The validity of the twin method depends on the equal environment assumption (EEA)--that monozygotic (MZ) and dizygotic (DZ) twins are equally correlated in their exposure to environmental factors of aetiological importance for the trait under study. Parents may treat MZ twins more similarly than DZ twins thereby potentially violating the EEA. We tested this hypothesis for four common psychiatric disorders (major depression, generalized anxiety disorder, phobia, and alcoholism) in a population-based sample of female-female twin pairs where analyses indicate sufficient statistical power meaningfully to test the EEA. Mother's and father's beliefs about their twins' zygosity disagreed with assigned zygosity in approximately 20% of cases, often because of what they were told about their twins' zygosity at their birth. By structural equation model-fitting, we found no evidence that mother's or father's perceived zygosity influenced twin resemblance for any of the disorders. Compared to parents of DZ twins, parents of MZ twins were more likely to report that, in rearing their twins, they emphasized their similarities more than their differences. However, by model-fitting, mothers' and fathers' approach to raising twins had no significant influence on twin resemblance for the four examined psychiatric disorders. These results suggest that the differential treatment of MZ and DZ twins by their parents is unlikely to represent a significant bias in twin studies of these major psychiatric disorders.