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)
References
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
