A Troublesome Inheritance

by Nicholas Wade

  The wealth was important because it enabled Jews to secure a considerable degree of reproductive success. Before the Industrial Revolution and the escape from the Malthusian trap, the rich had more surviving children, being able to provide better nutrition and warmer houses. The Ashkenazi population had grown from almost nothing in 900 AD to about 500,000 people by 1500 AD, and had reached 14.3 million by 1939.9

  From about 900 AD to 1700 AD, Ashkenazim were concentrated in a few professions, notably moneylending and later tax farming (give the prince his money up front, then extract the taxes due from his subjects). Because of the strong heritability of intelligence, the Utah team calculates that 20 generations, a mere 500 years, would be sufficient for Ashkenazim to have developed an extra 16 points of IQ above that of Europeans. The Utah team assumes that the heritability of intelligence is 0.8, meaning that 80% of the variance, the spread between high and low values in a population, is due to genetics. If the parents of each generation have an IQ of just 1 point above the mean, then average IQ increases by 0.8% per generation. If the average human generation time in the Middle Ages was 25 years, then in 20 human generations, or 500 years, Ashkenazi IQ would increase by 20 × 0.8 = 16 IQ points.

  There were of course Christian moneylenders who required the same cognitive skills as Ashkenazim. But the Christians married into a much larger community that included people in many other occupations. Natural selection may have been raising the intelligence of urban populations in general during the Middle Ages but exerted a much stronger effect on the smaller Jewish population. This was because any intelligence-enhancing genes that arose in a family in the general population would be diluted in the next generation, but could accumulate in the Jewish community because marriage to outsiders was deterred. This selective effect could not operate on Oriental Jews—those under Muslim rule—because their rulers for the most part confined them to unpopular occupations like tanning or butchery which required no particular intellectual skills. Oriental Jews and Sephardim are not overrepresented in cognitively demanding occupations and both have IQs comparable to Europeans, the Utah team says.

  The Utah researchers give short shrift to the other explanations that have been proposed for enhanced Jewish intelligence. One is that the series of massacres and expulsions that began at the time of the First Crusade in 1096 constituted a selective effect that only the more intelligent were able to survive. But the massacres and expulsions affected the whole Ashkenazi population and seem unlikely to have selected the more intelligent nearly as precisely as did the skills required for moneylending.

  Jewish folklore holds that marriages between the children of rabbis and rich merchants were the driver of enhanced intelligence. Talmudic academies, writes the anthropologist Melvin Konner, “culled the best minds in every generation of Jews for more than a thousand years. Rising stars among these bright young men would board with successful merchants, and matches would be made between them and the merchants’ daughters. Thus the smartest, most studious boys would join the wealthiest families.” 10

  Without any data as to how often such matches were made, this seems more like a scholar’s fantasy than a common arrangement. Rich merchants may have been more likely to see another merchant’s son as a more promising son-in-law than a poor rabbinical student. But even if such marriages did sometimes occur, there were not enough rabbis in the population—a mere 1%—to make a genetically significant difference, the Utah team says.

  The Utah researchers make a plausible enough general case that the selective pressure from a cognitively demanding occupational niche would have selected for higher intelligence among Ashkenazim. They then go on to identify what they believe are the causative genes. Their proposals, if confirmed, would give specific plausibility to the general argument but, if false, would not bring it down.

  The genetic argument concerns the mutations that cause Mendelian diseases. Mendelian, or simple, diseases are those that result from a mutation that disables a single gene, as opposed to complex diseases like cancer or diabetes, which are the product of several causative variant genes.

  Every population has its own pattern of Mendelian diseases. Among Jews, some Mendelian mutations, like familial Mediterranean fever, are very ancient, being shared with other Middle Eastern populations like Turks and Druze, while others are found among only Ashkenazim or Sephardim and so must have occurred after the two populations separated.

  The Utah team’s analysis focuses on a group of four Mendelian diseases that occur in Ashkenazim and affect an obscure biochemical function, the storage of fats known as sphingolipids. The four diseases are known as Tay-Sachs, Gaucher’s, Niemann-Pick and mucolipidosis type IV.

  Inheriting a single copy of any of these variant genes does no great harm: the good copy inherited from the other parent compensates for the defective allele. But inheriting a double dose of the variant alleles can cause serious impairment in the case of Gaucher’s and is lethal in the case of the other three diseases.

  The variant genes that cause the four diseases are found in relatively high proportions in the Ashkenazi population. When a version of gene is more common than expected, geneticists usually assume one of two causes. One is natural selection and the other is the influence known as a founder effect.

  Why should natural selection favor a variant gene associated with a lethal disease? This can happen when the variant, though lethal in a double dose, confers some advantage when inherited from only one parent. A well-known example is that of sickle-cell anemia. A person with one copy of the variant gene is protected from malaria, but those who inherit two copies suffer from a serious blood disease. The allele will be favored by natural selection because the many single-allele carriers, who are protected from malaria, far outnumber the carriers of two alleles, who die or suffer impairment.

  The other reason why a variant gene can be more common than expected is that it happened to occur at high frequency in a small population that later expanded. Any rare mutation carried by one of the population’s founders will be inherited by his or her descendants and attain a higher frequency in that population than in most others, a situation known as a founder effect.

  The geneticist Neil Risch has concluded that the Ashkenazi Jewish mutations are founder effects that arose around 1,000 years ago. Since the mutations all arose at the same time, they must have the same cause, and that must be a founder effect, Risch argues, because such a variety of mutations is unlikely to offer any specific advantage that natural selection might favor.11

  But this argument is neatly turned around by the Utah team. They agree that the Ashkenazi mutations arose within the past 1,000 years but argue that the mutations were indeed all favored by natural selection at the same time because they all promote intelligence.

  If the founder effect argument is rejected, a plausible reason for the commonness of Ashkenazi Mendelian mutations would be that they protect against some serious disease. But no such protective effect has yet been detected. In any case, Ashkenazi Jews and the European populations they lived among suffered from the same diseases, yet there is no similar pattern of mutations in Europeans.

  The only significant difference in the Ashkenazi way of life was that they worked in cognitively demanding occupations, the Utah team argues, so this must be the selective pressure that drove the Ashkenazi Mendelian mutations to such relatively high levels.

  Another reason for assuming natural selection is at work, rather than a founder effect, is that some of the Ashkenazi mutations occur in clusters. This is highly unusual because mutations strike at random throughout the genome so should not be concentrated in genes that all have the same function. One set of Ashkenazi mutations occurs in the cluster of genes that controls the sphingolipid storage pathway mentioned above. For four mutations to be found in a specific pathway is a strong indication of natural selection. The Utah team points to experimental evidence, though there is not very much of it, suggesting th
at disruption of sphingolipid storage induces neurons to make more connections than usual.

  A second cluster of four mutations is found in a DNA repair pathway. Two of the mutations occur in the BRCA1 and BRCA2 genes and are associated with breast and ovarian cancer. The other two mutations cause the diseases Fanconi’s anemia type C and Bloom syndrome. It is hard to see how disruptions of DNA repair systems could be beneficial in any context, especially in the case of the two BRCA mutations, which carry risk even when an individual has a single copy of the mutated gene. The Utah team notes that BRCA1 can limit cell proliferation in neuronal stem cells in the embryo and adult, so that impairing the gene could allow extra brain cells to be generated. They suggest there may be similar advantages, yet to be discovered, in the other DNA repair mutations.

  Though the exact role of the Mendelian mutations in promoting intelligence has yet to be clarified, they are strikingly common among Ashkenazim. Some 15% of Ashkenazi Jews carry one of the sphingolipid or DNA repair mutations, and 60% carry either these or one of the other Mendelian disease mutations special to Ashkenazim. As already noted, the mutations are harmless when inherited from just one parent. The Utah team’s explanation seems the best so far for this strange pattern of mutations, and in particular for those that exist in clusters. Moreover, it is a great virtue in a scientific hypothesis to be easily testable, as the Utah team’s theory is. The theory implies that people carrying one of the Ashkenazic mutations will be found to have higher IQ scores, on average, than people who do not. Anyone with access to a population of Ashkenazim could test the prediction that high IQ is associated with the Ashkenazic mutations. Strange to say, no one has yet done so or, if they have, they have not published their findings.

  Without being able to test a living population, the Utah team obtained indirect evidence that Gaucher’s disease raises IQ. They found that of 255 working age patients at a Gaucher’s clinic in Israel, one third were in fields like science, accountancy or medicine, which require high IQs, a far greater proportion than in the population as a whole.

  Advantages of Literacy

  A possible weakness of the Utah team’s proposal is the assertion that enhanced cognitive capacity is confined to the Ashkenazic branch of the Jewish population. Sephardim have given the world Spinoza, Disraeli, Ricardo and many other distinguished individuals. It is hard to find specific measurements of Sephardic IQ, and the Utah team offer none in their article. Measurements of IQ in Israel report that Ashkenazi IQ is higher than that of non-Ashkenazim, but the latter group includes Oriental Jews as well as Sephardim. The Utah team focuses on Ashkenazi Jews because the Mendelian mutations found in Ashkenazim seem to have originated around 1000 AD, after Ashkenazim and Sephardim became separate populations. But even if the Utah team’s thesis has merit, there is no reason why Jews should not have enjoyed special cognitive capacities from much earlier in their history; if so, these traits could later have been enhanced among Ashkenazim in the manner the Utah team describes.

  A new perspective on Jewish history has recently been developed by two economic historians, Maristella Botticini and Zvi Eckstein. Botticini specializes in medieval contracts and marriage markets and teaches at Bocconi University in Milan. Eckstein is a distinguished economist who has served as deputy governor of the Bank of Israel. Their interest in Jewish history is focused on population numbers and occupation. They allude hardly at all to intelligence or genetics, yet their economic history makes abundantly clear how selection pressures could have acted on the Jewish population so as to enhance cognitive capacity.

  The widely held conventional explanation for Jewish occupational history is that Jews were forbidden by their Christian host nations to own land and drifted into moneylending because it was the only profession open to them. Because of frequent expulsions and persecutions, according to this view, Jewish communities were dispersed in towns all over Europe and the Mediterranean world.

  Botticini and Eckstein reject this explanation, arguing with a wealth of historical detail that Jews were not forced into moneylending but rather chose it because it was so profitable, and that they generally dispersed not because of persecution but because there were jobs for only so many moneylenders in each town.

  But how did Jews come to choose this unusual occupation? Botticini and Eckstein develop a simple but forceful explanation that goes back to the beginnings of rabbinical Judaism in the 1st century AD.

  Before the rabbinical era, Israelite religion was focused on the temple in Jerusalem and on copious animal sacrifices. Its leaders promoted three major insurrections against Roman rule, the first of which culminated in the destruction of the Jerusalem temple in 70 AD. The loss of the temple strengthened the position of the Pharisees, one of several sects, and led them to develop a quite different version of Judaism in which the temple and animal sacrifice were replaced as central components of the religion by study of the Torah.

  The rabbinical form of Judaism that emerged from this movement emphasized literacy and the skills to read and interpret the Torah. Even before the destruction of the temple, the Pharisee high priest Joshua ben Gamla issued a requirement in 63 or 65 AD that every Jewish father should send his sons to school at age six or seven. The goal of the Pharisees was universal male literacy so that everyone could understand and obey Jewish laws. Between 200 and 600 AD, this goal was largely attained, as Judaism became transformed into a religion based on study of the Torah (the first five books of the Bible) and the Talmud (a compendium of rabbinic commentaries).

  This remarkable educational reform was not accomplished without difficulty. Most Jews at the time earned their living by farming, as did everyone else. It was expensive for farmers to educate their sons and the education had no practical value. Many seem to have been unwilling to do so because the Talmud is full of imprecations against the ammei ha-aretz, which in Talmudic usage means boorish country folk who refuse to educate their children. Fathers are advised on no account to let their daughters marry the untutored sons of the ammei ha-aretz.

  The scorned country folk could escape this hectoring without totally abandoning Judaism. They could switch to a form of Judaism Lite developed by a diaspora Jew, one that did not require literacy or study of the Torah and was growing in popularity throughout this period. The diaspora Jew was Paul of Tarsus, and Christianity, the religion he developed, seamlessly wraps Judaism around the mystery cult creed of an agricultural vegetation god who dies in the fall and is resurrected in the spring.12

  As evidence that many Jews did indeed convert to Christianity, Botticini and Eckstein cite estimates showing that the Jewish population declined dramatically from around 5.5 million in 65 AD to a mere 1.2 million in 650 AD. There is little else to account for such a dramatic decline other than a high rate of conversion away from Judaism.

  Botticini and Eckstein make no mention of the genetic forces that would have been brought into play by such a conversion. But if Jews who lacked the ability or commitment to become literate were shed from the community generation after generation, the propensity for literacy of those remaining would steadily rise. The rabbinical requirement for universal male literacy may thus have been the first step toward a genetic enhancement of Jewish cognitive capacity. A second step was to come later, when the literacy was put to great practical effect.

  By 650 AD, Jews had almost entirely disappeared from regions where Christianity was strong, including Syria, Lebanon, Egypt and even Israel itself. The center of the Jewish world shifted to Iraq and Persia. There was also a shift in Jewish occupations. Jews abandoned farming and moved to towns, where they entered trade and commercial activities or became shopkeepers and artisans.

  After the establishment of the Abbasid caliphate in 750 AD, Jews migrated to the newly prospering towns and cities. By 900 AD, almost all Jews were engaged in urban occupations, dealing with crafts, trade, moneylending and medicine. Why did Jews choose professions in these particular fields? Common belief is
they were forbidden to own land and denied entry to certain crafts. Botticini and Eckstein say there is little or no evidence for such prohibitions. Jews concentrated in professions like trade and moneylending, they argue, for a simple reason. In a world where most people were illiterate, the literacy of almost all Jews gave them a decided advantage in any occupation that required reading contracts or keeping accounts.

  Jews enjoyed another practical benefit conferred by their religion. Jewish communities were subject to law, as laid out in the Talmud, and rabbinic courts oversaw contract enforcement and disputes. Because of the presence of Jewish communities in many cities of Europe and the Near East, Jews had access to a natural trading network of their coreligionists. Both the network and the dispute resolution mechanism were unusual and gave Jews a special advantage in long-distance commerce.

  As trade and urbanization started to flourish in the Muslim world under the Abbasids, the “higher literacy of the Jewish people,” Botticini and Eckstein write, “gave the Jews a comparative advantage over non-Jews in crafts, trade, commerce and money-lending.” 13

  The sack of Baghdad by the Mongols in 1258 destroyed the political and cultural center of the Abbasid empire, and large regions of Iraq and Persia became depopulated. The population center of the Jewish community now shifted to Europe, where Jews increasingly specialized in moneylending.

  This occupational pattern had a profound demographic consequence. Because moneylending was so profitable, despite its high risks, Jews could afford to support large families and, like other wealthy people, could ensure that more of their children survived to adulthood. After the devastation of the Jewish communities in Iraq and Persia and the expulsion of European Jews from England, France and many regions of Germany, their total population fell to fewer than 1 million in 1500 AD. But propelled by their new wealth, the Jewish population started to increase rapidly and by 1939 had reached 16.5 million globally.