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?
"That's the general agreement on why this temperament survives in the gene pool," he says. "If you look back through history, some of our most esteemed public figures, the people who really made important political or scientific or philosophical or literary contributions, you'll find introverted people terribly over-represented relative to their prevalence in the population as a whole."
They may get sick a lot easier, but many of them are driven to do things "that are exceptional or remarkable," he says.
It took a deadly virus to point the way toward understanding the relationship between personality and health. During the early years of AIDS, researchers kept close tabs on patients who were suffering from that horrible disease, and they soon saw a correlation.
Among those infected with the virus, the "socially inhibited, shy, sensitive, introverted people got sick and died something like two to three years earlier" than others with the virus, Cole says.
"So we knew there was something going on there, a real relationship," he adds.
The participants in the recent study were given a series of psychological tests to determine which ones were truly introverted. Then all 54 men were subjected to various types of stress, like answering simple math questions and being chastised if they got the answer wrong.
Response to that stress was measured by monitoring such things as blood pressure, perspiration, and pulse, all conditions that are influenced by the autonomic nervous system which controls involuntary bodily functions.
The researchers also measured the rate at which the virus was replicated in each patient and other indicators of how the patient was responding to treatment.
"The study surprised us by showing that it's precisely the people who have these high nervous system responses, the shy, sensitive, introverted people, who fail to show complete benefit following drug treatment," Cole says. "There is something about their body that is supporting the virus more than in other patients."
The Next Step to Understanding
That shows there is an "exceptionally strong correlation" between personality and infectious diseases, he says, but one more step is needed.
The researchers have just embarked on a new project, and they hope that it will show whether the stress that is so inflamed in shy people is the villain. They aren't going to try to change anybody's personality, but they are going to try to inhibit the consequences of stress, and they can do that with off-the-shelf drugs used for heart disease, common beta blockers.
These drugs suppress the effects of stress by keeping such things as blood pressure in check. If patients who are diagnosed as introverted respond just as well as extroverts while their autonomic nervous system is suppressed then it clearly is stress, induced by their heightened sensitivity, that is the link between personality and infectious diseases, Cole says.
It will be a year or two before he knows if he is right. If he is, it should be possible to produce drugs that reduce the physiological impact of stress among ailing shy folks, thus helping them fight off an infectious disease.