MONDAY, March 9, 2015 (HealthDay News) -- Aggressive boys tend to develop more physical strength when they're teens than nonaggressive boys do, a new study finds.
Researchers examined data from twins in Minnesota whose levels of aggression and hand-grip strength were assessed at ages 11, 14 and 17. Hand-grip strength is closely associated with other types of muscle strength, the researchers explained.
Aggressive-antisocial behavior was assessed through teacher and self-report ratings.
At age 11, aggressive and nonaggressive boys were equally strong. But aggressive boys had larger increases in physical strength during their teens than nonaggressive boys, the study found. This was not the case among girls.
Height and weight did not play a role in the strength differences between aggressive and nonaggressive boys, according to the study published recently in the journal Psychological Science.
While a correlation was found between aggressiveness and strength in boys, the study didn't prove a cause-and-effect relationship.
There are a number of possible explanations for the findings, the researchers said. Muscle strength and aggressiveness could be the result of changing hormone levels as boys become teens. Or more aggressive boys might do activities that lead to larger increases in muscle strength.
Whatever the mechanisms, the link between aggressiveness and strength is likely based in evolution, the researchers surmised.
"The pubertal changes responsible for males' superior strength were likely shaped by intermale competition for mates," said study author Joshua Isen in a journal news release. He is a psychological scientist at the University of Minnesota.
This would explain why aggression is associated with strength only in males.
"Our findings indicate that other aggression-related characteristics -- including deceit, risk taking and lack of empathy -- predict future development of strength in males," Isen concluded.
The U.S. National Library of Medicine has more about teen development.
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