Weird as it seems, heartbeats may help predict who might become a criminal.

A new study, which analyzed data from 710,000 men, found that those whose hearts beat unusually slowly when they were around 18 were 49 percent more likely to be convicted of violent crimes and 25 percent more likely to be convicted of nonviolent crimes as adults than those with the most rapid beats. Those whose hearts beat slowly were also at higher risk to become assault victims and to be injured in accidents.

The study, which was published Wednesday in JAMA Psychiatry, found that the average young man's heart beat 72.2 times per minute. The research team from Sweden and Finland compared men in the lowest 20 percent - 60 beats per minute or less - with men in the highest quintile - 83 beats per minute and up.

Previous research has established that low resting heart rate correlates with antisocial and aggressive behavior in children and adolescents.

In an accompanying editorial, University of Pennsylvania professor Adrian Raine said researchers theorize low heart rate might lead to risky, antisocial behavior in two ways. Anxious people tend to have higher heart rates. Those with low rates may be more violent and take physical risks because they are less fearful of the consequences. Another theory, he said, is that life may feel "a bit dull" to people with low heart rates, so they seek excitement.

Raine, a pioneer in the field of neurocriminology, studies how biology affects criminal behavior. His 1981 doctoral thesis found that low resting heart rate in teenage boys correlated with antisocial and aggressive behavior.

In his editorial, he said the new study "establishes low resting heart rate as a marker for broad rule-breaking behavior in general, although it particularly predicts serious violence."

In an interview, he said it is unknown whether low heart rate actually causes bad behavior. "It could well be that the key process here is lacking fear," he said.

Heart rate usually goes down steadily during childhood and reaches its low point around 18, Raine said. This also happens to be a point when criminality surges. After that, heart rate goes up and criminal behavior tends to go down. Of course, other things that may affect behavior are also happening then, Raine said. Young men's brains are maturing and testosterone levels begin dropping after around 20.

What should parents do if they learn a son has a low resting heart rate? "Don't panic. Just do not panic," Raine said. Heart rate is one of many factors that correlate with behavior, including the quality of parenting, nutrition, and neighborhood influences. Parents, he said, might look at other factors they can control, "like, 'Do I beat him up?' " Emotional and sexual abuse, as well as neglect, are also related to crime.

Stimulant medications prescribed for attention deficit hyperactivity disorder increase heart rate and are also effective at improving behavior, Raine said, but he doesn't think they are a solution.

Might low resting heart rate have a bright side? Fearlessness might come in handy in jobs that require bravery and tolerance for stress. Raine said that hasn't been studied much, but it is known that military bomb-disposal experts have lower resting heart rates than other soldiers. Bomb experts who have been decorated for their bravery have even lower resting heart rates.

As for the criticism that law-abiding athletes have low resting heart rates, Raine said exercise lowers heart rate less than most of us think. He cited a study that found that 20 weeks of endurance training only lowered resting heart rate by two beats per minute. Marathoners may indeed have lower heart rates, he said, but the kind of self-control required to become a long-distance runner reduces the odds of violent crime.

Our legal system, he said, is facing an ethical dilemma as evidence mounts that some people are at biologically higher risk for rule breaking.

"Can the criminal justice system continue to turn a blind eye to the anatomy of violence?" he asked in his editorial.

Not coincidentally, he raised this issue in his 2013 book: The Anatomy of Violence, the Biological Roots of Crime.

sburling@phillynews.com 215-854-4944 @StaceyABurling