The big bullies of the world may have more than a muscle advantage over the 90-pound weaklings. Research now suggests they have stronger immune systems, too.
The reason may have to do with an evolutionary process in which hunters and warriors needed greater protection from disease, researchers report in the August issue of Psychosomatic Medicine.
In a study of more than 4,400 men who had served in the US Army, Dr. Douglas A. Granger and his colleagues found that those with a history of fisticuffs, run-ins with the law, and behavioral problems in school had immune systems that were equally aggressive. Specifically, the aggressive men had a greater number of white blood cells known as B cells and helper T cells, both key in the immune system's response to foreign invaders.
Granger, a professor of behavioral health at Pennsylvania State University in University Park, told Reuters Health that the link between aggression and immunity was strongest for men who were "moderately aggressive." For example, men who reported committing two aggressive acts in their lives were 70% more likely than passive men to be in the group with the most helper T cells. But the trend toward higher helper T cells leveled off once men had committed six or more aggressive acts.
"I don't think we'd want to encourage people to start being aggressive to strengthen their immune systems," Granger said. However, he added, unlike some research that has linked aggression to poorer health, these study findings suggest "it's not always bad."
Besides looking at aggressive behavior, Granger's team considered other factors that influence immunity, such as overall health and risky behaviors such as smoking, drinking excessively, and having many sex partners. The investigators also measured the men's testosterone levels because some research has linked the hormone to weakened immunity. Even when they accounted for these factors, though, aggression remained tied to stronger immune function.
The reason for the connection is unclear, but Granger speculated that it may have to do with evolution. "We're throwing out the hypothesis that this (connection) has been with us for a long time," he said.
Throughout history, Granger noted, men who were more aggressive may have been more likely to "be out" foraging, hunting, fighting, and otherwise exposing themselves to a high risk of injuries and infection. A strong immune response would have been an asset to these men.
Although the current study focused on men who engaged in bad behavior, Granger said that since stronger immunity was linked to moderate aggression, the findings may extend to men who merely have more aggressive personalities.