This mixed methods research study examined the effects of middle and high school mathematics experiences on students' choice of college major, particularly whether students decided to major in a STEM field. Social cognitive career theory was used to examine potential influences of mathematics self-efficacy and how those influences and mathematics self-efficacy levels affected students' career choices. The purpose of this study was to uncover middle and high school experiences that could be used to encourage more students to major in STEM fields due to the current shortage of students pursuing STEM majors in college. The modified Mathematics Self-Efficacy and Anxiety Questionnaire was administered to 433 college sophomores who responded by answering Likert-style and open-ended questions regarding their middle and high school mathematics self-efficacy and anxiety levels and their mathematics experiences. Follow-up interviews were conducted with eight participants, with half majoring in STEM and the other half majoring in non-STEM fields. The results from the data analysis showed that lower levels of mathematics anxiety, higher levels of mathematics courses completed in high school, positive teacher experiences, and multiple instances of exposure to STEM fields while in middle and high school increased the likelihood that students would choose a STEM major. In addition, lower levels of mathematics anxiety and being placed into higher-ability mathematics courses in middle and high school correlated with higher levels of mathematics self-efficacy. Finally, higher levels of mathematics self-efficacy in middle and high school led to increased instances of pursuing a STEM career. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]