This study examined the effects of group members' relative task ability on the relationship between group sex composition and member task motivation. Male and female subjects performed a simple motor task with an opposite-sex partner, a same-sex partner, or no partner. Partners had higher, equal, or lower task ability than subjects. When the partner was more able than the subject, both sexes did more work with an opposite-sex partner than when working individually, whereas subjects with a same-sex partner did no more work than individual controls. Group sex composition did not affect member motivation when the partner was equally or less able. These findings disconfirmed several models of the sex composition-motivation relationship. They supported an esteem maintenance model which holds that both sexes place greater value on demonstrating competence to opposite-sex partners than to same-sex partners.