Abstract We study how long-lived rational agents learn from repeatedly observing a private signal and each others’ actions. With normal signals, a group of any size learns more slowly than just four agents who directly observe each others’ private signals in each period. Similar results apply to general signal structures. We identify rational groupthink—in which agents ignore their private signals and choose the same action for long periods of time—as the cause of this failure of information aggregation.

We study contests in which contestants are homogeneous and have convex effort costs. Increasing contest competitiveness, by making prizes more unequal, scaling up the competition, or adding new contestants, always discourages effort. These results have significant implications: although often criticized as evidence of laxity or cronyism, muting competition (e.g., adopting softer grading curves or less high-powered promotion systems) can both reduce inequality and increase output. Holding promotion contests at the division level rather than the firm level can boost employees’ effort. Our results are also consistent with personnel policies that feature egalitarian pay systems and dismissal of worst-performing employees.

Stochastic dominance is a crucial tool for the analysis of choice under risk. It is typically analyzed as a property of two gambles that are taken in isolation. We study how additional independent sources of risk (e.g., uninsurable labor risk, house price risk) can affect the ordering of gambles. We show that, perhaps surprisingly, background risk can be strong enough to render lotteries that are ranked by their expectation ranked in terms of first-order stochastic dominance. We extend our results to second-order stochastic dominance and show how they lead to a novel and elementary axiomatization of mean-variance preferences.