Is IQ in the Genes? Twins Give Us Two Answers

days the heritability of intelligence is not doubt: Bright adults are more to have bright kids. The debate was always this calm. In 1970s, suggesting that IQ could be inherited at was a heresy in academia, punishable by the equivalent of burning at the .

More any other evidence, it was the study of twins brought about this . "Born Together—Reared Apart," a new book by Nancy L. Segal about the Minnesota study Twins Reared Apart (Mistra), narrates the history of the shift. In 1979, Thomas Bouchard of the University of Minnesota came a newspaper report about a of Ohio twins, separated birth, who had been reunited and proved possess uncannily similar habits. Dr. Bouchard began to collect case histories of twins raised and to invite them to Minneapolis for study.

By 1990, he, Dr. Segal and other colleagues were ready to publish their results Science magazine. By then they measured the IQ of 48 pairs of monozygotic, or identical, twins, raised apart (MZA) and 40 pairs of twins raised together (MZT). The MZA twins were 69% similar IQ, compared with 88% for MZT twins, both greater resemblances than for any other pairs of individuals, siblings. Other variables than genetics, as material possessions in the home, had little influence, was the degree of social contact between the twins in each pair associated their similarity in IQ.

The paper attracted plenty of the usual criticism, and years there was a quiet whispering campaign to discredit Mistra study on the that it relied on anecdotes, underestimated contact twins, ignored a tendency for reunited twins to exaggerate their similarities or assumed little similarity among the families which the twins were adopted.

Yet, as Dr. Segal records, the Mistra scientists were meticulous addressing these issues and more. politically incorrect to be funded by most government agencies, the study relied grants from sources like the Pioneer Fund, once the forefront of the eugenics movement. What counted, Dr. Bouchard argued, were the results of the research, the source of the twins' travel expenses.

Today, a third of a century the study began and with other studies of reunited twins reached the same conclusion, the numbers are striking. Monozygotic twins raised are more similar in IQ (74%) than dizygotic (fraternal) twins raised together (60%) and much 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 genes can explain this hierarchy.

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

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

IQ heritability in the middle class proves uncannily similar to the estimate by the very first study of twins raised apart, the British psychologist Cyril Burt between 1943 and 1966. He found that the similarity IQ between MZA twins was 77.1%. The fact that this number did not change as his sample grew an improbably large size to charges by the Princeton psychologist Leon Kamin in the 1970s that Dr. Burt (then dead) had committed fraud by making most of his results. To day, experts disagree on many of his data Dr. Burt invented, but his conclusion was not wrong much.


Adapted from: The Wall Street Journal, June 22, 2012.