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A study into the causes of clinical depression is being given £4.7 million in the hope that new treatments can be developed.
Over the next five years, researchers will look at groups of people at risk of depression and use a series of tests to discover if 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.
The University of Edinburgh 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 such as low birth weight or childhood psychological trauma.
The study, which has received the 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 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 and this information can be used to develop diagnostic tests and new therapies.
Clinical depression is a chronic worldwide health problem affecting millions of people and about 13% of the UK population, but little is known about what makes people vulnerable or resilient to the condition.
Lead researcher Andrew McIntosh, professor of biological psychiatry at the University of Edinburgh, said: "For many people, the symptom of low mood is the most understandable of reactions to loss or stress, yet we remain ignorant of its causes and mechanisms.
"This means that progress in discovering new and more effective treatments is slow.
"This Wellcome Trust grant will enable us to make significant progress with this common and disabling condition.''
Ian Deary, director of the university's centre for cognitive ageing and cognitive epidemiology, said: "It is good to see this support for our broadening-out of the research on depression, paying closer attention to the thinking skills that alter when people are depressed, and to the brain changes that occur.
"Our hope is that we will be able to study enough of these factors in a large-enough group of people to start to understand depression better.''