Women are more educated than men in the United States, but continue to lag behind in the traditionally masculine pursuits of STEM. This chapter focuses on individual-level as well as societal-level explanations for these differences. Individual factors include sex differences in the association of achievement with negative consequences, self-confidence, self-esteem, response to evaluative feedback, stereotype threat, and attributions for performance. In each of these areas, women are at a disadvantage compared to men when the domain is masculine. Social factors are rooted in the expectancy/value model of achievement which states that achievement pursuits are a function of one’s expectancy of success and the value that one attaches to the domain. Although women have lower expectancies than men in regard to STEM performance, it is the value component that predicts occupational pursuits. Gender socialization by parents, teachers, and peers contributes to expectations and values. Parents and teachers hold gender-related stereotypes about achievement that influence the opportunities they provide to girls and boys and how they respond to their achievements, both of which are linked to girls’ and boys’ self-concepts and ultimately achievement pursuits.