Violent behavior is a complex product of biology and upbringing, and when that violence involves murder and destruction to the extent that erupted at the Boston Marathon, the questions about what drives such aggression become all the more urgent.
Criminologist Adrian Raine, a professor at the University of Pennsylvania, has spent more than 35 years trying to answer such questions. TIME spoke with him recently about the Boston bombings and the seeds of violence, which he explores in his new book, The Anatomy of Violence.
The question on everyone’s minds now is why…
Most mass killers have mixed motives, but more often than not there is a fundamental grievance, a score that needs to be settled with society. For [the older brother], the earlier questioning by the FBI and rejection of his application for US citizenship could have been a contributing factor that got wrapped up with political ideology and a dissatisfaction with his own life. But likely a complex combination of factors created this toxic mix – likely a biological predisposition to violence combined with social triggers and mild mental illness.
What would you expect to see in these brothers’ brains and backgrounds?
You can either say that these two men had no pathology and were driven by ideology, or that something was wrong. On balance I suspect the latter.
If I could brain scan [them], I would expect to see good frontal lobe functioning that is needed for a carefully planned and regulated attack. But they would also show a reduction in the functioning and volume of the amygdala, which would predispose them to fearlessness and lack of conscience. As with the Unabomber, they may also have relatively low resting heart rates, a marker for violence and stimulation-seeking.
Mass killers come from a wide range of social and psychological backgrounds. But there is frequently social instability. I would also suspect some form of psychopathology. There’s a fine line between ideology and unshakable beliefs on the one hand, and clinical paranoia on the other.
The Unabomber had firmly held convictions that technology was harming society that some might agree with, and he was also a paranoid schizophrenic. While Tamerlan and [Dzhokhar] may not have been overtly psychotic, they likely harbored grudges and ill-feelings over time that later became expressed in these bombings.
What does your heart rate have to do with your risk of violent and antisocial behavior?
Lots and lots of studies show that antisocial people have lower resting heart rates than people who are not antisocial and that’s been found of violent offenders as well. It’s true of psychopaths and true of [antisocial] school kids.
It’s really speaking to the issue that violent offenders are cold-hearted literally. There are different possible explanations. One is that [it is connected with lack of fear]. Those people whose heart rates are relatively low, they’re relatively more fearless than the rest of the population. The simple idea is that if you lack fear, you don’t worry about robbing the 7-Eleven, you don’t have the anticipatory fear that holds the rest of us back. The is idea that they are actually showing a blunted stress response and low heart rate reflects [that]. [For example] bomb disposal experts have low heart rates.
Another explanation [for the connection] is stimulation seeking. Low heart rate is linked with low physiological arousal and some kids seek out stimulation to increase arousal. Like joining a gang, burglarizing a house — in some environments, that’s stimulating, it’s exciting.
So what would make someone with a low heart rate choose to use this for good, as a bomb disposal expert — as opposed to becoming a criminal?
There’s a whole environmental overlay on top. The research has never really grappled with it. This is a biomarker and a few studies show that low heart rate during adolescence is predictive of future offending. [But] the social context is critical. I don’t want people to run away with the idea that it’s all biology. It’s not true.
MORE: Brain Scans Can Predict Which Criminals Are Likely to Get Re-Arrested
Are there differences between criminals who commit impulsive acts of violence and those who plot and plan, as in the Boston case?
There’s a part of the brain called the amygdala, which is the seat of emotion. We find that part of brain to be structurally and functionally impaired in those people who will go out and lack remorse and empathy and use violence, [such as we see with] the more predatory, cold-blooded psychopaths. They know what they are doing and that it potentially could lead to someone being killed, but they don’t have those feelings that hold the rest of us back.
In contrast, with the more hot-blooded [types], we think it’s the other way around. Emotions are running out of control and the amygdala is overly responsive to mildly provocative stimuli. They lack the prefrontal regulatory control. The prefrontal cortex is right above the eyes, just behind the forehead and it’s involved in planning and regulating and controlling behavior. We think the more hot-blooded [types] are lacking the normal regulatory control of the prefrontal cortex and that’s why they act out impulsively.
MORE: Brother’s Keeper: Did Older Sibling Lure Bombing Suspect into Plot?
Many people are concerned about focusing on the biology of violence, given historical abuse of such research. What do you say to these critics?
You can go back to the Holocaust. We can recognize the bad use made of biological research, which fueled disastrous social policies, and focus on the dark nightmare. It makes people skittish and you have to recognize and realize that and say we need safeguards on how this research is used and that has to be uppermost in everyone’s mind.
But the counter side is have we learned a lesson and can we manipulate and change biology for the better? The downside is could we foreclose some benefit? Don’t close the door too quickly to what new research may promise, but [of course] let’s be cautious and tread carefully.
Do you think that the younger brother would have plotted these killings without his older brother’s influence?
Just as violence is a mix of social and biological factors, so too could the social relationship between these brothers have been a critical ingredient in what transpired. Of course one can act alone, but if you have a partner-in-crime they can give you the conviction and courage to go one step further than you would have by yourself.
Some see this as a mix of Columbine and 9/11. Do you see this as more political or personal?
I suspect this bombing is a mix of personal and political issues. Not all people dealt a rough hand in life, or who suffer under an oppressive regime, become terrorists. We have next to no scientific knowledge on what causes someone to become a terrorist, but my best educated guess is that there are common denominators to different forms of planned violence. At the personal level those factors include a cold-blooded psychopathic personality, combined with an aggressive, stimulation-seeking temperament.
Let’s say you have a child who has some of these traits, like a low heart rate and an apparent lack of empathy. What do you do? Understanding risk factors for violent and criminal behavior can be useful, but they can also harm.
There’s always that argument and that’s a sensible issue of labeling. Rather than talking about cold-blooded fledgling psychopaths, talk about people with some emotion regulation problems.
You could say let’s close door to this; many of kids will age out. Yes, that’s going to be true but they may age out because they have some social experience, which changes their behavior. Maybe what you do in treatment is put together a biosocial setting which will [help that change].
To me as a parent, I’d like to know. I’ve got two boys, both 11. If someone said, [one of them] has a really significant probability of aging into becoming a serious violent offender, but on the other hand, we’ve got new social and biological and psychological interventions that can change this and this is our chance of success or failure. I’d like as parent to make an informed decision. I want the information out there.