Properties of expectation biases: Optimism and overconfidence

This study examines the properties of expectation biases using 14 sets of panel surveys that required participants to forecast the NIKKEI 225 over three forecasting horizons: one-day, one-week, and one-month. Constructing proxies for optimism and overconfidence as the expectation biases, this study shows that participants, on average, had pessimistic beliefs for the one-day and optimistic beliefs for one-week and one-month horizons, while they had overconfident beliefs for all three horizons. It also shows that participants tended to become more optimistic and overconfident at longer horizons. Moreover, the degree of optimism or pessimism varied considerably across samples taken at different times, while overconfidence remained stable. Furthermore, this study finds a negative correlation between optimism and the return on the NIKKEI 225, demonstrating that participants became optimistic when the NIKKEI 225 decreased. A negative correlation would be expected if people formed expectations following a random walk; however, this study rejects this hypothesis.