Longtime Indiana University basketball coach Bob Knight was recently fired for allegedly being too rough on students. So it looks as if excessive aggression isn’t good for a man’s career these days. But that’s not all: excessive aggression does nothing for a man’s immune system either. When it comes to immunity, what might be more productive is moderate aggression, according to one interpretation of study results reported in the July/August Psychosomatic Medicine.

Back in 1985 and 1986, the Centers for Disease Control in Atlanta gathered psychological and physical information about 4,415 men who had served in the U.S. Army between 1965 and 1971.The psychological information included data about how aggressive the men were, and these data were derived from DSM-III symptoms assessing antisocial personality disorder. Twelve items measured behavior as a youth and as an adult. Although the items were designed to assess aggressive antisocial behavior for which there are social prohibitions, such as being expelled from school, fighting, or using a weapon, these acts, in a slightly different context, are similar to behaviors that have survival value, such as a willingness to travel to foreign territory or engage in conflict to gain needed resources. Of the 4,415 subjects, 22 percent reported no aggressive acts, 39 percent one or two, 27 percent three to five, and 12 percent six or seven. The median, it turned out, was two acts.

Then Douglas Granger, Ph.D., an associate professor of biobehavioral health at Pennsylvania State University; Alan Booth, Ph.D., a distinguished professor of sociology and human development and family studies at Penn State; and David Johnson, Ph.D., a professor of sociology at the University of Nebraska, used these data to see whether they could find any relationship between evidence of aggression in these subjects and various types of immune cells and antibodies in their bodies.

The researchers found a significant relationship between moderate aggression and high levels of certain types of immune cells, but not between excessive aggression and those cells, nor between lack of aggression and those cells.

Moderate aggression correlated with high levels of B cells—those immune cells that make antibodies—and with high levels of CD4 cells—T cells that wage the good fight against infections, such as HIV. What’s more, the link between moderate aggression and high levels of these immune cells remained even after taking into account possible confounding factors such as age, testosterone level, stress, smoking, drinking, taking drugs, and health status.

The researchers believe that this is the first study to link aggression at any level with immunity in humans.

How to interpret the findings is not clear. One explanation is that an aroused immune system leads to moderate aggression, Granger told Psychiatric News, or—more likely, he believes—that moderate aggression is bolstering the immune system.

Michael Irwin, M.D., a professor of psychiatry with the San Diego VA Healthcare System and an associate editor of Psychosomatic Medicine, tends to agree with Granger on this latter interpretation. The reason, he explained in an interview, is that a number of nonhuman primate studies have linked aggressive behavior not just with an increased number of immune cells, but with enhanced immune function. Irwin cautioned, however, that the study in no way proves that moderately aggressive men are less susceptible or more susceptible to disease since the study simply links moderate aggression with enhanced numbers of immune cells, not with heightened immune activity.

The investigation also leaves some questions unanswered. For instance, why is moderate aggression associated with increased numbers of immune cells, whereas excessive aggression is not? Granger admitted he does not know, although he has several possible explanations. If a less-aggressive man becomes pushier, will it pump up his immune system, and if an extremely belligerent man becomes less so, will it benefit his immunity? Once again Granger could give no firm answer. However, he opined, "We are not convinced that it is any individual aggressive action that causes change in immunity in the short run. We think this is more like a trait, a characteristic of an individual. So it really wouldn’t be appropriate, considering the information we now have, to speculate that if maybe you become a little [less or] more aggressive, you’d have a stronger immune system."

And does the study’s finding apply to women? Here too Granger admitted he doesn’t know. "However," he pointed out, "when we look at the animal literature, females tend to have stronger immune systems than males do. So it would be interesting to explore a possible relationship between aggression and immunity in females."

Even with the meaning of its findings murky, and even though certain questions remain unanswered, this study nonetheless has something powerful going for it—a large sample size (4,415 men). Thus, the link that was found between moderate aggression and enhanced levels of immune cells in men cannot be easily dismissed and calls for further exploration.