This study provides new insights on how investors form beliefs about future asset prices and how they use these beliefs for their trading decisions. Compared to the objective Bayesian benchmark, investors become overly optimistic when they face a paper loss. In addition, selling decisions are less sensitive to beliefs than purchase decisions. This difference is driven by selling behavior in the presence of paper losses. Our insights stem from a laboratory experiment in which participants are price-takers and trade a stock governed by a persistent two-state Markov chain. At each point in time, we elicit incentivized beliefs about the probability that the stock price will increase in the next period.