This study investigates the impact of an informal science outreach programme built around theories of identity formation and self-efficacy on middle school girls' science affinities. A lottery-based, randomised control trial was used to identify programme effects on four science affinity outcomes: science interests, efficacy with science, science attitudes, and science identity. A multivariate analysis of variance demonstrated that programme participants scored higher than their control group peers on weighted composite of post-programme affinity indicators. These results suggest that informal science education may offer a venue through which to support the formation of science identities and efficacy in girls. Implications for including psychosocial support elements into science classroom pedagogy and science education standards are discussed.