Saturday, May 31, 2014

Rice farming and gene-culture co-evolution

Rice paddies, China, circa 1917-1923 (source). To grow rice, you must cooperate with neighbors for irrigation and labor. Today, even with the shift to a post-agricultural society, Chinese from rice-farming areas display less individualism and more interdependence than Chinese from wheat-farming areas. Is this evidence of gene-culture co-evolution?

Human populations differ in genetic variants that influence a wide range of mental and behavioral traits. These differences are statistical, often being apparent only when one compares large numbers of individuals. Yet even a weak statistical difference can affect the way a culture develops. Furthermore, the way a culture develops may favor certain genetic variants over others.

East Asian cultures, for example, have diverged noticeably from European cultures, particularly those of Western Europe:

Western culture is more individualistic and analytic-thinking, whereas East Asian culture is more interdependent and holistic-thinking. Analytic thought uses abstract categories and formal reasoning, such as logical laws of noncontradiction—if A is true, then "not A" is false. Holistic thought is more intuitive and sometimes even embraces contradiction—both A and "not A" can be true. (Talhelm et al., 2014)

This is of course a generalization that ignores differences within each culture area. Historically, abstract thinking has been stronger among the French, whereas the English have tended toward empirical "bottom-up" thinking. A new study suggests that similar differences exist within East Asia, specifically between rice-farming areas and wheat-farming areas. In short, rice farming favors interdependence, whereas wheat farming is more conducive to individualism:

The two biggest differences between farming rice and wheat are irrigation and labor. Because rice paddies need standing water, people in rice regions build elaborate irrigation systems that require farmers to cooperate. In irrigation networks, one family's water use can affect their neighbors, so rice farmers have to coordinate their water use. Irrigation networks also require many hours each year to build, dredge, and drain—a burden that often falls on villages, not isolated individuals. (Talhelm et al., 2014)

Labor inputs are thus greater for rice growing. A husband and wife cannot farm a large enough rice paddy to support their family if they rely only on their own labor. This is not the case with wheat farming:

In comparison, wheat is easier to grow. Wheat does not need to be irrigated, so wheat farmers can rely on rainfall, which they do not coordinate with their neighbors. Planting and harvesting wheat certainly takes work, but only half as much as rice. The lighter burden means farmers can look after their own plots without relying as much on their neighbors. (Talhelm et al., 2014)

A study of 1,162 Han Chinese found differences between rice-farming and wheat-farming regions on three psychological measures: cultural thought, implicit individualism, and loyalty/nepotism.

Cultural thought

The triad task shows participants lists of three items, such as train, bus, and tracks. Participants decide which two items should be paired together. Two of the items can be paired because they belong to the same abstract category (train and bus belong to the category vehicles), and two because they share a functional relationship (trains run on tracks). People from Western and individualistic cultures choose more abstract (analytic) pairings, whereas East Asians and people from other collectivistic cultures choose more relational (holistic) pairings.

[...] People from provinces with a higher percentage of farmland devoted to rice paddies thought more holistically. [...] Northern and southern China also differ in several factors other than rice, such as climate, dialect, and contact with herding cultures. Therefore, we analyzed differences among neighboring counties in the five central provinces along the rice-wheat border. [...] People from the rice side of the border thought more holistically than people from the wheat side of the border.  (Talhelm et al., 2014)

Implicit individualism

Researchers measure how large participants draw the self versus how large they draw their friends to get an implicit measure of individualism (or self-inflation). A prior study found that Americans draw themselves about 6 mm bigger than they draw others, Europeans draw themselves 3.5 mm bigger, and Japanese draw themselves slightly smaller.

People from rice provinces were more likely than people from wheat provinces to draw themselves smaller than they drew their friends. [...] On average, people from wheat provinces self-inflated 1.5 mm (closer to Europeans), and people from rice provinces self-inflated -0.03 mm (similar to Japanese). (Talhelm et al., 2014)


One defining feature of collectivistic cultures is that they draw a sharp distinction between friends and strangers. A previous study measured this by having people imagine going into a business deal with (i) an honest friend, (ii) a dishonest friend, (iii) an honest stranger, and (iv) a dishonest stranger. In the stories, the friend or stranger's lies cause the participant to lose money in a business deal, and the honesty causes the participant to make more money. In each case, the participants have a chance to use their own money to reward or punish the other person.

The original study found that Singaporeans rewarded their friends much more than they punished them, which could be seen positively as loyalty or negatively as nepotism. Americans were much more likely than Singaporeans to punish their friends for bad behavior.

[...] People from rice provinces were more likely to show loyalty/nepotism [...]. In their treatment of strangers, people from rice and wheat provinces did not differ. (Talhelm et al., 2014)

Gene-culture co-evolution?

Interestingly, these findings come from people who have no connection to farming of either sort. If these psychological traits have survived the transition to a post-agricultural and largely urban society, how are they passed on? The question is raised by the authors:

[…] perhaps the parts of culture and thought style we measured are more resistant to change. Or perhaps modernization simply takes more generations to change cultural interdependence and thought style. However, most of our participants were born after China's reform and opening, which started in 1978. Furthermore, Japan, South Korea, and Hong Kong modernized much earlier than China, but they still score less individualistic on international studies of culture than their wealth would predict. (Talhelm et al., 2014)

The authors do not use the term "gene-culture co-evolution" but this seems to be the explanation they implicitly favor. Over many generations, rice farming has selected for a certain package of psychological traits, i.e., less abstract thinking and more functional "holistic" thinking; less individualism and more collectivism; and less impartiality toward strangers and more favoritism towards kin and friends.

The predominance of rice farming in East Asia may thus explain why East Asian cultures have developed their pattern of psychological traits:

The rice theory can explain wealthy East Asia's strangely persistent interdependence. China has a rice-wheat split, but Japan and South Korea are complete rice cultures. Most of China's wheat provinces devote less than 20% of farmland to rice paddies. None of Japan's 9 regions or South Korea's 16 regions has that little rice (except for two outlying islands). Japan and Korea's rice legacies could explain why they are still much less individualistic than similarly wealthy countries. (Talhelm et al., 2014)


Talhelm, T., X. Zhang, S. Oishi, C. Shimin, D. Duan, X. Lan, and S. Kitayama. (2014). Large-scale psychological differences within China explained by rice versus wheat agriculture, Science, 344, 603-607.

Saturday, May 24, 2014

The puzzle of European hair, eye, and skin color

Taylor Swift (photo by David Shankbone). The physical appearance of Europeans seems to result from a selection pressure that acted primarily on women and only secondarily on men. This is especially true for highly visible traits on or near the face—the focus of visual attention.


I have just published a paper on "The puzzle of European hair, eye, and skin color." The introduction is reproduced below (reference citations have been removed for ease of reading). The full text is available here.


Most humans have black hair, brown eyes, and brown skin. Europeans have a different color scheme, their hair being also brown, flaxen, golden, or red, and their eyes also blue, gray, hazel, or green. Finally, their skin is pale, almost like an albino's.

How did this unusual color scheme come about? Perhaps the genetic change that lightened the skin also affected the hair and the eyes. Yet the genes are different in each case. European skin lightened mainly through the appearance of new alleles at three genes: SLC45A2, SLC24A5, and TYRP1. European hair color diversified through a proliferation of new alleles at MC1R. European eye color diversified through a proliferation of new alleles in the HERC2-OCA2 region and elsewhere.

Light skin is associated with a few of the new hair and eye color alleles, particularly the ones for red hair or blue eyes. Conceivably, these alleles may be a side effect of selection for lighter skin. But why would such selection increase the total number of alleles for hair and eye color, especially when so many of them have little or no effect on skin color? And why have neither red hair nor blue eyes reached fixation in any human population, even those with milk-white complexions?

The European color scheme has another puzzling aspect. It seems to result from a selection pressure that acted primarily on women and only secondarily on men:

- Hair color varies more in women than in men. Redheads are especially more frequent among women.

- Eye color varies more in women than in men when both copies of the so-called blue-eye allele are present, the result being a greater diversity of female eye colors wherever blue eyes are the single most common phenotype, i.e., in northern and eastern Europe.

- Blue eyes are associated in men with a more feminine face shape.

- In all human populations, women are paler than men after puberty. This post-pubescent lightening is due to sexual maturation and not to differences in sun exposure. In women, lightness of skin correlates with thickness of subcutaneous fat and with 2nd to 4th digit ratio—a marker of prenatal estrogenization. Admittedly, this sex difference is not greater in Europeans than in other populations, although it could not easily be otherwise, since Europeans are so close to the physiological limit of depigmentation.

While women are more diverse than men both in hair and eye color, this greater diversity came about differently in each case. With hair color, women have more of the intermediate hues because the darkest hue (black) is less easily expressed. With eye color, women have more of the intermediate hues because the lightest hue (blue) is less easily expressed. 

In sum, European hair and eye color diversified through a selection pressure that acted on different genes via different pigmentary changes. The common denominator seems to be the creation of new visual stimuli on or near the face—the focus of visual attention.


Frost, P. (2014). The puzzle of European hair, eye, and skin color, Advances in Anthropology, 4, 78-88.

Saturday, May 17, 2014

Another Robert Chambers?

Robert Chambers (1802-1871). His anonymously published book, Vestiges of the Natural History of Creation (1844), helped pave the way for public acceptance of Darwin’s theory of evolution. (source)


I haven't yet read Nicholas Wade's book A Troublesome Inheritance. I will venture to say, however, that it will be remembered less for its actual content than for its role in encouraging discussion of a difficult topic. In particular, it will familiarize a broad audience with the following points:

1. Biological evolution did not slow down with the advent of cultural evolution. In fact, it speeded up, particularly when farming began to replace hunting and gathering some 10,000 years ago. At that time, the pace of genetic change may have risen a hundred-fold.

2. Cultural evolution diversified the range of human environments. Instead of adapting only to differences in climate or food sources, like other animals, our species also adapted to differences in social structure, in the division of labor, in the means of subsistence, in unwritten or codified norms of conduct, in the degree of sedentary living, and in many other human-made phenomena. Our ancestors reshaped their environments, and these human-made environments reshaped them via gene-culture co-evolution.

3. This gene-culture co-evolution persisted into modern times. The English population, for instance, evolved between the twelfth and nineteenth centuries in terms of certain behavioral traits, particularly future time orientation and distaste for violence as a means to settle personal disputes. As Gregory Clark has shown, this behavioral change resulted from a demographic change—the relative reproductive success of the middle and upper classes—which altered the composition of the English gene pool. So the mantra that "we, too, were once savages" does not, in fact, deny the reality of biological evolution. It affirms it.

4. Human populations thus differ not only anatomically but also in various mental and behavioral predispositions. These differences are statistical and often apparent only when one compares large numbers of people. But even a weak statistical difference can profoundly affect how a society will develop and organize itself.

5. Finally, Richard Lewontin was right when he reported that genes vary much more within populations than between populations. He was unaware, however, that genetic variability between populations is qualitatively different from genetic variability within a population. The more a gene has value, the more it will vary across a population boundary, since such boundaries usually coincide with barriers that separate different habitats, different environments, different means of subsistence and, hence, different selection pressures. Conversely, the less a gene has value, the more it will vary within a population, that is, among individuals who share similar conditions of life. The selection pressure is uniform but this uniformity will not level out the variability of such genes within the population—much as a steam iron will smooth a rumpled shirt—since this variability is less phenotypically significant, i.e., it produces fewer functional differences that natural selection can act on. 

Are there questionable points in Wade's book? Undoubtedly. But we should not wait until all issues are settled before we put pen to paper. Writing is a process where ideas are shared with a broader audience for debate. We may forget that The Origin of Species was written without any knowledge of Mendelian genetics. We may also forget, or simply not know, that Darwin’s path to public acceptance was cleared by an earlier book: Vestiges of the Natural History of Creation (1844). Although its anonymous author, Robert Chambers, had no understanding of natural selection, he nonetheless played a key role in familiarizing the public with the fossil record and the reality of biological change over time. As one historian pointed out:

It is customary among biographers of Darwin to speak of the excitement which greeted the appearance of the Origin and of Huxley's able defense of Darwin at Oxford in his clash with Bishop Wilberforce. Actually, however, by the time Darwin published, Robert Chambers had drawn much of the first wrath of the critics and the intelligent public was at least reasonably prepared to consider a more able, scientific presentation of the subject.

[…] The attacks which the scientific world launched upon the Vestiges have, in retrospect, a quite unreal character. They belabor minutiae and amateurish minor errors as though there was some subconscious recognition that the heart of the thesis was unassailable.

[…] With its publication and success as a best seller, the world of fashion discovered evolution. The restricted professional worlds of science and of theology both lost their ability to suppress or intimidate public thinking upon the matter.

[…] By 1859, when the Origin of Species was published, an aroused and eager audience was considerably prepared for the revelations of Charles Darwin. The great amateur disputant and the great professional scholar should always be remembered as having together won the public mind to evolution. (Eiseley, 1958, pp. 134, 138, 139)


Chambers, R. (1844). Vestiges of the Natural History of Creation, London: John Churchill

Eiseley, L. (1958). Darwin's Century. Evolution and the Men Who Discovered It, New York: Anchor Books.

Wade, N. (2014). A Troublesome Inheritance. Genes, Race and Human History, Penguin Books.


Saturday, May 10, 2014

A pathway to pro-social behavior

The digit ratio is the length of the index finger (2nd finger) divided by the ring finger (4th finger). It correlates with the degree of androgenization or estrogenization of fetal tissues, including the fetal brain. (source)


As small bands of hunter-gatherers gave way to larger and more complex societies of farmers and townsfolk, trusting relationships had to expand beyond the circle of close kin. This larger social environment posed a two-fold problem:

For trust to evolve our ancestors must have 1) overcome the incentive to defect when involved in cooperative activity, and 2) suppressed the proclivity to use violence to take resources from conspecifics, as is seen in nonhuman primates. (Gifford, 2013)

This in turn required "social rules of governance and implicit institutions that suppressed free riding, provided rules of orderly behavior that increased cooperation by making individual behavior predictable, and also protected the property rights of individuals" (Gifford, 2013).

But what, exactly, does one do with free riders and sociopaths? Traditionally, such people were excluded from society, either by ostracism or, in more serious cases, by execution. There was thus strong selection for pro-social behavior, i.e., acting honorably and peacefully with other members of society. This selection operated even when ostracism was far from permanent or total. Shunning, public shaming, or simply a bad reputation would hurt one's chances for survival and reproduction in many ways: reduced access to community goods, discrimination on the marriage market, reluctance by others to provide assistance, and so forth.

This process of selection had genetic consequences, since nearly all behavioral traits have a heritability of 40% plus or minus 20%. There was thus removal not only of antisocial individuals from society but also of antisocial predispositions from the gene pool. The corollary was that the gene pool became dominated by pro-social predispositions, particularly empathy, compliance with social rules, and high thresholds for expression of anger.

This evolution probably occurred incrementally through small changes at many genes. This is what we see with increases in human intellectual capacity, and it is probably a general rule for the evolution of complex traits. Big changes at single genes tend to have nasty side-effects elsewhere on the genome.

A recent paper has highlighted one possible evolutionary pathway: the relative degree of androgenization or estrogenization of the developing fetus (Branas-Garza et al., 2013). By varying the ratio of one to the other, it's possible to alter a wide range of behavioral tendencies. This prenatal priming of fetal tissues can be easily measured by the "digit ratio," i.e., the length of the index finger (2nd finger) divided by the length of the ring finger (4th finger). The lower your digit ratio, the more you have been androgenized before birth. The higher your digit ratio, the more you have been estrogenized before birth.

Branas-Garza et al. (2013) found that altruistic behavior is strongest among men with intermediate digit ratios:

We analyze the association between altruism in adults and the exposure to prenatal sex hormones, using the second-to-fourth digit ratio. We find an inverted U-shaped relation for left and right hands, which is very consistent for men and less systematic for women. Subjects with both high and low digit ratios give less than individuals with intermediate digit ratios. We repeat the exercise with the same subjects seven months later and find a similar association, even though subjects' behavior differs the second time they play the game.

Different environments favor different degrees of altruism. In one setting, an altruist may be admired and enjoy preferential access to community goods. In another, the same person may be ridiculed and ruthlessly exploited. Thus, according to the context, the right balance has to be struck between altruism and selfishness:

One possible interpretation of the above findings comes from stabilizing selection. Since sharing with others is socially beneficial, selfish individuals are socially excluded and their fitness affected negatively. If individuals who are exposed too much or too little do not share with others, there is an evolutionary pressure on these non-altruistic individuals, which in turn generates an indirect evolutionary pressures on the degree of exposure to prenatal sex hormones by raising survival probabilities of individuals with intermediate levels of exposure. This hypothesis is supported by observed distributions of 2D:4D in the literature, which are universally concentrated around the median values. (Branas-Garza et al., 2013)

From one population to the next, digit ratios tend to cluster around different means, perhaps because altruism has been favored or disfavored to different degrees. This social selection may have targeted other behavioral traits, notably thrill-seeking. Kornhuber et al. (2013) have found that low digit ratios are associated with video game addition. This kind of addiction may tap into a male need for risk and adventure, which may likewise have been more adaptive in some environments than in others.



Brañas-Garza, P., J. Kovárík, L. Neyse (2013). Second-to-fourth digit ratio has a non-monotonic impact on altruism. PLoS ONE 8(4): e60419. 

Gifford Jr., A. (2013). Sociality, trust, kinship and cultural evolution, The Journal of Socio-Economics, 47, 218-227 

Kornhuber, J., E-M. Zenses, B. Lenz, C. Stoessel, P. Bouna-Pyrrou, et al. (2013). Low 2D:4D Values are associated with video game addiction. PLoS ONE 8(11): e79539.

Saturday, May 3, 2014

What happened in the 1980s to reaction time?

A steady increase in reaction time seems to begin circa 1980 in Sweden, Great Britain, and the United States (h/t to hbd* chick)

Has reaction time been steadily increasing from generation to generation? This was the finding of a paper last year, which argued that mean IQ had fallen in Britain by 13 points since Victorian times (Woodley et al., 2013). The problem here was not the extrapolation from reaction time to IQ, which in any case should not have changed over the past century. The problem was the possibility of sampling bias. The early samples (from the Victorian age) were slanted toward people of elite origin. In one case, they were University of Chicago students; in the other, museum visitors who had paid to take the reaction test. In contrast, the recent samples were much more representative, largely because the educational system had become more universal. Thus, the drop in reaction time over time may be largely, if not wholly, an artefact of better sampling of the general population (hbd* chick, 2013).

This criticism seems less applicable to a study of this drop in more recent times. This study was presented as a conference paper and only the abstract is available. But it does seem interesting:

Here, we show that change in genetic intelligence can be estimated, independently of the Flynn effect, by way of simple reaction time (RT). Data from three studies with different samples from Sweden, UK, and USA converge at an RT increase of 0.7-0.9 ms per year, which corresponds to a decrease in intelligence of between 4 and 5 IQ points per generation, or 1.3-1.7 points per decade in these countries. (Madison, 2014)

That’s a big jump in reaction time. Moreover, most of the increase seems to be squeezed into the last six years of the study, from 1980 to 1985 (see above chart). No selection pressure could have produced such a change over such a short time. So what could possibly be going on here?

Keep in mind that Sweden, the United Kingdom, and the United States are not closed systems. All three countries are open to the world, and the early 1980s corresponded to a time when they became much more open. Consequently, we are not looking at change within a population, but rather the replacement of one population by another, and this change would have been most noticeable in the school classrooms where these tests were conducted.

The other finding of this study is that the Flynn Effect has been masking a decline in intellectual capacity. Yes, we’re getting better at giving standardized answers to standardized questions, but this change doesn’t reflect an actual increase in intelligence. We’re just allocating more and more mental resources to the task of test-taking. The reaction time data suggest that real intellectual capacity has been declining since circa 1980.

At the same conference, Armstrong (2014) likewise argues that the Flynn Effect may be illusory for the most part:

However, "general intelligence", the biological substrata which cause the positive manifold amongst different IQ tests, has not increased, since the sizes of Flynn effects on different tests are inversely related to those tests' g loadings. The same pattern holds amongst items. Thus, for example, vocabulary size (generally the most or among the most g loaded tests) has shown a small Flynn effect, and by some measures even a decline. However, the "Coding" test (from the Wechsler) or the "Draw-a-Man" test both have low g loadings and have shown very large Flynn effects.

To date, Madison’s paper is unavailable. All we have is an abstract that raises more questions than it answers. We will have to wait for the full paper before a thorough assessment can be made.


Armstrong, E. (2014). LCI14 Elijah Armstrong on Rule Dependence, London Conference on Intelligence, Psychological comments, May 1  

Hbd* chick (2013). a response to a response to two critical commentaries on woodley, te nijenhuis & murphy (2013), hbd* chick, May 27  

Madison, G. (2014). Increasing simple reaction times demonstrate decreasing genetic intelligence in Scotland and Sweden, London Conference on Intelligence, Psychological comments, April 25
#LCI14 Conference proceedings  

Woodley, M.A., J. Nijenhuis, and R. Murphy. (2013). Were the Victorians cleverer than us? The decline in general intelligence estimated from a meta-analysis of the slowing of simple reaction time, Intelligence, 41, 843-850.