Being socially isolated may lead to developing more tumors—and tumors of a more deadly type—than living in a social group, according to a study conducted on female rats by researchers at Yale University and the University of Chicago and published in the December 6 issue of Proceedings of the National Academy of Sciences. "There is a growing interest in relationships between the environment, emotion and disease. This study offers insight into how the social world gets under the skin," said Gretchen Hermes, first author of the paper and a resident in the Neurosciences Research Training Program in the Yale Department of Psychiatry. The cause is thought to be stress triggered by being separated from a group. Stress has been linked to many negative health outcomes, including the activation of cancer-promoting genes. The research team, led by senior author Martha K. McClintock at the University of Chicago, had previously shown that fearful and anxious rats were more prone to tumors and death. The new study shows that social isolation and neglect can trigger the fear and anxiety responsible for this susceptibility to cancer. Researchers followed the development of spontaneously occurring mammary tumors in rats that lived in groups or in isolation. Although both sets of animals developed tumors, the isolated rats developed 84 times the amount of tumors as those living in groups, and their tumors were also more malignant. The results show that the health effects of isolation need to be studied more closely, and in a broad range of human disease, particularly psychiatric disorders, Hermes said. "The costs of social neglect have unique relevance for psychiatric patients, the natural history of psychiatric illness and the profound co-morbidities associated with mental disease," Hermes said. "The results of this study make a physiological link between loss of the social network and disease states."