This study explores the dynamics of imitation behavior in angel investments. Drawing from signaling theory and the social proof principle, we investigate the impact of external and internal social proof on investment decisions. Our findings, based on an analysis of 77,312 decisions by 469 angel investors and an experiment involving 1,092 investor assessments, show that inexperienced investors largely depend on easily accessible but less useful external social proof, whereas experienced investors largely rely on more difficult-to-obtain but also more valuable internal social proof. Moreover, we show that reliance on internal social proof correlates with superior investment returns, but external social proof only produces irrational herding behaviors. These results contribute to the understanding of early-stage investment networks, the dynamics of social information in uncertainty, and signaling theory in the context of angel investing.