At most points in human history, being alone meant mortal danger and was to be avoided at all costs. It is only in recent decades that our risky experiment with loneliness has become almost mainstream.
A high incidence of single households is now a mark of a wealthy society. This is partly a consequence of avoiding or delaying marriage and childbirth, and of single housing becoming more affordable, but it has serious implications.
When we live alone we are more likely to engage in unhealthy behaviour, such as smoking or eating badly, because no one else can see. Studies show that people who live by themselves are more likely to suffer from alcoholism, high blood pressure, insomnia and weak immune systems.