We study the dynamics of the selection problem in economic experiments. We show that adding dynamics significantly complicates the effect of the selection problem on external validity and can explain some contradictory results in the literature. We model the dynamics of the selection problem using a network model of diffusion in which agents’ participation is driven by the two channels: the direct channel of recruitment and the indirect channel of agent interaction. Using rich recruitment data from a large public university, we find that the patterns of participation and biases are consistent with the model. We find evidence of both short- and long-run selection biases between student types. Our empirical findings suggest that network effects play an important role in shaping the dynamics of the selection problem. We discuss the implications of our results for experimental methodology, design of experiments, and recruitment procedures.