Chronic stress can lead to depression and anxiety in humans. Scientists working with Herwig Baier, Director at the Max Planck Institute of Neurobiology in Martinsried, recently discovered a very similar link in fish. Normally, the stress hormone cortisol helps fish, as in humans, to regulate stress. Fish that lack the receptor for cortisol as a result of a genetic mutation exhibited a consistently high level of stress. They were unable to adapt to a new and unfamiliar situation. The fishes’ behaviour returned to normal when an antidepressant was added to the water. These findings demonstrate a direct causal link between chronic stress and behavioural changes which resemble depression. The findings could also open the door to an effective search for new drugs to treat psychiatric disorders.