Whether people become depressed after stressful experiences is influenced by their genes, researchers say.
Some people become depressed after stressful events
A study has shown that the likelihood of becoming depressed is partly determined by which version of a specific gene a person has.
Someone with the least protective version of the gene is over twice as likely to suffer depression as a person with the most protective form , researchers found.
They say their findings explain why some people cope well with experiences such as divorce or unemployment, and others do not.
It could lead to the development of a genetic test for the gene, though the scientists caution discovering more genes related to depression would make such a test more accurate.
The finding could also mean patients with a genetic susceptibility to depression could be given psychotherapy or medications - if research proves they have a preventative effect.
The researchers say the combined effect of 'nature' and 'nurture' could also have implications for other conditions, such as heart disease, where a person's genes plus their lifestyle combine to determine their risk of becoming ill.
The study looked at a gene called 5-HTT, which helps to control the serotonin, a brain chemical which passes messages between brain cells and affects mood.
The gene comes in two versions, short and long. Everyone has two copies of the gene.
The researchers looked at a group of 847 people born between April 1972 and March 1973 in Dunedin, New Zealand.
The group has been followed since birth.
Researchers were able to look at which version of the gene they had, and what stressful events they had experienced.
They focussed on those who had suffered several stressful experiences in the five-years between the ages of 21 and 26.
It was found that those 265 people who had two short versions of the gene were more likely to become depressed - 43% of them had developed depression after stressful life events.
In contrast, only 17% of the 147 who had two long copies of the gene were affected by depression.
Terrie Moffitt, professor of social behaviour and development at the Institute of Psychiatry at King's College London, told BBC News Online: "The most exciting thing about this research is that we've found that the risk of depression is halved for people who have the long-long genes."
But she stressed: "We are not reporting a gene that causes a disease.
"Instead, we believe the gene helps influence whether people are resistant to the negative psychological effects of the unavoidable stresses of life."
She added: "Until now, we've only been able to treat depression after it has become so severe that it's brought them into contact with a doctor.
"It would be really wonderful if there was a way of preventing it."
Professor Moffitt said the research opened new avenues for genetics.
"Genetic research has concentrated on studying rare genetic defects that cause rare diseases.
"Scientists have been looking at one-to-one correlation."
But she said researchers should now look at the interaction between genes and the environment.
Marjorie Wallace, chief executive of mental health charity SANE, welcomed the study.
"What this study shows is the importance of finding who may be particularly susceptible to the damaging combination of genetic vulnerability to depression and stressful life events, so that depressive breakdown might eventually be prevented.
"We hope that through studies like this, more effective medication and other treatments targeted to individual response will be developed."
The research is published in the magazine Science.