Explicit attention to recent theory and research on psychology and evolution can sometimes help criminologists generate productive hypotheses and avoid blind alleys. Homicide research illustrates this contention in that interpersonal conflicts are engendered by interactions among individuals whose psychological states have evolved through natural and sexual selection to make them effective competitors and effective nepotists (kin-benefactors). These considerations suggest numerous testable hypotheses about who is likely to kill, for example, and how the demography of homicide perpetration and victimization may vary among victim-killer relationships. An evolutionary psychological perspective is employed to criticize recent criminological discussions of sex differences and age patterns in criminal offending, theories that contrast rational choice with emotional or impulsive behavior, and the medicalization of antisocial behavior as a pathology. 175 references, 1 table, and 3 figures