This study examined the effects of birth order on interpersonal trust and sex role orientation among firstborn and only children in late adolescence. The following hypotheses were posed: (1) only children will demonstrate higher degrees of interpersonal trust than firstborn children; (2) only children will show higher degrees of androgyny than firstborns; and (3) female only children will show higher degrees of androgyny than male only children. A demographics questionnaire, the Bem Sex Role Inventory, and the Rotter Interpersonal Trust Scale were administered to 128 undergraduate students. The hypothesis that female only children are more androgynous than male only children received significant statistical support. The hypothesis that only children are more androgynous than firstborn children was not supported. Additionally, female only children approached significance on being more androgynous than female firstborns. The hypothesis that only children are more trusting than firstborn children did not receive any statistical support. Birth order effects do appear to exist, but in conjunction with gender effects. Female only children were the only group found to vary significantly from other groups on sex role orientation. This group warrants further investigation, especially in terms of their stereotypic feminine behaviors. These women differed significantly because they had high femininity scores, while the other three groups did not. Gender research may need to focus more on the predictive power of stereotypic feminine behavior on personality. (LLL)