The Intelligence Paradox: Why the Intelligent Choice Isn't Always the Smart One - Satoshi Kanazawa 2012
Other Possible Consequences of Intelligence
Another evolutionarily novel value is vegetarianism. Humans are naturally omnivorous, and anyone who eschewed animal protein and ate only vegetables in the ancestral environment, in the face of food scarcity and precariousness of its supply, was not likely to have survived long enough and stayed healthy enough to have left many offspring. So such a person is not likely to have become our ancestor. Anyone who preferentially ate animal protein and fat in the ancestral environment would have been much more likely to live longer and stay healthier. They are therefore much more likely to have become our ancestors.
Vegetarianism would therefore be an evolutionarily novel value, as well as a luxury of abundance. The Intelligence Paradox would predict that more intelligent individuals are more likely to choose to become a vegetarian than less intelligent individuals.
This indeed appears to be the case.3 Among the NCDS sample, those who are vegetarian at age 42 have significantly higher childhood general intelligence than those who are not vegetarian. Vegetarians have the mean childhood IQ of 109.1 whereas meat eaters have the mean childhood IQ of 100.9. The difference is large and highly statistically significant.
Figure 13.2 Association between childhood general intelligence and adult vegetarianism (NCDS)
The association between childhood general intelligence and adult vegetarianism holds among both women and men separately. Among women, vegetarians have a mean childhood IQ of 108.0 while meat eaters have the mean childhood IQ of 100.7. Among men, vegetarians have the mean childhood IQ of 111.0 and meat eaters have the mean childhood IQ of 101.1, a 10-point difference!
Figure 13.3 Association between childhood general intelligence and adult vegetarianism: women (NCDS)
Figure 13.4 Association between childhood general intelligence and adult vegetarianism: men (NCDS)
The fact that the difference in childhood IQ between vegetarians and meat eaters is larger among men than among women makes sense in light of the historical division of labor between the sexes. Throughout evolutionary history, men have traditionally hunted animals for their meat while women have traditionally gathered plant food. So vegetarianism—a complete and total eschewal of animal meat—should be even more evolutionarily novel for men than for women. Women are 60% more likely to be vegetarians than men are (3.33% vs. 2.07%).
Childhood general intelligence has a significantly positive effect on the likelihood of vegetarianism at age 42, even net of a large number of social and demographic factors, such as sex, whether ever married, whether currently married, education, income, religion, religiosity, social class at birth, mother's education, and father's education, both in the full sample and among men and women separately.
There appears very little doubt that more intelligent children are more likely to grow up to be vegetarian as adults in the United Kingdom. One standard deviation (15 points) increase in childhood IQ increases the odds of adult vegetarianism by 37% among women and by 48% among men.
Figure 13.5 Association between childhood intelligence and adult vegetarianism (Add Health)
Interestingly, the strong association between childhood intelligence and adult vegetarianism is not replicated with Add Health data in the US. American vegetarians in early adulthood do have significantly higher childhood intelligence in junior high and high school, but the difference is not large (101.5 vs. 99.3). And it is only significant among women (101.4 vs. 98.5), not among men (101.7 vs. 100.1). This is very strange given the historical division of labor by sex that I note above. The significant effect of childhood intelligence on adult vegetarianism among Americans disappears entirely once mother's or father's education or religion is statistically controlled.
Figure 13.6 Association between childhood intelligence and adult vegetarianism: women (Add Health)
Figure 13.7 Association between childhood intelligence and adult vegetarianism: men (Add Health)
It is not at all clear to me why the difference in childhood intelligence between vegetarians and meat eaters is so much larger and stronger in the UK than in the US. As I note in Chapter 11, when I discuss the divergent effects of childhood intelligence on the adult consumption of tobacco, there are two principal differences between NCDS and Add Health: the national differences between the UK and the US; and the generational differences between those born in March 1958 and those born between 1974 and 1983. I am not sure if it is the national differences or the generational differences, or something entirely different, that account for the observed differences in the association between childhood intelligence and adult vegetarianism in the UK and the US.