Genes, Brains, and Human Potential The Science and Ideology of Intelligence

42 PRETEND GENES

At any rate, the 1918 paper became hugely infl uential and soon was ap-
plied by psychologists or behavioral ge ne ticists to human mental traits—
but only by adopting additional assumptions. As we shall see, today’s
researchers into the ge ne tics of intelligence are still neglecting “compli-
cations” and employing “unfortunate short- cuts.”

THE RISE OF BEHAVIORAL GE NE TICS

It was the determined Cyril Burt who brought Fisher’s “solution” into the
domain of human mental abilities. As Burt explained in a paper in 1956,
his aim was to reveal the true causes of variation in traits like human
intelligence. He defi ned intelligence as innate cognitive ability and as-
sumed it to be well mea sured by IQ scores. His prob lem, again, was that
of separating the ge ne tic from the environmental eff ects on individual
diff erences in IQ scores.
Controlled breeding experiments, especially for complex functions
like cognitive ability, cannot be achieved in humans. Apart from not
knowing the ge ne tic backgrounds of individuals, or what environmental
factors to control, they would be unethical. However, as noted by Fisher
(and even by Galton), the known ge ne tic resemblances among relatives,
especially twins, suggested a natu ral experiment. In par tic u lar, identical
twins will share all the genes that make a diff erence in a trait, because
they arise from a single egg— they are monozygotic (MZ); nonidenti-
cal twins share only half of them, on average, because they arise from two
diff er ent eggs— they are dizygotic (DZ). Likewise for resemblances be-
tween parents and off spring, sibling pairs, and so on. Cousins share only
a quarter of those genes that make a diff erence in a trait, whereas pairs of
individuals chosen randomly from the population would not share any.
Comparing resemblances among such relatives might, in eff ect, separate
the ge ne tic from the environmental eff ects. A heritability for mental po-
tential or intelligence could be calculated in that way. Or so Burt and his
followers have claimed.
Th e simplest approach, it was realized, would be to compare MZ twins
who had been reared apart. Th is would seem to control for the eff ects of
the environment on resemblances while allowing the ge ne tic diff erences

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