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?
A field called epigenetics has finally begun to address some of these issues. Its practitioners study how tiny molecules stick to, or become unstuck from, two main targets in a cell's nucleus: the DNA in and around a gene and the histones—the proteins around which chromosomes spool. These tiny molecules are known as methyl and acetyl groups, and their presence or absence at target sites controls whether particular genes can generate proteins, the workhorses of most physiological processes.