Partners of people with depression are more likely to suffer from chronic pain, research has found.
The study shows that the two conditions share common causes - some of which are genetic whilst other causes originate from the environment that partners share.
Experts say their findings shed new light on the illnesses and could one day help to develop better diagnostic tests and treatments.
Researchers led by the University of Edinburgh studied information from more than 100,000 people taking part in large nationwide health studies.
The team analysed people's genetic background as well as details about their experiences of pain and depression.
Their findings revealed that chronic pain is caused partly by someone's genetic make-up and partly by as yet unidentified risk factors that are shared jointly by partners or spouses.
They also identified significant overlaps between the risk factors for chronic pain and depression.
Chronic pain is a common cause of disability but little is known about what causes it. Scientists say the research will bring a new understanding of why some people suffer from the condition and not others.
The research used data from the Generation Scotland and UK Biobank projects - major studies investigating genetic links to health conditions.
Researchers from the Universities of Edinburgh, Dundee, Aberdeen and Glasgow collaborated on the project. The study, published in the journal PLOS Medicine, was funded by Wellcome.
Professor Andrew McIntosh, Chair of Biological Psychiatry at the University of Edinburgh, said: "We hope our research will encourage people to think about the relationship between chronic pain and depression and whether physical and mental illnesses are as separate as some believe."
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McIntosh AM, Hall LS, Zeng Y, Adams MJ, Gibson J, Wigmore E, et al. (2016) Genetic and Environmental Risk for Chronic Pain and the Contribution of Risk Variants for Major Depressive Disorder: A Family-Based Mixed-Model Analysis. PLoS Med 13(8): e1002090. journals.plos.org/plosmedicine … journal.pmed.1002090