Women are underrepresented in most scientific disciplines, with achievement gaps expanding throughout the career trajectory. Gender differences in perceptions of scientific competence are evident at an early age. We examined the extent to which college-level instructor gender affects the confidence of males and females in their scientific abilities. The presence of a woman in either a primary or secondary instructional role (or both) improves female students' levels of scientific confidence. However, the absence of women in an authoritative role impedes significant gains in confidence levels for female students. We discuss this work in light of recent studies of women in the sciences, innate versus learned abilities, the role of confidence in career choices, and implications for enhancing diversity in the sciences overall. (Contains 2 tables.)