Child bullies ought to be strong and imposing figures before they think about taking a classmate's lunch money. From Biff in Back to the Future to Nelson in The Simpsons—and even the real-life guy who beat up author Allen Kurzweil in boarding school—if a bully couldn't pack a punch first, he or she wouldn't be able to push anyone around. Or so conventional wisdom holds. But a new study suggests that such thinking isn't quite right; it's often the aggression, not the muscle, that comes first.

"It's so intuitive that bigger, stronger people can more easily reap the benefits of being confrontational," writes Joshua Isen, a postdoctoral fellow at the University of Minnesota and lead author of the new study, in an email exchange. From that observation, it's no leap of logic to suppose that physical strength is a prerequisite for aggressive, anti-social tendencies.

But something didn't quite make sense to Isen. "I started questioning this assumption when I realized that the age trends in aggression and physical strength run in opposite directions," he writes. "As children approach adulthood, they become much less aggressive, while simultaneously developing greater physical strength." To Isen, this indicated that perhaps aggression precedes strength, not the other way around.

Eleven-year-old boys who rated high on aggression were also substantially stronger later in life, the team found, despite having the same strength as others when they were younger.

To investigate, Isen and professors Matthew McGue and William Iacono of the University of Minnesota examined data on about 2,500 children who'd taken part in the Minnesota Twin Family Study, which tracks kids on a number of physical and psychological attributes at ages 11, 14, and 17. Among those are hand grip strength and aggressive or antisocial tendencies, which the study measures using both teacher surveys and the kids' assessments of their behavior—for example, how strongly they agreed with statements such as "If I didn’t like someone, I might try to hurt him or her just for the heck of it." (The twin aspect wasn't important for their latest study, but it will make it possible to study the environmental and genetic determinants of aggression and strength in the future, Isen explains.)

Eleven-year-old boys who rated high on aggression were also substantially stronger later in life, the team found, despite having the same strength as others when they were younger. By age 17, Isen explains, the most aggressive boys had grip strengths about seven pounds stronger than the average boy. "Put another way, a very aggressive boy will tend to be 7.5 percent stronger than his average male peer at age 17," Isen writes. No such differences emerged in girls.

Though the results can't rule it out completely, they do cast doubt on the idea that muscles give rise to aggression, Isen writes; certainly, that hypothesis doesn't explain what he, McGue, and Iacono discovered. A better explanation, Isen writes, may be that the same underlying genetic factors that lead to aggression also lead to physical strength later on—something Isen hopes to explore in the future using the Minnesota twin data.

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