Background. Twin studies have long been used to disentangle the role of genetic and environmental factors in the aetiology of psychiatric disorders. However, the validity of the twin method depends on the equal environment assumption – that monozygotic (MZ) and dizygotic (DZ) twins are equally correlated in their exposure to environmental factors of aetiological importance for the disorder under study.

Methods. Both members of 822 female–female twin pairs from a population-based registry previously assessed for a range of psychiatric and substance use disorders were asked 12 questions assessing the similarity of their environmental experiences in childhood and adolescence. We examined whether the similarity of environmental experiences predicted concordance for psychiatric and substance abuse disorders by both a ‘pair-wise’ and ‘individual’ method utilizing logistic regression. We also examined smoking initiation, where prior evidence suggested a role for adolescent social environment.

Results. Three factors were derived from these items: ‘Childhood treatment’, ‘Co-socialization’ and ‘Similitude’. Members of twin pairs agreed substantially in their recollections of these experiences. Compared with DZ twins, MZ twins reported comparable resemblance in their childhood treatment, but socialized together more frequently and reported that parents, teachers and friends more commonly emphasized their similarities. None of these three factors significantly predicted twin resemblance for major depression, generalized anxiety disorder, panic disorder, phobias, nicotine dependence or alcohol dependence. However, co-socialization significantly predicted twin resemblance for smoking initiation and perhaps for bulimia.

Conclusion. Differential environmental experiences of MZ and DZ twins in childhood and adolescence are unlikely to represent a substantial bias in twin studies of most major psychiatric and substance dependence disorders but may influence twin similarity for the initiation of substance use.