In this article, we describe a study in which a validated motivational survey was used to investigate the effect of working in mixed or same-gender groups on physics self-efficacy and self-reported peer influence on self-efficacy in a calculus-based introductory physics course in which women are severely underrepresented both in our sample and in the United States overall. Likely due to societal stereotypes and biases, we found that men tended to have higher physics self-efficacy and reported higher peer influence on self-efficacy than women both before and at the end of the physics course. Additionally, all students except those who worked in same-gender groups had a decrease in their average physics self-efficacy from the beginning to the end of the semester. Our findings suggest that instructors should implement classroom policies that encourage equitable and inclusive group work so that all students can thrive.