Since Galton's time, critics of the twin method have rejected the evidence of genetic differences in human behavior, because the twin method assumes that identical and fraternal pairs have equally similar environments. Twins whose genetic similarity is misperceived by themselves and others provide a critical test of the adequacy of this assumption. The relative effects of perceived and actual genetic similarity on cotwin differences in cognitive, personality, and physical development were assessed in a sample of young, adolescent twins whose genetic similarity was often misperceived. Twins' responses to questions about their own and others' judgments about their zygosity and physical similarity, and the ratings of similarity by eight judges, were used to estimate the perceived similarity of the twins. Actual zygosity was established by matching cotwins on 12 or more blood group loci. Perceived zygosity and perceived similarity by self and others were found to be insignificant biases in the twin study method.