You can spend every day surrounded by other people and still feel lonely. You can also spend long periods alone without finding the experience distressing.

That difference may help explain why loneliness affects health differently from social isolation. In a new study, my colleagues and I found evidence that loneliness may contribute to poorer mental health and well-being, and to some measures of poorer general health. Social isolation appeared to have a narrower influence, with the clearest evidence involving poorer well-being.

Around 1 in 6 people worldwide experiences loneliness, according to the World Health Organization. Loneliness is the distress someone feels when their relationships do not provide the closeness or connection they want. Social isolation describes the number and frequency of a person’s social contacts. The experiences can overlap, although a person may experience either one without the other.

Previous research has linked loneliness and social isolation with poorer mental and physical health, including depression and cardiovascular disease. An overview of systematic reviews also found associations with poorer general health. These patterns cannot establish cause and effect. Loneliness might contribute to ill health, while illness can restrict someone’s activities and relationships. Poverty, disability and childhood circumstances may also influence both health and social connection.

Our study used several statistical approaches with different strengths and sources of bias. Taken together, the results suggest that loneliness may contribute to depression and increase the risk of self-harm or a suicide attempt. It may also reduce life satisfaction and other aspects of well-being. We found evidence that the relationship can operate in the opposite direction. Depression may increase loneliness, while poorer well-being may increase both loneliness and social isolation. Evidence that depression contributes to social isolation was less consistent. Previous longitudinal research has also found a two-way relationship between loneliness and common mental health problems.

Loneliness was linked to poorer general health, including multimorbidity, which means living with two or more long-term health conditions. We found no consistent evidence that loneliness or social isolation affected the specific physical health outcomes examined. These were coronary artery disease, heart failure, stroke, systolic blood pressure, which is the upper number in a blood-pressure reading, and type 2 diabetes.

This does not rule out an effect. Some estimates were too imprecise to support a firm conclusion, and previous longitudinal studies have reported links between loneliness, isolation and physical illness.