Women make less than men in some science, technology, engineering, and math (STEM) fields. While explanations for this gender pay gap vary, they have tended to focus on differences that arise for women and men after they have worked for a period of time. In this study we argue that the gender pay gap begins when women and men with earned degrees enter the workforce. Further, we contend the gender pay gap may arise due to cultural beliefs about the appropriateness of women and men for STEM professions that shape individuals’ self-beliefs in the form of self-efficacy. Using a three-wave NSF-funded longitudinal survey of 559 engineering and computer science students that graduated from over two dozen institutions in the United States between 2015 and 2017, we find women earn less than men, net of human capital factors like engineering degree and grade point average, and that the influence of gender on starting salaries is associated with self-efficacy. We find no support for a competing hypothesis that the importance placed on pay explains the pay gap; there is no gender difference in reported importance placed on pay. We also find no support for the idea that women earn less because they place more importance on workplace culture; women do value workplace culture more, but those who hold such values earn more rather than less. Overall, the results suggest that addressing cultural beliefs as manifested in self-beliefs — that is, the confidence gap — commands attention to reduce the gender pay gap.