Loneliness may run in the family, researchers have suggested.
Cavemen, like Stig of the Dump, often wanted to be alone
Teams from the Free University in Amsterdam and the University of Chicago looked at data on 8,000 identical, and non-identical, twins.
They found genetics had a significant influence on loneliness.
The researchers, whose study appears in Behavior Genetics, said it showed helping lonely people was not simply a matter of changing their environment.
Loneliness has been linked to heart disease as well as emotional problems, such as anxiety, self-esteem problems and sociability.
The researchers suggest that loneliness may stem from prehistoric times, where hunter-gatherers may have deliberately shut themselves away from others so they did not have to share food.
That would have meant they were better nourished and therefore better able to survive and have children.
But they added that the strategy had a downside, in that it also developed dispositions towards anxiety, hostility, negativity and social avoidance.
In the study, the twins, who have been surveyed regularly since 1991 when they were aged 13 to 20, were asked if they agreed or disagreed with certain statements, such as "I lose friends very quickly" and "nobody loves me".
The researchers compared the responses of adults in identical, and non-identical, twin pairs, all of whom had been brought up in the same households.
They found less difference in loneliness ratings between identical twins.
They suggest this means that genes play a major role in determining whether or not people will experience the feeling throughout their life.
Professor Dorret Boomsma, who led the study, told the BBC News website: "This kind of knowledge will help because it shows it's not as simple as saying that if you change someone's environment, it will have the same effect on everyone."
But Dr Arthur Cassidy, a psychologist at the Belfast Institute, said people could learn behaviours from their families.
"They may have a very pessimistic outlook and interpret things in a very negative way, so people can learn to become pessimistic and therefore to become lonely.
"But it is possible to unlearn behaviour, using Cognitive Behaviour Therapy (CBT) and psychotherapy."