Pervasive developmental disorder in monozygotic twins.

Monozygotic twin brothers were diagnosed as having pervasive developmental disorder, not otherwise specified (DSM-III-R). They show different levels of cognitive and behavioral impairment, consistent with a greater number of suboptimal pre-, peri-, and postnatal factors present in one of the twins. This case study lends support to the hypothesis that genetic factors are of importance in the expression of a pervasive developmental disorder, but that degree of developmental delay, its severity, and certain clinical features may be determined by nongenetic suboptimal pre-, peri-, and postnatal events.