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

60 PRETEND GENES

and contrast... and hence look out for diff erences (which) oft en leads
to the exaggeration of character traits that in fact lie well within the nor-
mal range... parents and friends soon learn these ste reo types and, per-
haps unconsciously, respond accordingly. More serious, however, is the
fact that children tend to live up to parental expectations of being the
one who is ‘good’ or ‘naughty,’ ‘quiet’ or ‘noisy.’ ”^28
Related to this is the way that many DZ twins actually strive to make
themselves diff er ent from one another. Th is is likely to be an active, not
merely a passive, pro cess, in which twins consciously shape diff er ent
identities. It has been shown that DZs form individual identities earlier
than MZs; and that there is less cooperation among DZs than MZs. Th e
development of polarized identities could, of course, shift motivations
away from similar learning in literacy, numeracy, academic, and other
test- related domains.^29
Th ese social and cognitive dynamics are rarely considered by twin re-
searchers and could well explain some of the puzzling correlations that
crop up in twin research. Such diff erence- creating environmental eff ects,
greater for DZs than for MZs, would be expected to increase with age.
Accordingly, the average correlation diff erences between MZ and DZ
pairs increases with age. Unfortunately, twin researchers keep reporting
this increasing gap as evidence that the ge ne tic eff ects— the heritability—
increases with age.
Th is is rather typical of the tendentious interpretations of murky data.
Another example is the way that, instead of accepting that the unequal
environments are fatal to the classical twin studies, behavioral ge ne ticists
instead argue that it is the more similar genes of MZ twins that produce
(or evoke) the more similar environments. Th ere is, of course, no direct
evidence for that. But it is not precision science, and all conclusions from
twin studies are best taken with a large pinch of salt.

STUDIES OF ADOPTED CHILDREN

Th ere is another way in which behavioral ge ne ticists have tried to show
that variation in human potential is largely due to diff erences in genes. Th is
is to fi nd adopted children and estimate the degree to which their IQs

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