Informal STEM education programs have become venues wherein girls can improve their sense of belonging and potential success (STEM identity) through interactions with role models and seeing how STEM fields are relevant to them. Despite decades of advocacy for single-sex programs' role in improving girls' STEM identity, few studies have found definitive results. To explore the role that a single-sex environment can have on adolescent girls' STEM identity development, this study compares participating girls' STEM identity from pre- to post-test using linear regression and hierarchical linear modeling to determine whether participants have a larger identity growth in an all-girls informal STEM education summer camp (STEM GIRLS) or a co-educational informal STEM education summer camp (STEM STARS). Results indicate that STEM Self-Efficacy and STEM Identity are positively correlated, however, the model is currently incomplete and could use more clarity to determine the role one plays on the other. Despite this, our study indicates the value in addressing self-efficacy by giving girls opportunities to struggle through challenges. This study also found that both camps were similarly beneficial in impacting STEM Identity and STEM Self-Efficacy, further supporting research that highlights the gendered aspect of the camp is less impactful than the practices used.