The ongoing rash of police using deadly force against minority citizens has triggered a search for a universal cause—most commonly identified as racism. Such soul searching is understandable, especially in light of the racist e-mails uncovered in the Ferguson, Mo., police department by the U.S. Department of Justice's investigation into the death of 18-year-old Michael Brown.

To whatever extent prejudice still percolates in the minds of a few cops in a handful of pockets of American society (nothing like 50 years ago), it does not explain the many interactions between white police and minority citizens that unfold without incident every year or the thousands of cases of assaults on police that do not end in police deaths (49,851 in 2013, according to the FBI). What in the brains of cops or citizens leads either group to erupt in violence?

An answer may be found deep inside the brain, where a neural network stitches together three structures into what neuroscientist Jaak Panksepp calls the rage circuit: (1) the periaqueductal gray (it coordinates incoming stimuli and outgoing motor responses); (2) the hypothalamus (it regulates the release of adrenaline and testosterone as related to motivation and emotion); and (3) the amygdala (associated with automatic emotional responses, especially fear, it lights up in response to an angry face; patients with damage to this area have difficultly assessing emotions in others). When Panksepp electrically stimulated the rage circuit of a cat, it leaped toward his head with claws and fangs bared. Humans similarly stimulated reported feeling uncontrollable anger.

The rage circuit is surrounded and modulated by the cerebral cortex, particularly the orbitofrontal cortex, wherein decisions are made about how you should respond to a particular stimulus—whether to act impulsively or show restraint. In her 1998 book Guilty by Reason of Insanity, psychiatrist Dorothy Otnow Lewis notes that when a cat's cortex is surgically detached from the lower areas of its brain, it responds to mildly annoying stimuli with ferocity and violence, not unlike a convicted killer improbably named Lucky, who had lesions between his cortical regions and the rest of his brain. Lewis suspects that Lucky's lesions were responsible for his savage stabbing of a store clerk.

In healthy brains and under normal circumstances, cortical self-control usually trumps emotional impulses. In certain conditions that call for strong emotions, such as when you feel threatened with bodily injury or death, it is prudent for the rage circuit to override the cortex, as in a case of a woman named Susan described by evolutionary psychologist David M. Buss in his 2005 book The Murderer Next Door. As her cocaine-fueled abusive husband advanced on her with a hunting knife screaming, “Die, bitch!” Susan kneed him in the groin and grabbed the knife. What happened next is what sociologist Randall Collins calls a “forward panic”—an explosion of violence akin to the wartime massacres at Nanking and My Lai and the beating of Rodney King by Los Angeles police officers. “I stabbed him in the head and I stabbed him in the neck and I stabbed him in the chest and I stabbed him in the stomach,” Susan testified at her murder trial, explaining the 193 stab wounds resulting from her uncontrollable urge to avenge her abuse. Such emotions evolved as an adaptation to threats, especially when there is not time to compute the odds of an outcome. Fear causes us to pull back and retreat from risks. Anger leads us to strike out and defend ourselves against predators or bullies.

A charitable explanation for why cops kill is that certain actions by suspects (running away, or resisting arrest, or reaching into the squad car to grab a gun) may trigger the rage circuit to fire with such intensity as to override all cortical self-control. This may be especially the case if the officer is modified by training and experience to look for danger or biased by racial profiling leading to negative expectations of certain citizens' behavior.

Future police training should include putting cops in threatening situations and giving them techniques for diffusing the outcome. In their 2011 book Willpower, Roy F. Baumeister and John Tierney describe methods for suppressing such impulses. In turn, citizens should remember that cops are working to protect us from threats to our security.