Over the past three decades, the number of people living on their own in the US has doubled to one in three people.
In women, one-third of the increased risk of depression was linked to factors such as low income and lack of education.
In men, risk factors included heavy drinking and a lack of support in the workplace or in their private lives.
Authors of the study in the journal BMC Public Health say other risk factors may include feelings of alienation from society, lack of trust or difficulties caused by major life challenges.
