A £4.7 million study is to be carried out into the causes of clinical depression in the hope new treatments can then be developed.

Over the next five years, researchers will look at groups of people at risk of depression and test whether specific patients correspond to certain disorders.

Experts say that rather than being one disease, ­clinical depression is a collection of different ­disorders with one common symptom - low mood.

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The Edinburgh University study will use data from Generation Scotland - a large, family-based sample of more than 21,000 people.

Scientists will look at groups of people who have known depression risk factors, including family history of low mood, diseases like heart disease and diabetes and early-life problems.

The study, which has received funding from the Wellcome Trust, will then use memory, reasoning and mental speed tests as well as brain imaging to find out whether specific subgroups of patients actually match specific disorders.

The team, which will also involve researchers from the University of Aberdeen, hope that by studying the groups in this way they will be able to identify the causes then develop diagnostic tests and new therapies.

Clinical depression is a chronic worldwide health problem affecting millions of people.

About 13% of the UK population is affected.

Lead researcher Andrew McIntosh, professor of biological psychiatry at Edinburgh University, said: "For many people, the symptom of low mood is the most understandable of reactions to loss or stress, yet we still remain ignorant of its causes and mechanisms."