such as Tay-Sachs being side effects of genes that facilitate intelligence. The researchers say that it's highly unlikely that mutated genes responsible for these illnesses could have reached such high levels in Ashkenazim if they did not influence cognitive performance.
While the link is difficult to prove, there is some evidence that Gaucher disease does increase IQ. Around one in three people of working age who were patients of the Gaucher Clinic at the Shaare Zedek Medical Centre in Jerusalem had professions requiring an average IQ of more than 120 (the standard figure is 100). They included scientists, academics, physicians, and accountants.
So if natural selection is responsible for above-average intelligence in Ashkenazi Jews, could the same apply to other ethnic groups?
Co-author Gregory Cochran said, "The logical place to look for something similar—an occupationally specialized, reproductively isolated group that might have been shaped by recent natural selection—would be India. In particular, the Parsis."
Parsis are descendents of the Zoroastrians who emigrated from Persia to India in the 8th century. As in medieval Jewish communities in Europe, Parsis marry almost wholly within their own group. They are also economically successful with a long history as traders and businessmen.
Parsis are also especially vulnerable to diseases not seen in their neighbors, including Parkinson's disease, breast cancer, and tremor disorders.
Modern-day Ashkenazim are now far more likely to marry outside their ethnic group. Cochran says he would expect that both a tendency for higher IQ and associated genetic disorders to become less marked over time.
The idea that some ethnic groups are inherently more intelligent than others is controversial, and researchers are divided on the issue.
The Human Genome Project, completed in 2000, suggested that humans are 99.9 percent genetically identical across all races. At the time, the project's findings were hailed as evidence that the human genome is color-blind.
But the journal Psychology, Public Policy and Law recently published a study that reviewed research into the differences in average IQ between Asians, whites, and blacks.
J. Philippe Rushton, from the University of Western Ontario, and Arthur Jensen, an educational psychologist at the University of California, Berkeley, concluded these differences are 50 percent genetic in origin.
"Race differences show up by three years of age, even after matching on maternal education and other variables," Rushton said. "Therefore they cannot be due to poor education since this has not yet begun to exert an effect."
Writing in response to their findings, University of Michigan psychologist Richard E. Nesbitt said that "there is not a shred of evidence" in the research to suggest the IQ gap between races has a genetic basis