According to a study published in BioMed Central's public health journal, people of working age who live alone increase their risk of depression by up to 80%.
(This is compared with people living in families. The worrying thing is that the proportion of one-person households in Western countries has increased during the past three decades, with one in every three people in the US and the UK living alone. Living alone, the study said, could be linked with feelings of isolation and a lack of social integration and trust, which are risk factors for mental health. Of course with the internet, no one needs to be alone for long, just log in and talk.)