From The New York Times:
For decades, researchers have ransacked the genetic pedigrees of people with mental illness, looking for common variations that combine to cause devastating conditions like schizophrenia and bipolar disorder. The search has stalled badly; while these disorders may involve genetic disruptions, no underlying patterns have surfaced — no single gene or genes that account for more than a tiny fraction of cases. So scientists are turning their focus to an emerging field: epigenetics, the study of how people’s experience and environment affect the function of their genes.
Genes are far more than protein machines, pumping out their product like a popcorn maker. Many carry what are, in effect, chemical attachments: compounds acting on the DNA molecule that regulate when, where or how much protein is made, without altering the recipe itself. Studies suggest that such add-on, or epigenetic, markers develop as an animal adapts to its environment, whether in the womb or out in the world — and the markers can profoundly affect behavior.