The present research investigates how people use observed decision time to form expectations of others' behavior in social dilemmas. In four studies, participants received information about others' decision times (fast or slow) and were asked to estimate how much they contributed to a common pool. People believe fast decisions are more extreme than slow decisions; in other words, they assume that fast decisions are either extremely selfish or extremely cooperative. People also believe that fast deciders are less moral (Studies 1 and 2) and less conflicted (Study 2) than slow deciders. Beliefs about decision time depend on whether time can be attributed to self-paced reaction times or external time constraints. When decisions are made under external time constraints, time has inconsistent or heterogeneous effects on behavioral expectations (Study 2). Decision time also moderates the effects of other informational cues: Positive facial expressions and perceptions of trustworthiness have stronger effects on expectations when paired with fast decisions (Study 3). Finally, observed decision time also has behavioral consequences – people make more extreme decisions when interacting with a partner who decided quickly (Study 4). Observed decision time plays a crucial role in how expectations of others' behavior are formed.