A wealth of literature documents how the common failure to think about the self-interests of others contributes to suboptimal outcomes. Yet sometimes, an excess of cynicism appears to lead us to over-think the actions of others and make negative attributions about their motivations without sufficient cause. In the process, we may miss opportunities that greater trust might capture. We review the research on when people expect too little or too much self-interest in the intentions of others, as contrasted with rational behavior. We also discuss the antecedents and consequences of these naïve and cynical errors, as well as some potential strategies to buffer against their effects and achieve better outcomes in competitive contexts.