Some young men who have a low resting heart rate may be more likely to engage in violent criminal behavior later on, a new study suggests.
The finding comes from researchers who tracked criminal activity among more than 700,000 Swedish men whose resting heart rate was recorded at age 18.
"It is important to stress that the vast majority of men who have low resting heart rates do not commit crimes," said study author Antti Latvala, a postdoctoral researcher in the department of public health at the University of Helsinki in Finland.
But, the study only found an association rather than a cause-and-effect link.
Nevertheless, Latvala added, "it can be observed that those with a lower heart rate tend to be more likely in the future to commit crimes and engage in other risky behaviors."
This study isn't the first to explore the biological roots of aggression. Evidence that resting heart rate could be a marker for antisocial behavior has already been identified in young children and teens, the researchers said in background notes.
Experts have theorized that those with a low resting heart rate may be naturally less averse to fear, stress or the consequences of aggressive activities. They also may be more prone to want to heighten their relatively low arousal levels with risky behavior.
Your heart rate, or pulse, is the number of times your heart beats a minute. Normal resting heart rate -- when you're relaxed and not exercising -- is between 60 and 100 beats a minute, according to the American Heart Association.
For this study, published in the Sept. 9 online edition of JAMA Psychiatry, the research team focused on more than 710,000 Swedish males born between 1958 and 1991. At age 18, all had their resting heart rate calculated as part of a physical conducted just before beginning compulsory military service.
Those in the lowest resting heart rate grouping had a heart rate of no more than 60 beats a minute, while those in the highest grouping had their rates pegged at 83 beats or more a minute.
All criminal convictions between 1973 and 2009 were tallied from Sweden's crime register. Those included violent crimes such as murder, assault, kidnapping, robbery and rape, as well as nonviolent crimes such as drug dealing, theft and some traffic violations.
Injury histories were gleaned from a national patient register, which tracked medical care provided after an assault, murder, car crash, fall or accidental poisoning.
The result: More than 40,000 men were convicted of a violent crime at some point in the roughly 35 years that followed their teenage resting heart rate reading. And the men in the lowest resting heart rate group were nearly 40 percent more likely to be in that convicted pool than those in the highest resting heart rate group.
The lowest heart rate group also faced a 25 percent greater risk for engaging in nonviolent crime, and an almost 40 percent bump in their risk for either unintentional injury or assault-related injuries.
Latvala said it's not clear if a similar association would be found among girls and women. But he said prior research indicated that resting heart rate has the same impact across gender.
As for practical implications, he said, "in principle, physiological markers such as heart rate may be useful, in combination with other well-known risk factors, for prevention and intervention efforts by helping to identify individuals who have an elevated risk for antisocial behavior and violence."
But Latvala and his colleagues said more research is needed.
Adrian Raine, a professor of criminology, psychiatry and psychology at the University of Pennsylvania, said the study is "a very strong piece of research.
"They really have shown that this linkage is beyond dispute," he said. "It's now beyond reasonable doubt."
However, it's important to clarify that this doesn't show that a low resting heart rate causes someone to become violent, added Raine, author of an editorial accompanying the study. "There are many things that contribute to violence, not just biological factors like heart rate," he said.
"Yes, having a low heart rate raises the odds of committing a crime, and not just violent crime, all kinds of criminal and reckless behavior," Raine said. "But it doesn't mean that you will."