When a loved one dies, families usually gather together to grieve. While some members cope with the loss, others sink into depression.

Who will experience the telling signs of this mental illness after a stressful life event depends in part on genetic makeup, according to a new study.

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Researchers from the University of Wisconsin in Madison, King's College London, and the the University of Otago in New Zealand focused on two forms of a gene called 5-HTT in about 850 young adults in New Zealand.

They found that among people who suffered multiple stressful life events over five years, 43 percent with a "short" version of a gene developed depression, compared to only 17 percent with the "longer" version of the gene.

The study was partially funded by the National Institute of Mental Health and is published in Friday's issue of the journal Science.

According to the researchers, the long form of 5-HTT seems better able to control the brain chemical serotonin. Serotonin previously has been found to have an effect on mental state. Antidepressant drugs, such as Prozac, work by increasing serotonin levels.

Those with the "short," or stress-sensitive, version of the serotonin transporter gene were also at higher risk for depression if they had been abused as children. But no matter how many stressful life events they endured, people with the "long," or protective, version experienced no more depression than people who were totally spared from stressful life events.

People carry two copies of the gene -- either two copies of the short variant, two copies of the long variant, or one copy of each. While both genes are common among humans, nearly two-thirds of the population carries at least one copy of the short gene, said Terrie Moffitt, a researcher from the University of Wisconsin, Madison.

Although the short gene appears to predict who will become depressed following life stress, it's not yet ready for use as a diagnostic test, said Moffitt said.

If confirmed, such a test may eventually be used in conjunction with other, yet-to-be-discovered genes that predispose for depression in a "gene array" test that could help to identify people who might benefit from intervention. And discovering how the long variant exerts its protective effect may also lead to new treatments, Moffitt said.

The World Health Organization estimates that 121 million people worldwide currently suffer from depression, which causes periods of constant sadness, disinterest in activities once enjoyed, decreases in energy, difficulty thinking, feelings of worthlessness, recurrent thoughts of death, and changes in eating and sleeping patterns.

"The World Health Organization ranks depression as the world's fourth leading cause of disease burden -- years that humans live with disability," Moffitt said. "If current trends continue, by the year 2020, depression will be the first cause of disease burden worldwide and, in the developed world, will be second only to heart disease."

Science