Prior experience informs ensemble encoding.

People quickly form summary representations that capture the statistical structure in a set of simultaneously-presented objects. We present evidence that such ensemble encoding is informed not only by the presented set of objects, but also by a meta-ensemble, or prototype, that captures the structure of previously viewed stimuli. Participants viewed four objects (shaded squares in Experiment 1; emotional expressions in Experiment 2) and estimated their average by adjusting a response object. Estimates were biased toward the central value of previous stimuli, consistent with Bayesian models of how people combine hierarchical sources of information. The results suggest that an inductively learned prototype may serve as a source of prior information to adjust ensemble estimates. To the extent that real environments present statistical structure in a given moment as well as consistently over time, ensemble encoding in real-world situations ought to take advantage of both kinds of regularity.