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.