The Anatomy of Violence (19 page)

Now, it’s important to note that these studies do not demonstrate causality, and nobody is arguing that we can tell who exactly in a classroom of kids is going to become antisocial on the basis of heart rate alone. But it’s a factor, and by teasing out the temporal ordering of the variables in question
through research that follows young children into adulthood, we move one step further in support of the causal model that low heart rate early in life raises the
odds
of someone becoming a future offender.

Could it be that social factors cause both crime and low heart rates—giving the false impression that low heart rates cause crime?
David Farrington, of
Cambridge University, one of the world’s leading criminologists, examined this issue in establishing the best independent early predictors of convictions for violence. He found that out of forty-eight predictors (family, socioeconomic position, academic
attainment, and personality—everything from low social class to low IQ to impulsivity), only
two
were related to violence independent of all other risk factors: low resting heart rate and poor concentration.
¹⁹
Indeed, low heart rate was even more strongly related to measures of violence than having a criminal parent—one of the best social predictors of later crime.
²⁰
These findings led Farrington to conclude that “low heart rate may be one of the most important explanatory factors for violence.”
²¹

Let’s look at this relationship coming from the other direction. While a low heart rate raises the odds that someone will become antisocial, a
high heart rate actually
reduces
the odds of later crime. I conducted a study of English schoolboys who were antisocial at age fifteen
but who desisted from adult crime at age twenty-nine. I then matched them against seventeen antisocial adolescents who had become criminal by age twenty-nine and also with seventeen non-antisocial, noncriminal controls. The ones who desisted from crime had significantly
higher
resting heart rates relative to both criminal and control groups, indicating that a high heart rate protects against adult crime.
²²

On the treatment side, medications like
stimulants that raise heart rate reduce antisocial behavior.
²³
Studies are also showing that heart rate may help predict which children will benefit from therapy—and which won’t. One study from Germany found that children who before treatment had low heart rates were
less
responsive to behavior therapy.
²⁴
Interventions may be more effective in antisocial children with normal or high heart rates in whom the causes of their antisocial behavior may be more environmental than genetic. Knowledge of resting heart rate may not just help predict which children are more at risk for later criminal behavior, it may also provide invaluable knowledge in treatment programs.

Again on the medical side, one of the big problems is that it’s nearly impossible to find a biomarker that is diagnostically specific to just one psychiatric disorder. For example, there are many biological correlates of
depression, but they are also found in patients with anxiety and other mental illnesses. An unusual and important feature of the low heart rate–antisocial relationship is its diagnostic specificity. While other psychiatric conditions, including
alcoholism, depression,
schizophrenia, and
anxiety disorders, have if anything been linked to a
higher
resting heart rate, no psychiatric condition other than
conduct disorder—i.e., antisocial and aggressive behavior—has been associated with a
low
resting heart rate.
²⁵

The above studies have largely focused on violent criminals, psychopaths, and conduct-disordered children. But how much of a transgressor do you have to be to have a slower heart rate?

I was pondering this issue on the sabbatical I took with my family at
Hong Kong University. In Hong Kong it is rare for pedestrians to cross the road on a red light, even when the coast is clear. But there are always a few who do. Whenever I would take my boys out to the park, we’d inevitably come upon a crossing and they would see some adults breaking the rule. They would point at them and call them “naughty penguins”—after
Pingu, a cartoon they watched about an adventurous
but mischievous little penguin. So it occurred to me—do naughty penguins also have low resting heart rates?

With the help of eight undergraduates, I collected heart-rate data on 622 Hong Kong students and asked them about their habits, including how many times they ever crossed the road on a red light. We found a difference. It was not big, just two beats a minute, but it was statistically significant and in the right direction. Naughty penguins really do have lower resting heart rates! Of course, this minor infraction is just the tip of the antisocial iceberg, but it indicates that low heart rate covers the whole spectrum of antisocial acts down to the smallest transgression.

Taking all these points together, it’s hard to deny that a true, replicable relationship exists between low cardiovascular arousal and violence. When one line of scientific evidence supports a hypothesis, it is persuasive. But when many separate lines of evidence from different perspectives converge on the same conclusion, the argument becomes truly compelling.

Indeed, this body of evidence has raised the intriguing possibility that low heart rate could be considered a
biomarker
for the diagnosis of conduct disorder.
²⁶
Currently, conduct disorder and almost all clinical disorders like schizophrenia are defined not in terms of biology, but in terms of symptoms that are obtained in an interview with a clinical practitioner. So the clinical symptoms of conduct disorder are things like lying, stealing, fighting, and cruelty to animals. These are all behavioral in nature and rely on subjective verbal reports from caregivers of the children themselves. There are two good reasons that biomarkers are not included in psychological diagnoses. First, they are not found to be diagnostically specific—they apply also to other disorders. Second, in everyday practice it’s not that easy for a doctor to scan a patient to assess
brain functioning—to say nothing of the extra financial burden scanning would present.

Heart rate is different on both counts. It
is
diagnostically specific, and it is extremely cheap and quick to assess. Think of it yourself. What happens first when you go to your doctor’s office? You have your blood pressure and heart rate taken. Adding an objective biomarker to a subjective diagnosis is the holy grail that psychiatry and clinical psychology are searching for in all mental illnesses. Of course, not everyone with a low heart rate becomes a violent offender. My heart rate in my
mid-twenties was 48 beats per minute, and the same will be true for a number of you. Yet at an admittedly imperfect level, low
heart rate is a telltale sign of transgressors.

So low resting heart rate represents one of the best replicated, most easily measured, and most promising biological correlates of antisocial and aggressive behavior. But
why
does it predispose someone to antisocial behavior? Even with simple biological measures, unfolding the “mechanism of action”—how low heart rate produces antisocial and aggressive behavior—is highly complex. Let’s examine a few of the
prevailing explanations.

One is
fearlessness theory.
²⁷
A low heart rate is thought to reflect a lack of fear.
²⁸
Although we talk about “resting” heart rate, the term is misleading. In research studies, subjects are brought into a novel environment, met by strangers, and have electrodes slapped on them. This is less like “resting” and more like experiencing a mild stressor. Timid, anxious children will have higher heart rates. Those lacking fear will have lower heart rates.

As outlined above, there are some particularly fearless individuals such as
bomb-disposal experts who function perfectly well in society and also have particularly low heart rates.
²⁹
After all, it takes nerves of steel to defuse a bomb. By the same token, antisocial and violent behavior requires a degree of fearlessness. If a boy lacks fear, he is more likely to get into a fight because he is not afraid of getting hurt. Similarly,
punishments like prison do not motivate many offenders to desist from violence because this punishment does not hold fear for them.

Fearlessness theory receives support from research showing that low heart rate provides the underpinning for a fearless, uninhibited temperament in infancy and childhood,
³⁰
and that the more uninhibited a preschooler is, the more aggressive he or she will be later in life.
³¹
Adolescents with low heart rates are also better able to stand stress, indicating that such individuals are more insensitive to social stressors, including socializing punishments.
³²

Another theoretical explanation of the low heart rate–antisocial behavior connection lies in
empathy. Children with low heart rates are less empathic than children with high heart rates.
³³
Children who lack
empathy are less able to put themselves into another person’s shoes and to imagine what it must feel like to be
bullied and hit. Those with low empathy may be more aggressive because they have no concern for the feelings of others. Certainly children lacking empathy are more
antisocial and aggressive.
³⁴

Another explanation for how low
heart rate produces antisocial and aggressive behavior is
stimulation-seeking theory. This theory argues that low arousal represents an unpleasant physiological state, and that those who display antisocial behavior seek stimulation to increase their arousal levels to an optimal level.
³⁵
We all have an optimal level of arousal at which we can operate effectively and comfortably.
³⁶
Think of times you come back home and really need some stimulation—you turn on the TV, brew some coffee, turn up the music, get on your cell phone, or go out and party. You are bored and need a buzz. Yet there are other times you instead come home and leave the TV off, turn off your cell phone, and retreat into your own quiet space. The day has been too much and you’re over-aroused.

The same need you have applies to kids with chronically low levels of arousal. Preschool boys with low heart rates not only are more antisocial and
hyperactive, but they also choose to watch videotapes depicting intense anger more often than kids with more normal heart rates.
³⁷
In my own research, resting heart rate at the age of three characterizes both stimulation-seeking behavior at that same age
³⁸
and aggressive behavior at the age of eleven. Kids with chronically low levels of arousal may get an arousal boost in life by beating someone up, shoplifting, joining a gang, or getting involved in drugs. The harsh reality is that breaking any
rule is fun for most kids—just think back to the days when you were a teenager. Living on the edge may not be what parents want for their teenagers, but for the kids themselves it’s exciting and gives meaning to life. Perhaps it’s not too surprising, then, that resting heart rate is at its lowest in life during adolescence, when stimulation-seeking
³⁹
and antisocial behavior are at their highest.
⁴⁰
And that craving for an arousal boost in adolescence may be part of the reason
violence
peaks in the late teenage years.

If you have ever experienced this craving for stimulation, as I did when I was a kid, you get into a state of really just not knowing where to put yourself. You experience an intense feeling of restlessness and emptiness that can peak in a sense of agitation, and a real need to
release some type of hard-to-describe, built-up tension. I have that feeling right now. You want to move around. Once you can find something to do in order to “shift gears,” you feel better.

These feelings are exactly what a significant number of serial killers report experiencing prior to their homicides. The intense tension and restlessness. The need to go out in search of a victim. The consequent excitement of the abduction,
torture,
rape, and killing. And then the sense of relief and release from tension.

Why would that be? I suspect it’s explained partly by having physiological under-arousal and a
stimulation-seeking personality. The important message I really want to convey is a simple medical fact: low heart rate is a significant risk factor for antisocial behavior. Of course, it is not the only process within the autonomic nervous system that has gone awry in antisocial and violent individuals. To put that statement into a societal context, we need to take a trip to
Mauritius.

Mauritius is one of the most beautiful tropical islands in the world and a destination for those seeking a luxury holiday with its consequent peace, quiet, and harmony. It’s also not a bad place to do research. In the past twenty-five years I have had to drop in on the island thirty-nine times. One could, of course, research violence in Detroit, but on balance I slightly prefer Mauritius. “It’s so delicious,” as the advertisements say along the road going from the airport to La Pirogue beach hotel, where I always stay. The sun, the palm trees, the beaches, the volcanic mountains, and some of the warmest and most gracious people I have ever met make for an exotic mix.

Mauritius is a small island in the Indian Ocean near the Tropic of Capricorn, lying to the east of Madagascar. It extends thirty-eight miles from north to south, and twenty-nine miles from east to west. Part of the
African continent, it is a multiracial democratic nation that gained independence from British rule in 1968 and became a republic within the Commonwealth of Nations in 1992. With a population of 1.28 million as of July 2009, it is the third most densely populated country in the world. At the initiation of our longitudinal study, in 1972, Mauritius was a developing country, but now it is developed and widely viewed as a model African country.