This lack of power may have precluded the detection of qualitative or quantitative sex differences in the variance components for withdrawal and abuse/dependence. Hence, while the very low DOS twin pair correlations for both variables point to possible differences in the sources of variance between sexes, the sample was not adequately powered to detect sex-limitation in the source of genetic or shared environmental influences. By equating the same sex DZ twins with the OS twins we may have inflated the heritability estimates and underestimated the role of shared environment. Furthermore, for males but not for females, the same-sex twin pair correlations point to a substantial influence of C factors on individual differences in withdrawal, but our sample size did not enable us to differentiate between male and females ACE parameters, resulting in a lower C and larger A estimate for withdrawal than expected based on the same sex twin pair correlations.
