ABSTRACT Although initially believed to contain orthogonal dimensions, the Big Five personality taxonomy appears to have a replicable higher-order structure, with the metatrait of Plasticity reflecting the shared variance between Extraversion and Openness/Intellect, and the metatrait of Stability reflecting the shared variance among Neuroticism, Agreeableness, and Conscientiousness. These higher order traits have been theorized to relate to individual differences in the functioning of the dopamine and serotonin systems, respectively. As dopamine is associated with exploration and incentive-related action, and serotonin with satiety and constraint, this neuropharmacological trait theory has behavioral implications, which we tested in 307 adults by examining the association of a large number of behavioral acts with multi-informant reports of the metatraits. The frequencies of acts were consistently positively correlated with Plasticity and negatively correlated with Stability. At the broadest level of description, variation in human personality appears to reflect engagement and restraint of behavior.