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A *common effects G E model* can also be fitted to
covariance matrices computed conditionally on environmental exposure
by simply fixing the environment-specific effects of the general model
to zero, and comparing the two using a difference test.
The information from pairs discordant for environmental exposure
allows for this comparison.
A critical sub-model of the common effects G E model
is one which tests the hypothesis that exposure group heterogeneity is
*solely due* *to
heteroscedasticity*, or *group
differences* *in random environmental variance*, rather than
*group differences in genetic variance*. To fit this model, the
genetic parameters are simply equated across groups, while allowing
the random environmental effects to take on different values. If this
model does not fit worse than the full common effects model, then
there is evidence for heteroscedasticity.
A second sub-model of the common effects G E
interaction model is one which constrains the *environmental*
parameters to be equal across exposure groups, while allowing the
genetic variance components to differ. If this model is not
significantly worse than the full common effects model, then there is
evidence to suggest that the environmental interaction only involves a
differential expression of genetic, but not environmental, influences.

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** Index**
Jeff Lessem
2002-03-21