Doctors have known for a long time that feeling lonely can make you physically sick, but until now they did not know why. The answer may be in our genes.

Researcher Steven Cole of the University of California, Los Angeles, and his colleagues there and at other institutions found that chronic loneliness triggers a change in gene activity. The initial results published last year showed that people who scored in the top 15 percent of the U.C.L.A. Loneliness Scale, a self-administered psychiatric questionnaire for measuring the emotion, exhibited increased gene activity linked to inflammation and reduced gene activity associated with antibody production and antiviral responses. These patterns of gene expression were specific to loneliness, not to other negative feelings such as depression.

But what could cause these changes? In a new study of 1,023 Taiwanese adults, Cole analyzed data from a variety of lonely people and found that the hormone cortisol was not doing its job of suppressing the genes associated with inflammation. Inflammation is a known risk factor for a variety of serious illnesses, such as heart disease and cancer. Recent animal studies from Cole’s group confirm the link: cortisol receptors stopped working in rhesus monkeys that were socially stressed.

Yet questions still remain. Cole and his colleagues are now working with patients in Chicago to try to determine how different degrees of loneliness affect health. Do all lonely people suffer some damage, or is there some threshold at which feeling isolated starts affecting the body? “We are just touching the tip of the iceberg” in our understanding of loneliness, says University of Chicago team member John Cacioppo.

This story was originally printed with the title, "So Lonely It Hurts".