Australian researchers suggest that the answer to the question may lie in one's genetic make up. They said that some people are genetically predisposed to develop depression when confronted with stressful life events such as a marriage breakup. The researchers found the influence of adverse life events on the onset of depression was significantly greater for those who inherited a short version of the serotonin transporter gene, known as 5-HTTLPR, from both parents. The individual is more than twice as likely to become depressed as others, given similar circumstances.
Mental health experts conducted the study on 127 graduates from a Sydney teachers' college for the past 25 years. 42 % reported at least one episode of major depression in that time. The researchers compared their DNA profiles with their lifetime history of major stress to find that a particular gene, 5-HTTLPR interacted with the environment to bring on depression. They reported their findings in the British Journal of Psychiatry.
Geneticist Peter Schofield said that those with the genetic predisposition had an 80 % chance of becoming depressed if they had three or more negative events in five years. On the other hand others had only about 30 % chance of developing the mental illness given the similar circumstances.
Lead researcher Kay Wilhelm, of Sydney's St Vincent's Hospital and the University of New South Wales (NSW) said that the current research proved the 2003 New Zealand study which looked at a larger sample of more than 800 people between ages 21 and 26.
The Australian researchers found that about 21 % of their sample, now aged 48, had inherited the short version of the serotonin transporter gene from both parents. Prof Wilhelm, a key researcher at Sydney's Black Dog Institute, said that these genes are less efficient at stopping unwanted emotional messages and hence result in a state of depression.
They produce reduced protein responsible for transporting the brain chemical serotonin, which plays a key role in mood and pain regulation, appetite and sleep. This research is of great significance and one day result in individualized treatments for depression based on a person's genetic makeup.
But Prof Wilhelm warned people against getting their genes checked to assess their susceptibility to depression as it is a very preliminary study. He also said that depression is such a complex condition that the cause was unlikely to be found in just one gene. He said that instead people can be aware of how to handle stress and need to be educated about some relaxation techniques such as meditation.