IQ, height, and homicide rate in Japan (red = lowest value, light blue = highest value) (Kura, 2013). The longer and harsher the winter, the greater the need for cognition and male solidarity?
The Japanese are descended from the intermixture 2900 to 1500 years ago of indigenous hunter-gatherers, the Jomon people, with incoming farmers from East Asia, the Yayoi people. Even today, the Japanese still show an east-west genetic cline, with East Asian haplogroups being most frequent in western regions that face the Korean Peninsula (Kura, 2013).
In addition to this east-west cline, there is also a north-south cline for physical traits like height and skin color. Kura (2013) shows that the Japanese are taller and lighter-skinned the farther north they are. Interestingly, the same study shows a north-south cline in mental and behavioral traits, notably IQ, homicide rate, and divorce rate. In Japan, latitude correlates positively with IQ and negatively with homicide rate and divorce rate.
This north-south cline seems independent of the east-west one. If it likewise has a genetic basis, the reason cannot be varying degrees of Yayoi/Jomon intermixture. The reason must be differences in the way natural selection has acted on the Japanese over the past 1500 years.
Of course, this north-south cline can also be explained on cultural grounds. The climate is milder in southern Japan, and farmers have less need to store food for the lean times of winter and early spring. Consequently, they tend to live more in the present and don’t plan ahead as much. Men are also less necessary as hunters who can bring food when the ground is frozen. Because women are more self-reliant food-wise, they depend less on their male partners. For the same reason, the cost of supporting a second wife is lower for men. There is thus more polygyny, more male-male rivalry for access to females, and less male solidarity. State formation becomes more difficult, and there is no longer a higher authority that can impose a monopoly on violence and thereby pacify social relations.
But this culturalist explanation hardly applies to modern Japan, or even the Japan of the past century or so. To the extent that culture has created this mental/behavioral cline, it has probably done so through gene-culture co-evolution. This is the explanation that Kura (2013) favors. The longer and harsher the winter, the greater the demands on cognition and the sexual bond … and the higher the likelihood of survival and reproduction for those who can meet such demands.
Kura also sees this Japanese north-south cline as evidence that evolution did not stop with the emergence of behaviorally modern humans some 100,000 years ago. Evolution has actually been proceeding at a faster rate:
This conclusion would be in line with the Hawks, Wang, Cochran, Harpending, and Moyzis (2007) idea of ever-accelerating human evolution. They insist that more and more beneficial mutations swept populations, after the advent of agricultural civilizations with metallurgy, letters and complex hierarchical organizations. The Japanese north–south gradient in height and intelligence can be evidence that modern humans have evolved to higher intelligence within the last two millennia. (Kura, 2013)
Hawks, J., E.T. Wang, G.M. Cochran, H.C. Harpending, and R.K. Moyzis. (2007). Recent acceleration of human adaptive evolution. Proceedings of the National Academy of Science, 104(52), 20753–20758.http://www.pnas.org/content/104/52/20753.full
Kura, K. (2013). Japanese north–south gradient in IQ predicts differences in stature, skin color, income, and homicide rate, Intelligence, 41, 512-516.http://www.sciencedirect.com/science/article/pii/S0160289613000949