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.