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.

The latest findings suggest the stress of social isolation may be the trigger for ill health.

The study, done in conjunction with Martha McClintock of the University of Chicago, found rats in both groups developed breast tumors but many more and larger tumors were found in the isolated rats.

The team also found the isolated rats produced more of a stress hormone, corticosterone, and they found receptors for stress hormones in breast tissue.

Hermes believes the stress hormones may directly feed breast tumors.

McClintock, who studies the impact of social isolation on breast cancer, said the findings could help explain why many women living in high-crime neighborhoods, and especially black women in these settings, develop breast cancer earlier than other women.

The work explains the role of a social network in protecting health, Hermes said.

She said social isolation may help explain why so many patients with psychiatric disorders have a shortened lifespan.

I do feel it goes well beyond breast cancer, she said.