GUY RAZ, host:

If you've ever seen the 1955 film "The Night of the Hunter," you'll have come across one of the most psychopathic characters in movie history. Robert Mitchum plays the serial killer Reverend Harry Powell.

The basic story is that Powell's looking for a hidden stash of money. And over the course of the film, we watch as he takes pleasure in the suffering and fear of others.

(Soundbite of movie, "The Night of the Hunter")

Mr. ROBERT MITCHUM (Actor): (as Reverend Harry Powell) I can hear you whispering, children, so I know you're down there. I can feel myself getting awful mad. I'm out of patience, children. I'm coming to find you now.

RAZ: That scene never gets any less creepy. Anyway, scientists may now have a better understanding for what motivates psychopaths. It was long thought that they lack the certain brain function, the one that enables empathy, for example. But it turns out, psychopathic behavior may be tied to the brain's reward system, the same one that causes addiction.

That study has just been published in the journal Nature Neuroscience and one of the authors is Josh Buckholtz. He's a graduate student in neuroscience at Vanderbilt University and he joins me from Nashville.

Welcome to the program.

Mr. JOSH BUCKHOLTZ (Graduate Student, Vanderbilt University): Thank you, Guy.

RAZ: So, describe the difference between the brain of a psychopathic person and a normal person.

Mr. BUCKHOLTZ: We can think of psychopathy as a personality disorder. It's a collection of related traits. There's this emotional and interpersonal dimension. They've been superficial and they lack fear, they lack empathy. And they also exhibit profound socially deviant behavior. They're impulsive. They need excitement. They have poor behavioral controls.

And what prior research has shown is that psychopaths have changes in their brain in regions, like the amygdala and the orbital frontal cortex that are involved in generating emotional experiences. And it's these brain changes that are thought to underlie the first dimension, their inability to experience things like empathy or regret and remorse. So, what wasn't clear, though, is what drove them in the first place. So, we think that these changes in the brain's reward system might promote a focus on reward, on obtaining a reward, be it money or sex or status.

RAZ: Give us an example of how your research would apply to, say, a serial killer like Ted Bundy.

Mr. BUCKHOLTZ: So, what this research lets us know is that there's a new brain system that wasn't previously known to be involved in psychopathy. It's called the mesolimbic dopamine system. What the mesolimbic dopamine system is important for is organizing motivated behavior to obtain a reward.

And because we now know that this mesolimbic dopamine reward system is involved in variation in these impulsive anti-social traits, this might lead the way for future studies that target the system as a way of reducing aggression and anti-social behavior.

RAZ: Just clarify something for us, is it that you either are a psychopath or you are not a psychopath?

Mr. BUCKHOLTZ: That is an area of great debate in the field of psychopathy. Currently, it's thought that psychopathic traits operate along a continuum. So, you can measure a range of psychopathic traits. And this is important for the purposes of our study because the people in our study were community volunteers with no diagnosable psychiatric disorders.

So, the people in our study might be your Machiavellian mother-in-law, your bullying boss, your conniving co-worker, but none of these people were out there committing violent crimes. None of these people had the same level of psychopathy as an incarcerated violent criminal.

RAZ: Why is it important, from your perspective, for us to have a better understanding of this?

Mr. BUCKHOLTZ: Well, there's a very compelling economic reason and that is that crime is extremely expensive. And psychopaths commit more crime than anybody else. And so, we get to make a decision as a society about where we want to spend our money. We can invest upfront in, you know, understanding what makes a psychopath a psychopath so that we can better know how to intervene before they begin to engage in all of these very costly behaviors, or we can spend our money on incarcerating them.

RAZ: That's Josh Buckholtz. He's a graduate student in neuroscience at Vanderbilt University and he's the co-author of a new study that links the brain's reward system to violent behavior in psychopaths.

Josh Buckholtz, thanks so much.

Mr. BUCKHOLTZ: Thank you, Guy.

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