Mind: Is IQ in the Genes? Twins Give Us Two Answers

For the rich and well-fed, genes cause most of the variance [in intelligence]. But for the poor, environment is key.

Today, a third of a century after the study began and with other studies of reunited twins having reached the same conclusion, the numbers are striking. Monozygotic twins raised apart are more similar in IQ (74%) than dizygotic (fraternal) twins raised together (60%) and much more than parent-children pairs (42%); half-siblings (31%); adoptive siblings (29%-34%); virtual twins, or similarly aged but unrelated children raised together (28%); adoptive parent-child pairs (19%) and cousins (15%). Nothing but genes can explain this hierarchy.

But as Drs. Bouchard and Segal have been at pains to point out from the start, this high heritability of intelligence mainly applies to nonpoor families. Raise a child hungry or diseased and environment does indeed affect IQ. Eric Turkheimer and others at the University of Virginia have shown that in the most disadvantaged families, heritability of IQ falls and the influence attributed to the shared family environment rises to 60%.

In other words, hygienic, well-fed life enables people to maximize their genetic potential so that the only variation left is innate. Intelligence becomes significantly more heritable when environmental hurdles to a child's development have been dismantled.

For more, see Is IQ in the Genes? Twins Give Us Two Answers, June 22, 2012 at WSJ.com.