Map of life expectancy at birth from Global Education Project.

Monday, June 08, 2015

Nature or Nurture?

You may have heard about this meta-analysis published Nature Genetics which combined data from all the twin studies ever done to make the claim that our "nature" -- personality, intelligence, all that --  is about 50% due to heredity and 50% due to our environment. It appears that a lot of people who should know better, including Jeremy Dean, author of "Psyblog" to which I link above, fell for this claptrap unquestioningly.

A feature-length deconstruction by Jonathan Kaplan at Scientia Salon will reward your time if you want to know more about both meta-analysis and heritability, but I will give you the short version and say a bit about why the pervasive misconception about this subject is so pernicious.

To say that a trait is 50% "heritable" is to say, roughly, that 50% of the variation in a specific, observed population is associated with the genes a person is born with. In humans, these conclusions are usually derived from twin studies -- researchers compare identical (monozygotic) with fraternal (dyzygotic) twins and see how much difference there is in the variability of some trait. The idea is that twins, identical or not, are raised in very similar environments, so you can parse out the effect of genes vs. environment. This is questionable, of course -- parents and others probably treat identical twins differently than they do fraternal twins. Every child in a family lives in a different micro-environment.

But leaving that aside, it does not follow that genes explain 50% of the variability of the trait in humans in general. That's because genes are associated with a particular phenotype (the actual characteristics of an organism) only given a particular environment. Your genes won't make you tall if you are malnourished as a child, and they won't make you able to bench press 250 pounds if you don't work out. For children born into the same family, of the same socioeconomic status, in the same culture and community, 50% of the variability in IQ might be associated with genetic predisposition. I don't think the twin studies really prove this (see Kaplan) but even if conceding the point for the sake of argument, average IQ in the U.S. has risen by some 20 points since widespread testing began. The average American in 1940 would be a special needs student today.

This is of course one reason -- not the only one -- why Charles Murray and his racist followers (a group which includes Jeb Bush, by the way) are wrong. More generally, it strikes at the heart of political conservatism, including its so-called "libertarian" variant. The liberal view of human nature is that it is what we make of it. That is my view -- we can become better.

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