Extraordinary advances in neurochemistry are both transforming our understanding of human nature and creating an urgent problem. Much is now known about the ways that neurotransmitters influence normal social behavior, mental illness, and deviance. What are these discoveries about the workings of the human brain? How can they best be integrated into our legal system?

These explosive issues are best understood by focusing on a single neurotransmitter like serotonin, which is associated with such diverse behaviors as dominance and leadership, seasonal depression, suicide, alcoholism,...