BEIJING — A new study in mice published in U.S. lately suggests that chronic stress may be a cause – not an effect of – depression. After years of circumstantial evidence, the new findings relating to the human stress hormone cortisol are being seen as the ‘smoking gun’ of what causes mood disorders for some people. Scientists already knew that many people with depression have high levels of cortisol but it was not clear whether this was a cause or an effect of the problem. The new study conducted by a research team from Harvard Medical School suggests that long-term exposure to cortisol may actually contribute to depression. The study involved exposing mice to their equivalent of the stress hormone, corticosterone, for short (24 hours) and long (17 to 18 days) periods. The mice given corticosterone for more than two weeks showed less willingness to explore a new environment and dulled reactions to startling stimuli, both symptoms of depression. The findings, published in the April issue of the journal Behavioral Neuroscience, could help improve treatments for depression, the researchers say.