Impostor Phenomenon (IP) - a psychological experience marked by self-doubt and the fear of being exposed as a fraud - affecting 46% of 18-24-year-olds and 1 in 3 Americans, leads individuals to doubt their accomplishments, attributing success to luck (Cokley et al.,2015; Neureiter & Traut-Mattausch, 2016). In STEM (Science, Technology, Engineering, and Mathematics) fields, minority groups face representation and support challenges, aggravating IP. This study investigates the IP, specifically focusing on gender and ethnic disparities. Using survey data from 140 undergraduate STEM majors at a Southern California public university, the research analyzes how IP varies across gender and ethnic groups and identifies key factors contributing to these feelings. The study finds that women experience higher levels of IP than their male peers, highlighting IP's prevalence among women in STEM. Significant predictors of IP include "insecurity of skills, abilities, and knowledge" and "lack of representation in educational materials. "The results highlight the need for systemic changes to improve representation and support within STEM education and professional environments, aiming to reduce IP and enhance inclusivity.