Aggression is often depicted as the automatic result of hostile situations (e.g., an insult or angry stare). These situations involuntarily activate hostile thoughts in memory, which in turn can elicit thoughtless or uncontrolled aggression. In contrast, we have found that individuals high in the personality trait of agreeableness use effortful control (i.e., the ability to inhibit one response in favor of another) to restrain aggressive impulses. These individuals activate pro-social thoughts when faced with hostility and thus are less likely to act aggressively. In this project, we will build upon this strategy in order to reduce aggression and enhance pro-social actions. We will examine three related but independent aims in six studies by having experimental participants engage in effortful thought training that focuses on the activation of altruistic thoughts (e.g., forgive) in situations that normally create hostility (e.g., provocation). We expect our paradigm will increase pro-social thoughts and actions as well as decrease aggressive thoughts and actions when trained participants are exposed to subsequent hostile situations (Aim 1). Furthermore, we hypothesize that our training paradigm will reduce aggression and increase pro-social actions over the long term (Aim 2). Finally, we expect that our training paradigm can modify the behavior of habitually aggressive individuals (Aim 3). Our aims will be examined in lab contexts and in participants' daily lives. The project is expected to produce significant outputs (e.g., four journal articles, three conference presentations, and press releases), outcomes (e.g., a greater awareness from academics, clinicians, and the general public about the conscious processes that can decrease aggression and increase pro-social behavior), and enduring impacts (e.g., frequent citations, the discussion of our work in psychology textbooks, theory modifications, and potential intervention strategies for aggressive individuals).