PRETEND GENES 57Equal Environments Assumption
Th e fi rst of these fl aws is fatal and insurmountable. Simply taking the dif-
ference between MZ and DZ correlations as an index of ge ne tic eff ects
makes a big assumption. Th is is that the environments experienced by
MZ pairs are no more similar during development than for DZ pairs. Th is
is called the equal environments assumption (EEA). It is unlikely to be
valid for a number of reasons.
For one thing, even the environment in the womb before birth is likely
to be more similar for MZ twins than for DZ twins. Most MZ twins share
a placenta, whereas all DZs have one each. One indication of this derives
from an examination of the epige ne tic markers in diff er ent kinds of
pairs. Th ese are chemical signals from the mother imprinted on specifi c
genes in embryos before birth. Th ey refl ect environmental experiences
of the mother or the early fertilized egg and strongly infl uence the way
that the genes are used in subsequent development (see chapter 4). Such
signals are found to be much more variable for DZ twins compared with
MZs.^21 Th ese environmental eff ects could reduce the DZ correlations in
comparison with the MZ correlations. But they will be mistakenly read
in the blind statistical model as “ge ne tic.”
More obviously, environmental aspects of the home and other experi-
ences are found to be markedly more similar for MZ twins than for DZ
twins. For example, parents treat MZ twins more similarly, and the twins
are more likely to dress alike, share bedrooms, friends, activities, and so on.
In a review in 2000, David Evans and Nicholas Martin, themselves leading
behavioral ge ne ticists, reported that “ there is overwhelming evidence that
MZ twins are treated more similarly than their DZ counter parts.”^22
In another review, Jay Joseph cites questionnaire studies revealing very
large diff erences between MZ and DZ pairs in experiences such as iden-
tity confusion (91 percent versus 10 percent), being brought up as a unit
(72 percent versus 19 percent), being inseparable as children (73 percent
versus 19 percent), and having an extremely strong level of closeness
(65 percent versus 19 percent). It is also known that parents hold more
similar expectations for their MZ than DZ twins.^23
In other words, this is prima facie evidence that it is environmental
similarities that, at least in part, make MZ twin pairs more alike in
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