Biological research on aggression is increasingly consulted for possible answers to the social problems of crime and violence. This paper reviews some contrasting approaches to the biological understanding of behavior—behavioral genetic, social-environmental, physiological, developmental—as a prelude to arguing that approaches to aggression are beset by vagueness and imprecision in their definitions and disunity in their measurement strategies. This vagueness and disunity undermines attempts to compare and evaluate the different approaches empirically. Nevertheless, the definitions reveal commitments to particular metaphysical views concerning (...) behavior. Alternative understandings of aggression suggest even different research approaches than those most commonly found in the literature. (shrink)