We administer a theory-driven, lab-in-the-field experiment to study the disposition effect among financial professionals. Our novel design identifies, at the individual participant level, key behavioral drivers of the disposition effect: reference-dependent risk attitudes (“tastes”), second-order uncertainty attitudes (including “ambiguity”), and subjective likelihood assessments (“beliefs”). Among the 237 professionals in our sample, 34% exhibited the disposition effect, which seems to be primarily driven by non-Bayesian beliefs. Our experimental results suggest that, when faced with new information about their asset’s performance, financial professionals failed to update their beliefs sufficiently leading them to sell the asset that gained (lost) value more (less) readily.

Combining experimental data sets from seven individual studies, including 255 asset markets with 2,031 participants, and 36,326 short-term price forecasts, we analyze the role of heterogeneity of beliefs in the organization of trading behavior by reproducing and reconsidering earlier experimental findings. Our results confirm prior evidence that price expectations affect trading behavior. However, heterogeneity in beliefs does not seem to drive overpricing and asset market bubbles, as suggested by earlier studies, and we find no indication of short-term beliefs being better determinants of trading behavior than longer-term beliefs.