PSYCHOLOGISTS, psychiatrists and neuroscientists have jousted for years over how much of our behavior is driven by our genes versus the environments in which we grow up and live. Arguments have persisted because there has been little hard evidence to answer basic questions: How exactly do genes and environment interact to determine whether someone will become depressed, say, or schizophrenic? And can environmental interventions such as drugs or psychotherapy really alleviate disorders that are largely determined by genes?