"Well, son, you'll either be a violent criminal or a triathlete. Only time will tell".

For the video version of this post, click here. Predicting future crime is a cool idea one that has seen play from dystopian novels to Hollywood movies, but the results never seem to work out that well. Thats the case in a study appearing in JAMA psychiatry that well be discussing in the next 150 seconds. This study examines the relationship between resting heart rate and future crime. It is 100 times larger than all prior studies of this phenomenon combined. And what it says is, yes, your heart rate is associated with your risk of future violence. But, Im going to argue, that it doesnt matter. Here are the details.

Swedish researchers looked at a cohort of around 750,000 Swedish men reporting for mandatory military conscription evaluation at the age of 18. Using Swedens robust national reporting system for health and crime, they were able to follow these individuals for as many as 35 years. They collected data on violent crime, non-violent crime, being a victim of violence, and even unintentional injury.

What they found was that those with lower heart rates were more likely to experience all of these outcomes. If your heart rate was less than 60 beats per minute, your risk of committing a violent crime was 25% higher than someone whose heart rate was above 83. Thats before adjusting for things like cardiorespiratory fitness and socioeconomic factors, but accounting for those confounders actually increased the risk - to about a 50% increased likelihood of violent crime.

The authors offer two explanations. One, that low resting heart rate is a marker of chronically low physiological arousal - those with low resting heart rate might engage in risky behaviors to bring themselves up to a more normal level. Or two, its a marker of fearlessness - in the stressful situation of a conscription exam, low resting heart rate suggests youre not easily frightened. If thats the case, maybe you dont fear the consequences of your actions as much in the future. These theories cant be teased out in the context of the study, but they are certainly intriguing.

Where things go a bit off the rails though is in the introduction, discussion, and accompanying editorial, where the prognostic value of resting heart rate is seriously considered. The authors imply that, perhaps, we should be paying special attention to those with low RHR. Aside from the fact that this rankles my libertarian sensibilities, I dont believe this is at all supported by the data.

The authors dont give us enough data to assess how good a test low resting heart rate is, but I made some rough estimates and heres what I found.

If we had a million 18-year olds, according to this study roughly 58,000 would commit a violent crime, and 200,000 would have a resting heart rate less than 60. If we targeted that group, wed capture roughly 10,100 future criminals, and 190,000 future innocents. Wed be right about 5% of the time. Interestingly, if we just picked a random 200,000 people and labeled them aspotential criminals, wed be right 5.8% of the time. A test that works better when you dont do it is not a very good test. So no, we should not identify these adolescents as being at risk, as the authors suggest, or consider resting heart rate as a mitigating factor in criminal trials as the editorialists suggest. Doing that would, well, really get my heart rate up.