For both the externalizing and internalizing spectra, we could not constrain the parameters of our multivariate twin model to equality across the sexes. In addition to the reliably observed large prevalence differences in the sexes for these two broad classes of condition (internalizing disorders are consistently more common in females than males and the reverse for externalizing disorders) (Wender, 1971; Kessler et al. 1994, 2005), the nature of the disorders and traits that optimally index the genetic and environmental liabilities to these spectra also differ in the sexes. These results are at odds with prior analyses of internalizing and externalizing disorder in our twins, where we found that the same parameters could be applied to both sexes (Kendler et al. 2003). However, that study examined only seven disorders across both spectra and so had much less power to detect sex differences than the present investigation.
