The rest is caused by life circumstances like leaving home or losing a spouse, the Chicago Sun-Times reported.

Researchers in the Netherlands and the University of Chicago examined surveys of 8,387 young adults in the Netherlands Twin Register.

Fraternal and identical twins and their families responded to mailed surveys every two or three years. Twins were asked whether they agreed with such statements as "I feel lonely," "I like to be alone," "Nobody loves me," and "I try to have as little as possible to do with other people."

More Than One Loneliness Gene?

Thirty-five percent of the men and 50 percent of the women reported having moderate to extreme feelings of loneliness. And the researchers found less difference in loneliness ratings between identical twins.

There probably is more than one loneliness gene, although no such gene has yet been identified, said University of Chicago psychologist John Cacioppo, a co-author of the study, which is published in the journal Behavior Genetics.