Teens who have a low resting heart rate may be at increased risk of committing violent crimes as adults, a new study from Sweden suggests.
In the study, the researchers analyzed information from more than 710,000 men who had their heart rate measured when they were about 18 years old, as part of a test for military service that was mandatory in Sweden until 2009.
The researchers divided the men into five groups based on their heart rate. They found that the men in the group with the lowest resting heart rates (60 beats per minute or less) were 39 percent more likely to be convicted of a violent crime over the next few decades, compared with the men in the group with the highest resting heart rate (83 beats per minute or more). The men's violent crimes included murder, assault, robbery and arson, as well as several other crimes.
The findings held even after researchers took into account factors that might affect heart rate, such as the men's height, weight or cardio-fitness level, and factors that might affect a person's risk of committing violent crimes, such as his socioeconomic status.
The study also found that men with the lowest resting heart rates were 25 percent more likely to be convicted of nonviolent crimes (such as those involving drugs) and 39 percent more likely to be injured by an assault or an accident like a car crash, compared to the men with highest resting heart rates.
The new findings agree with previous research in children that linked a low resting heart rate with anti-social behaviors.
"Our results confirm that, in addition to being associated with aggressive and antisocial outcomes in childhood and adolescence, low [resting heart rate] increases the risk for violent and nonviolent antisocial behaviors in adulthood," the researchers, from the University of Helsinki, wrote in today's (Sept. 9) issue of the journal JAMA Psychiatry. [Top 10 Controversial Psychiatric Disorders]
There are two theories as to why having a low resting heart rate may increase the risk of violent behavior. One is that people with a low resting heart rate have unusually low levels of what psychologists call arousal, or the feeling of being awake and alert. It could be that people with lower resting heart rates seek out stimulating experiences (such as risky behaviors and crime) to boost their arousal, the researchers said.
It could also be that people with a low resting heart rate have less of a reaction to mildly stressful experiences like getting your heart rate checked, meaning they are more fearless, and less afraid of risks and their consequences.
The new study does not provide evidence for one theory over the other, the researchers said. In addition, the study found only an association, and does not prove that a low resting heart rate can cause violent behavior. Other factors, such as a person's genes and environment, likely play a role, and should be studied, the researchers said.
The new findings raise wider questions, such as the extent to which a person is responsible for his behavior, Adrian Raine, a professor at the University of Pennsylvania who studies the neuroscience of anti-social behavior, wrote in an editorial accompanying the study.
On the one hand, Raine noted that the men in the study were at increased risk for injury by assault, which most people would not see as their fault. "They can hardly be blamed for having a low RHR that puts them at risk. Yet, if we accept this logic, should the legal system in turn accept low RHR as a mitigating factor for the commission of serious violence?"
Raine said the new findings "put the case beyond reasonable doubt," that a low resting heart rate increases a person's risk of committing future crimes.
However, Dr. Brandon Korman, chief of neuropsychology at Nicklaus Children's Hospital in Miami, disagreed, and said that the effect of resting heart rate on violent crimes found in the study was so small that it was not meaningful.
Korman pointed out that, among men with the highest resting heart rate, 5 percent committed violent crimes in adulthood, compared with 5.8 percent of men with the lowest resting heart rate — less than half a percentage point difference. Although the finding was statistically significant, "in the real world, it has very little meaning," Korman told Live Science.
People should be careful how they interpret the findings, because they have potentially dangerous implications, Korman said. "You can't start predicting who is going to be a criminal offender based on a 0.8 percent difference between the highest and the lowest groups," he said.
There are likely other, environmental factors that play a role in the link, he said. For example, some people with low resting heart rates may have trouble with their peers and get involved with gangs, which are risk factors for violent behavior, Korman said.
Still, Korman said the effect of resting heart rate on unintentional injuries was larger: about 36.5 percent of men with a low resting heart rate experienced unintentional injuries, compared with about 30 percent of men with the highest resting heart rate. This finding would be in line with previous studies that found people with lower heart rates tend to be greater risk-takers, he said.
The researchers noted that the new study did not include women, and so the results apply only to men.