On the outside, violent offenders come in all shapes, sizes, colors and ages. But on the inside, research finds that they may share some traits. Here's a look at some of the biological risk factors psychologists and others have linked to violence — and the interventions they're testing to reduce that risk.

Brain structure and function

The amygdala — a part of the brain involved in fear, aggression and social interactions — is implicated in crime. Among the research that points to this link is a neuroimaging study led by Dustin Pardini, PhD, of the University of Pittsburgh. His team found that 26-year-old men with lower amygdala volumes were more than three times more likely to be aggressive, violent and to show psychopathic traits three years later than men of the same age with more normal-sized amygdalas — independent of factors including history of violence and social background (Biological Psychiatry, 2013).

Other research, such as an fMRI study led by psychologist Andrea Glenn, PhD, of the University of Alabama, suggest that amygdala functioning — not just size — is also more likely to be reduced among those with psychopathic tendencies (Molecular Psychiatry, 2009).

At least one study indicates that such deficits may appear long before people commit crimes. Adrian Raine, DPhil, of the department of criminology at the University of Pennsylvania, led a study with Yu Gao, PhD, at CUNY-Brooklyn that examined fear conditioning, which is dependent on amygdala function, in a group of 1,795 3-year-olds. The researchers put electrodes on the children's fingers while repeatedly playing two tones: one that was followed by a loud, unpleasant sound and another that was played alone. Subsequently, the difference in sweat responses to each tone by itself yielded a measure of each toddler's fear conditioning. Twenty years later, the team identified participants who had gone on to commit crimes and compared them with noncriminal counterparts, matching them on gender, ethnicity and social adversity. They found that those children who went on to commit crimes had "simply failed" to demonstrate fear conditioning, Raine says. In other words, they were fearless when most of us would be fearful. This finding suggests that deficits in the amygdala, which are indirectly identifiable as early as age 3, predispose to crime at age 23 (The American Journal of Psychiatry, 2010).

The anterior cingulate cortex (ACC), which plays a major role in behavior regulation and impulsivity, has also been linked to crime. Psychologist Kent Kiehl, PhD, and colleagues at the University of New Mexico used fMRI to look at the brains of nearly 100 adult male inmates while they completed a cognitive task involving inhibitory control. They found that prisoners with lower ACC activity were twice as likely to reoffend four years after they left prison than prisoners with higher ACC activity (PNAS, 2013). While such studies need replication and extension, Raine says, they are "proof of the concept that there may be added value with bringing on board neurobiological information, including brain imaging information, for future prediction of violence."

Change brain to change behavior

If we know that certain brain characteristics may predispose some people to violence, what can we do about it? Intervene — and the earlier, the better, says Raine, author of "The Anatomy of Violence" (Random House, 2013).

In one intervention, for example, he and colleagues found that 3-year-olds who had been assigned to an enrichment program focused on nutrition, exercise and cognitive skills had better brain functioning at age 11 and a 34 percent reduction in criminal activity at age 23 when compared with a control group that did not receive the intervention (American Journal of Psychiatry, 2003). Intervening even earlier, David Olds, PhD, of the University of Colorado, has found that pregnant low-income mothers who were visited regularly by home nurses who talked to them about health, education and parenting were less likely to have children who were arrested by age 15 (Infant Mental Health Journal, 2006).

Even simple interventions may make a difference. In one preliminary study, prisoners assigned to a 10-week yoga class improved their impulse control (Journal of Psychiatric Research, 2013). In an earlier randomized-controlled trial of British prisoners, those who received vitamin, mineral and essential fatty acid supplements committed an average of 26.3 percent fewer offenses than those who had received the placebo. They also showed a reduction in offenses of more than 35 percent, while the placebo-taking prisoners' records remained stable (British Journal of Psychiatry, 2002). A study in the Netherlands replicated the effect, and now Raine is testing a similar intervention for children.

The bottom line, he says, is that "biology is not destiny. We can change the biological roots of crime and violence — there's no question about it."

—Anna Miller

Hear Dr. Adrian Raine's plenary address on this topic at APA's 2014 Annual Convention in Washington, D.C., Aug. 7–10.