Third, etiologic heterogeneity is likely in psychiatric disorders, so our heritability estimates inevitability reflect an averaging across potentially different etiologies. For example, as noted above, certain clinical aspects of major depressive disorder appear associated with higher heritability, including recurrence and early age-at-onset (Levinson et al., 2003, Sullivan et al., 2000). However, it is not clear that problems of etiologic heterogeneity would impact heritability estimates differentially for disease-like versus behavioral conditions.
