College STEM (science, technology, engineering, and math) major matriculation and attrition decisions may be influenced by experiences occurring much earlier in students’ lives than their contemporary college experiences. This research was guided by social cognitive career theory (SCCT; Lent, Brown, & Hackett, 1994), which describes how students develop career interests, make choices as a result of these interests, interpret their successes and failures, and ultimately make career decisions. Although many studies of STEM interest, self-efficacy, and academic behavior have been conducted on high school and college students, childhood experiences have been less studied, despite the emphasis that SCCT places on early life experiences. Using a biodata measure that incorporated background influences and learning experiences factors previously developed from semi-structured interviews with STEM students, I conducted a two-part study. Study 1 entailed an exploratory factor analysis of the biodata measure and other constructs on underprepared STEM students at Rice (N = 154). After refining the biodata measure, a confirmatory factor analysis and structural equation model were conducted on a separate sample of Rice freshmen in Study 2 (N = 175). Findings from both studies provide support for a seven-factor model and a reasonable fit with a modified structural equation model based on SCCT. These seven factors comprise three background influence factors (general parent support, STEM parent support, and teacher support) and four formative experience factors (math extracurricular activities and competitions, science extracurricular activities and competitions, proactive behavior, and STEM knowledge). All were significant in the full structural equation model except teacher support. Further research will be necessary to refine and validate this measure on other samples and develop greater insight into the relationships between this new measure and STEM self-efficacy, STEM interest, career outcome expectations, initial college major, and intended college major. Ultimately, these studies may guide future research that uses a theoretical approach to explore the STEM experiences of students at all education levels, as well as define factors that schools and program administrators may find valuable to explore in future STEM interventions.