Heredity helps determine why some adults are persistently lonely, show research co-authored by psychologists at the University of Chicago.

Working with colleagues in The Netherlands, the scholars found that 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, thus 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 writes in the article.

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

Scholars are becoming increasingly interested in the role loneliness plays in health. Other work by John Cacioppo, a Professor at the University of Chicago and a member of the research team, shows that loneliness is a risk factor for heart disease. Loneliness is also at the base of a number of emotional conditions, such as self-esteem, mood, anxiety, anger and sociability.

The researchers write that loneliness may have developed early in human evolution as a response by hunter-gathers facing conditions of undernourishment 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 theorized. In developing loneliness as an adaptation to survival, these early humans also developed dispositions toward anxiety, hostility, negativity and social avoidance, they said.