Parents of 702 twin pairs, ages 4 through 12, completed a sex-role behavioral preference questionnaire for each cotwin. Data were analyzed to determine the effects of gender, zygosity, and age on behavioral similarities and differences between cotwins. Among same-sex cotwins, male MZ pairs were reported to behave the most similarly. Girl-boy pairs were the most dissimilar. Sex and zygosity contributed significantly to co-twin behavioral differences, with female pairs varying more on sex-typed behaviors than male pairs and DZ pairs varying more than MZ pairs. Age of twins was not a major source of differences within twin pairs.
