Neuroscience. New brain cells prompt new theory of depression.

No one knows what causes depression, although many neuroscientists blame an imbalance of brain chemicals, so-called neurotransmitters, especially those that affect the brain's pleasure responses. Now a few neuroscientists are converging on a radical, but complementary, theory: that depression may be caused by a lack of new cell growth in the brain. The recent discovery that the brain keeps producing neurons into adulthood has given this highly speculative theory at least one leg to stand on.