People of working age who live alone are significantly more likely to be depressed, a study has shown.
They are 80% more likely to be taking antidepressants than people in any kind of social or family group, scientists found.
Researchers in Finland followed the progress of 3,500 working men and women for seven years while monitoring their antidepressant use.
The number of people living alone has doubled over the last three decades, reaching one in three in the UK and US.
Lead scientist Dr Laura Pulkki-Raback, from the Finnish Institute of Occupational Health, said: "Our study shows that people living alone have an increased risk of developing depression.
"Overall there was no difference in the increased risk of depression by living alone for either men or women. Poor housing conditions (especially for women) and a lack of social support (particularly for men) were the main contributory factors to this increased risk."
She pointed out that studies of this kind usually underestimated risk. This was because people most at risk of depression tended to be those who were least likely to complete the study. Rates of untreated depression were also unknown.
The findings are published in the online journal BMC Public Health.
Living alone was already known to increase the risk of mental health problems for the elderly and single parents, but its effect on the working population was unclear.
While the study identified some reasons why people living alone became depressed, more than half the increase in risk was unexplained.
Possible additional factors may include feelings of alienation, lack of trust, or difficulties arising from critical life events, say the researchers.