In this article, the authors propose and test an interactionist model of personality functioning. The model maintains that many traits function in a threshold-like manner, such that less situational strength is needed to evoke a trait-relevant response in people who are high on the trait than in those who are low on the trait. Because of these different sensitivities, people who are high on a trait are more reactive to moderate Provocation than are those who are low on a trait, but the opposite is true when strong provocation is compared to moderate provocation. Three studies are reported showing how the model can be used to understand the nature of aggression.