Boys who are aggressive tend to develop into physically stronger teens than boys who are not aggressive, according to a new study published in the journal Psychological Science.
“This work was motivated by a long-standing controversy over the relationship between physical development and personality,” said psychological scientist Joshua Isen, Ph.D., of the University of Minnesota, Minneapolis.
“The physiques of boys and girls increasingly diverge during adolescence, leading to a profound sex difference in physical strength, and there’s also an observable sex difference in personality traits like physical aggression and risk-taking.”
Prior studies have suggested a link between male upper-body strength and aggressive actions, but the reasons for this are not well understood.
“Very little is known about how this association unfolds developmentally,” said Isen. “Our study is unique because we used a prospective longitudinal design to examine whether male-typical behavioral tendencies are related to pubertal change in physical strength.”
For the study, researchers looked at data from two large samples of twins collected as part of the Minnesota Twin Family Study. The participants in the study began at age 11 and researchers followed up with them every three years.
The researchers were specifically interested in evaluating the children’s levels of aggression as well as their physical strength at ages 11, 14, and 17.
Aggressive-antisocial tendencies were measured through a combination of teacher and self-report ratings, while strength was measured using hand-grip strength, a measure highly correlated with overall muscular strength. To gauge hand-grip strength, the children were told to squeeze a dynamometer as hard as they could in both their left and right hands.
The findings showed that boys who showed high levels of aggression and those who showed low levels of aggression were equally strong at age 11; however, the more aggressive boys became much stronger physically during adolescence than their less aggressive peers. And this difference could not be attributed to participants’ weight or height, which the researchers had accounted for in their analyses.
Among girls, the data showed no connection between aggressive-antisocial tendencies and development of physical strength.
There are a couple of possible mechanisms that could explain the findings, researchers said.
First, it’s possible that muscular strength and aggressive-antisocial traits are both mediated by changing hormone levels from childhood through adolescence. Or it could be that more aggressive boys participate in activities that lead to greater development of physical strength.
In either case, the researchers believe that the developmental relationship between aggressive traits and physical strength is likely to have an evolutionary basis.
“The pubertal changes responsible for males’ superior strength were likely shaped by inter-male competition for mates,” said Isen, which would explain why competitive personality traits correlate with physical strength among males only.
“Our findings indicate that other aggression-related characteristics — including deceit, risk-taking, and lack of empathy — predict future development of strength in males.”