Ever felt blue for no rhyme or reason? Ever felt lonely in a crowd? Ever felt like nobody loves you?

It could be your genes, says new research which has found that heredity might be the reason behind strange feelings of loneliness. This may help determine why some of us are persistently lonely.

Working with colleagues in Holland, psychologists at the University of Chicago, USA, found about 50 percent of identical twins and 25 percent of fraternal twins shared similar characteristics of loneliness.

Research on twins is a powerful method to study the impact of heredity because twins raised together share many of the same environmental influences as well as similar genes, making it easier to determine the role of genetics in development.

'An interesting implication of this research is that feelings of loneliness may reflect an innate emotional response to stimulus conditions over which an individual may have little or no control,' the research team wrote in an article in the Behavior Genetics journal.

Psychologists had previously thought loneliness was primarily caused by shyness, poor social skills or an inability to form strong attachments with other people.

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The role loneliness plays in health is increasingly becoming a hot academic topic. "The genetics of social behaviour is an intriguing and expanding area of research," said Jeffrey W Elias, cognitive aging specialist at the National Institute on Aging.

Other work by John Cacioppo, a member of the research team, shows that loneliness is a risk factor for heart disease.

Studies have shown loneliness is also at the base of a number of emotional conditions, such as self-esteem, mood, anxiety, anger and sociability.

A caring environment can help lonely people overcome their feelings. But the research also shows that in some cases, the impact of heredity is stronger, said Cacioppo, a professor of psychology at the University of Chicago, who was joined in the study by Louise Hawkley, a senior research scientist in psychology at the university.

The study was based on data from 8,387 twins in Holland, who have been surveyed regularly since 1991.

Smaller, earlier studies done with children suggested that the tendency toward loneliness could be inherited. The Dutch-US study is the first to be done on adults and shows that heredity persists in playing a role in loneliness as people age.

As part of the study, the twins were asked to rate to what extent certain descriptions -- like 'others don't like me', 'I lose friends very quickly', 'I feel lonely', and 'nobody loves me' -- applied to them.

The researchers write that loneliness may have developed early in human evolution as a response by hunter-gatherers facing conditions of under-nourishment who may have decided not to share their food with their families.

By surviving a famine, those early ancestors would be able to propagate during periods of plenty, the researchers theorised. In developing loneliness as an adaptation to survival, these early humans also developed dispositions toward anxiety, hostility, negativity and social avoidance, they said.

The study suggests there may be a genetic component to loneliness, and that people with a predisposition to loneliness process social interaction and information differently.