Evidence has been accumulating that the serotonergic system may be involved in the processes underlying at least some types of aggressive behavior. Most of this research in humans has been correlational. It has sought to demonstrate relationships between a history of aggressive behavior or trait measures of hostility on the one hand, and, on the other, various markers of serotonergic function. The latter have included plasma concentration of the 5-HT precursor tryptophan, CSF concentration of its metabolite 5-HIAA, and endocrine responses to serotonergic agents (for review see Verhoeven and Tuinier 1997). Findings from these studies include higher levels of plasma tryptophan in men who had committed violent crimes than non-offender controls (Eriksson and Lidberg 1997); and in men with personality disorders, negative correlations between history of aggression and CSF 5-HIAA (Brown et al. 1979), and between the Assault subscale of the Buss-Durkee Hostility Inventory (BDHI) and the prolactin response to d-fenfluramine (Coccaro et al. 1996). However, in correlational studies, it is difficult to disentangle the possible influence of other variables. For example, individuals who score high on history of aggression often