The frustration-aggression hypothesis was developed by Dollard et al. (1939) and is based on the psychodynamic explanation of catharsis. Freud believed that the drive for aggression was innate, like the drive for food. He believed that the only way to reduce aggression is to engage in an activity which released it. Dollard et al. (1939) claim that when humans experience frustration, this leads to aggression; the aggression is a cathartic release of the frustration. Furthermore, Dollard explains that if an individual is prevented from achieving a goal by some external factor, then this will lead to frustration, which will always lead to aggression.