CHICAGO (Reuters) - Lonely, stressed-out rats were far more likely to develop breast tumors than rats living in a social group, a finding that suggests loneliness can have a profound effect on health, researchers said on Monday.

They said rats that were separated from a social group shortly after birth had a three times higher risk of developing breast tumors than did rats living in a social group, and the tumors in the isolated rats were more deadly.

"The leading suspect is poorly regulated stress," Gretchen Hermes, a researcher at Yale University in New Haven, Connecticut, who led the study, said in a telephone interview.

Hermes said many studies have suggested loneliness has a negative impact on human health.

"The effects are equal to or greater than the effects of cigarette smoking — that includes a significantly shortened life span," said Hermes, whose study appears in the journal Proceedings of the National Academy of Sciences.

Stress has been shown to trigger cancer-causing genes in humans. Prior studies by the research team showed that fearful, anxious rats were more prone to tumors and death.