Hostile sexism expresses derogation of women's competence and emphasizes that women will exploit men's relational dependence. Men who endorse hostile sexism perceive their female partners more negatively, but do these negative perceptions stem from motives for dominance or insecurities about dependence? We tested both perspectives by assessing bias in perceptions of partners' behaviors that challenge dominance (criticism, instruction, taking over) versus affirm partners are dependable (love, care, availability). Both members of 100 heterosexual couples reported how much they received and enacted these behaviors in (a) a lab-based discussion and (b) six monthly retrospective reports about an ongoing important goal. In both support contexts, men's hostile sexism was associated with underestimating dependability-relevant support, particularly when partners reported providing low support. This pattern did not emerge for dominance-relevant behaviors. These results indicate that men's hostile sexism involves insecurities about dependence, producing perceptions that female partners are less dependable than they actually are.