In this excellent survey article, Siddhartha Mukherjee looks at the latest developments in depression research. SSRI drugs that seek to restore “chemical imbalances”, he suggests, seem to work only for severely depressed patients, particularly those with a family history of depression. In any case, the interaction of SSRI drugs with the brain appears to be far more complex than previously thought. Mukherjee concludes:

John Gribbin, a historian of science, once wrote that seminal scientific discoveries are inevitably preceded by technological inventions. The telescope, which situated the earth and the planets firmly in orbit around the sun, instigated a new direction in thinking for astronomy and physics. The microscope, taking optics in a different direction, ultimately resulted in the discovery of the cell.
We possess far fewer devices to look into the unknown cosmos of mood and emotion. We can only mix chemicals and spark electrical circuits and hope, indirectly, to understand the brain’s structure and function through their effects. In time, the insights generated by these new theories of depression will most likely lead to new antidepressants: chemicals that directly initiate nerve growth in the hippocampus or stimulate the subcallosal cingulate. These drugs may make Prozac and Paxil obsolete — but any new treatment will owe a deep intellectual debt to our thinking about serotonin in the brain. Our current antidepressants are thus best conceived not as medical breakthroughs but as technological breakthroughs. They are chemical tools that have allowed us early glimpses into our brains and into the biology of one of the most mysterious diseases known to humans.