"The effects are equal to, or greater than, the effects of cigarette smoking, that includes a significantly shortened life span, said Gretchen Hermes, the Yale researcher who led the study.
Stress has been show to trigger cancer-causing genes in humans. The latest findings link the stress of social isolation to ill health.
Researchers found isolated rates produced more of a stress hormone, corticosterone. They found receptors for stress hormones in breast tissue.
Hermes believes the stress hormones may directly feed breast tumors.
The findings could help explain why many women living in high-crime neighborhoods develop breast cancer earlier than other women, according to study researcher Martha McClintock of the University of Chicago.
"The work explains the role of a social network in protecting health, " Hermes said.
She added social isolation my help explain why so many patients with psychiatric disorders have a shortened lifespan.