PRETEND GENES 59mentioned above, that “ there is very little knowledge of the trait- relevant
environments that infl uence IQ.” In a review in 1995, Robert Sternberg
concluded that “[psychologists] do not have a very good understanding
of the factors that aff ect IQ. Neither does anyone else.”^27 Th at situation
has not changed.
In other words, such indirect studies are hopelessly inadequate as tests
of the EEA, because they evade the fundamental requirements of a scien-
tifi c study. Only the following will do. First, identify and characterize the
phenotype, so that every one knows and agrees what we are talking about.
Second, prove that the mea sure used is actually mea sur ing it (i.e., has
validity in the same way that a blood pressure or breathalyzer test has).
Th ird, describe in detail the specifi c environments that demonstrably— not
merely supposedly— create individual diff erences in the trait in question;
that is, the environments that are trait relevant. Fourth, assess for inter-
actions with each other and with diff er ent genotypes (a particularly
demanding stipulation). Fift h, and most impor tant, comprehensively
assess whether average levels of these trait- relevant environments are in-
deed equal across twin groups.
Until those conditions have been fulfi lled, we have no right to be pro-
mulgating defi nitive conclusions about the EEA and the validity of the
classical twin method. Rough and ready approximations simply will not
do, especially when there is abundant de facto evidence that, in general,
environments are much more similar for MZ twins than for DZ twins;
and, of course, when the conclusions may have profound consequences
for countless children.
Making DZ Twins Diff er ent
Th ere are a number of other ways in which simply comparing MZ/DZ
twin correlations is not as simple as it seems. One of these is the way that
parents may try to reinforce resemblances in MZ twins but do the oppo-
site with DZ twins— that is, they amplify diff erences. (Ironically, they
may do this because of what they have been told by behavioral ge ne ticists
about the dominance of ge ne tics). In his 1992 study of twins, Arthur Bry-
ant pointed out that “it is inevitable that parents should constantly compare
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