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Matt Ridley is the author of provocative books on evolution, genetics and society. His books have sold over a million copies, been translated into thirty languages, and have won several awards.

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High IQ heritability would testify to environmental equality

How twin studies silenced their critics

My latest Mind and Matter column for the Wall Street Journal:

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

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

By 1990, he, Dr. Segal and other colleagues were ready to publish their results in Science magazine. By then they had measured the IQ of 48 pairs of monozygotic, or identical, twins, raised apart (MZA) and 40 pairs of such twins raised together (MZT). The MZA twins were 69% similar in IQ, compared with 88% for MZT twins, both far greater resemblances than for any other pairs of individuals, even siblings. Other variables than genetics, such as material possessions in the home, had little influence, nor was the degree of social contact between the twins in each pair associated with their similarity in IQ.

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

Yet, as Dr. Segal records, the Mistra scientists were meticulous in addressing these issues and more. Too politically incorrect to be funded by most government agencies, the study relied on grants from sources like the Pioneer Fund, once at the forefront of the eugenics movement. What counted, Dr. Bouchard argued, were the results of the research, not the source of the twins' travel expenses.

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%.

(Image from Discover magazine)

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.

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