Do human populations vary statistically in their mental abilities and predispositions? And if so, why? Such questions have lately refocused from the ‘macro’ to the ‘micro’ level—from differences that may have developed in prehistory between large continental populations to those that may have arisen in historic times between much smaller groups.
In part, the focus is now on the higher intellectual performance of Ashkenazi Jews (Cochran et al., 2006; Murray, 2007). It is also on the attitudinal and behavioural changes that ultimately sparked England’s Industrial Revolution. In a newly released book, Gregory Clark (2007) argues that natural selection gradually raised the English population to a threshold that made this economic sea-change possible, specifically by selecting for middle-class values of non-violence, thrift and foresight. The question remains, of course, as to why this threshold was first reached in England.
This new focus reflects a growing recognition that natural selection does not need aeons of time to change a population significantly (Eberle et al., 2006; Harpending & Cochran, 2002; Voight et al., 2006; Wang et al., 2006). Change can occur over a dozen generations, certainly during the last six millennia of history. This possibility is all the likelier given that these millennia have seen humans specialize in a wide range of occupations, some more mentally demanding than others. Charles Murray, for one, has argued that selection for intelligence was historically weaker in farming and stronger in sales, finance and trade.
He may be right. But the reason, I believe, lies not so much in the occupation itself as in its relations of production.
In the Middle Ages and earlier, farmers had little scope for economic achievement and just as little for the intelligence that contributes to achievement. Most farmers were peasants who produced enough for themselves, plus a surplus for the landowner. A peasant could produce a larger surplus, but what then? Sell it on the local market? The possibilities there were slim because most people grew their own food. Sell it on several markets both near and far? That would mean dealing with a lot of surly highwaymen. And what would stop the landowner from seizing the entire surplus? After all, it was his land and his peasant.
The situation changes when farmers own their land and sell their produce over a wide geographical area. Consider the "Yankee" farmers who spread westward out of New England in the 18th and 19th centuries. They contributed very disproportionately to American inventiveness, literature, education and philanthropy. Although they lived primarily from farming, they did not at all have the characteristics we associate with the word "peasant".
Conversely, trade and finance have not always been synonymous with high achievement. In the Middle Ages, the slow growth economy allowed little room for business expansion within one's immediate locality, and expansion further afield was hindered by brigandage and bad roads. Furthermore, the static economic environment created few novel situations that required true intelligence. How strong is selection for intelligence among people who deal with the same clients, perform the same transactions and charge the same prices year in and year out?
This point has a bearing on the reported IQ differences between Ashkenazi and Sephardic Jews. Charles Murray, like others, believes that the Ashkenazim were more strongly selected for intelligence because more of them worked in sales, finance and trade during the Middle Ages. Now, we have no good data on the occupations of medieval Ashkenazim and Sephardim. But the earliest censuses (18th century for Polish Jews and 19th century for Algerian Jews) show little difference, with the bulk of both groups working in crafts.
There was, however, one major demographic difference. While the Sephardim grew slowly in numbers up to the 20th century, the Ashkenazim increased from about 500,000 in 1650 to 10 million in 1900. The same period saw strong population growth among Europeans in general. This boom used to be attributed to falling death rates alone, but demographers now recognize that rising birth rates were also key, in some countries more so. England, in particular, saw a rise in fertility that contributed two and a half times as much to the increase in growth rates as did the fall in mortality, largely through a younger age of first marriage. This was how England overtook France in total population.
The baby boom was particularly strong among one class of people: semi-rural artisans who produced for the larger, more elastic markets that developed with the expanding network of roads, canals and, later, railways. Their family workshops were the main means for mass-producing textiles, light metalwork, pottery, leather goods and wood furnishings before the advent of factory capitalism. Unlike the craft guilds of earlier periods, they operated in a dynamic economic environment that had few controls over prices, markets or entry into the workforce. "They were not specialized craftsmen in life-trades with skills developed through long years of apprenticeship; they were semi-skilled family labour teams which set up in a line of business very quickly, adapting to shifts in market demand" (Seccombe, 1992, p. 182). Their workforce was their household. In more successful households, the workers would marry earlier and have as many children as possible. In less successful ones, they would postpone marriage or never marry.
In Western Europe, these cottage industries were located in areas like Ulster, Lancashire, Yorkshire, Brittany, Flanders, Alsace, Westphalia, Saxony, the Zurich uplands, the Piedmont and Lombardy. In Eastern Europe, they were concentrated among Ashkenazi Jews. Selection for intelligence among the Ashkenazim may thus have been part of a larger European-wide selection for intelligence among cottage industry workers. These entrepreneurial artisans had optimal conditions for selection: 1) tight linkage between success on an intelligence-demanding task and economic achievement; 2) broad scope for economic achievement; 3) tight linkage between economic achievement and reproductive success; and 4) broad scope for reproductive success. Such artisans were a minority in Western Europe. Among the Ashkenazim, they appear to have been the majority.
In the late 19th century, cottage industries gave way to factories and the tight linkage between economic achievement and reproductive success came undone. Entrepreneurs could now expand production by hiring more workers. Henry Ford, for instance, produced millions of his Model T but had only one child.
Thus, it is not the type of occupation that drives selection for intelligence. It is the relations of production. In particular, do people own their means of production? Do they operate in a large, elastic market that rewards progressively higher levels of ability with commensurate increases in production? Finally, do they meet the increased demand for labour by increasing the size of their families?
Clark, G. (2007). A Farewell to Alms: A Brief Economic History of the World. Princeton (NJ): Princeton University Press.
Cochran, G., Hardy, J., & Harpending, H. (2006). Natural history of Ashkenazi intelligence. Journal of Biosocial Science, 38, 659-693.
Eberle, M.A., M.J. Rieder, L. Kruglyak, D.A. Nickerson. (2006). Allele frequency matching between SNPs reveals an excess of linkage disequilibrium in genic regions of the human genome. PLoS Genet 2(9), e142
Harpending, H., & G. Cochran, G. (2002). In our genes. Proceedings of the National Academy of Sciences, 99(1), 10-12.
Murray, C. (2007). Jewish Genius. Commentary, April.
Seccombe, W. 1992. A Millennium of Family Change. Feudalism to Capitalism in Northwestern Europe, London: Verso.
Voight B.F., Kudaravalli, S., Wen, X., &Pritchard, J.K. (2006). A map of recent positive selection in the human genome. PLoS Biol, 4(3), e72
Wang, E.T., Kodama, G., Baldi, P., & Moyzis, R.K. (2006). Global landscape of recent inferred Darwinian selection for Homo sapiens, Proc. Natl. Acad. Sci USA, 103, 135-140.