Scientists believe they are close to answering a question that has baffled them for centuries:
Why are people who are introverted, or shy, more vulnerable to infectious diseases, including AIDS, than people who are extroverted and more outgoing?
Ever since the second century physicians have wondered why personality should have any impact on health, particularly why someone of "melancholic temperament," as it was called in the days of ancient Greece, should get sick easier, and have a tougher time recovering, than your typical happy-go-lucky life of the party.
"Physicians who had a keen eye spotted this many, many years ago," says Steve Cole of the AIDS Institute at the University of California, Los Angeles. Cole and his colleagues have been searching for the biological mechanism that explains that, and they think they've found it.
Reserved Racked by Stress
While studying the replication of the AIDS virus in 54 men who were in the early stages of the disease, the researchers found a rather startling fact. The men who were clinically diagnosed as introverted did not respond nearly as well to AIDS drugs as those who were more outgoing.
In fact, when given AIDS medications, the shy men's "viral load," or replication of the virus, shot up as much as 100 times faster than the more outgoing patients.
Further research has demonstrated that stress, or how people respond to stress, is the key to understanding the mystery. Shy people do not handle stress as well extroverts, and stress causes the body to release a chemical called norepinephrine that leaves the person more vulnerable to viruses.
"It looks as though sensitive people are simply wired to respond to stress more strongly than resilient people," says Bruce Naliboff of the UCLA Neuropsychiatric Institute, one of the authors of a paper reporting the research in the Dec. 15 edition of Biological Psychiatry.
One theory that has been making the rounds for years now holds that shy people "are born with essentially more sensitive brains and nervous systems and they find normal social existence to be more stressful than they are comfortable with," says Cole. "They are just kind of high-strung."
Chicken and Egg
Efforts to test that have produced mixed results, but one fact has emerged quite clearly, he adds.
"As people were testing that theory it did become clear that introverted people certainly have higher nervous system responses to stress," he says.
That begs the question of which came first, the chicken or the egg. Are shy people more vulnerable because they are shy, or are they shy because they are more vulnerable?
"That's not clear," Cole says, at least at this point. "But we can say there's a correlation there. It's sensitivity to stress."
Putting Introverts in Perspective
Cole emphasizes that being shy, or introverted, is not all bad. We probably need a fair percentage of people around who are more likely to be cautious in the face of danger, or more thoughtful about what's going on around them, for our species to survive. If everybody's the life of the party, who's going to watch the fox that's watching the henhouse?