Hannibal Lecter | via Comic Vine

Serial killers are defined as people who murder three or more people, in two or more separate events over a period of time, for primarily psychological reasons. Amongst America’s most infamous killers are James Holmes, the Colorado movie killer, Jeffrey Dahmer who killed and ate some of his 17 victims, Ted Bundy who murdered more than 36 women, Juan Vallejo Corona who dispatched more than 25 ranch laborers, and John Wayne Gacy who had 33 victims to his “credit.”

Mass murderers, on the other hand, murder many people, typically simultaneously or over a relatively short period of time. In fact, the FBI defines mass murder as “murdering four or more persons during an event with no “cooling-off period” between the murders. As we have become all too familiar, acts of mass murder frequently end with the killer killing himself, often with a gunshot wound to the head.

John Wayne Gacy
John Wayne Gacy | via Wikipedia

You might think that it is easy to spot people like them because of their strange behavior, odd appearance, or history of mental illness. Let me disabuse you of that notion.

Gacy was, in fact, a pillar of the community. Not only did he perform as a clown at children’s parties, but he was also a married, successful businessman. He did volunteer work in his neighborhood and was active in the Jaycees.

In private, however, Gacy was a serial rapist and murderer of young boys. On three occasions, he even killed two individuals on the same day. When the police finally caught up with him, they found the bodies of 28 young men and boys in the crawl space under his house.


The Sociopath Next Door

If you think that such people are an aberration, coming to us from some alien culture, think again. According to Martha Stout, in her book The Sociopath Next Door, 1 in 25 Americans is a sociopath who has no conscience.

The good news is that the vast majority of these sociopaths aren’t inclined to be violent. They can be found in all walks of life, even in Congress.

Obviously, something is wrong with them, however. But what?


Is it in their genes?

In 1993, scientists reported on a Dutch family, 14 members of whom were sociopaths, involved in aggressive crimes such as bullying, physical violence, rape, and arson. They all had in common a mutation in a gene that makes an enzyme called MAOA.

The function of this enzyme is to break down neurotransmitters such as serotonin, norepinephrine, and dopamine. The ready conclusion was a defective enzyme caused elevated level of the neurotransmitters, driving brain circuits that serve aggressive behavior into a frenzy of overactivity. Case closed? Not so fast.

Several longitudinal studies, following a case for several years and with multiple measurements along the way, have shown that men (the trait is X-linked) with low MAOA were more likely to engage in persistent fighting, bullying, cruelty, and violent crime. But not all of them; only men who had been neglected or abused as children fit the bill. Men who grew up in a normal environment exhibited none of the violent traits.


What about testosterone?

Testosterone is universally maligned as responsible for men’s aggression. The level of testosterone is considered within normal limits if it is between 20% and 200% of the mean. That’s a huge range of normal; however, if the level of testosterone exceeds 400% of the mean, men with these levels are more prone to violence.

In fact, testosterone is an equal opportunity hormone; in spotted hyenas, the first newborn in a litter, be it male or female, will try to eat the rest of the brood within days of birth. It turns out that this vicious sibling has inordinately high levels of testosterone in its brain, much higher than the other hapless siblings.


Could it be in the brain?

The amygdala, two almond-shaped structures deep inside the brain, is the seat of emotions such as rage and fear, which constitute the emotional basis of the fight or flight reaction. Luckily, these seemingly overwhelming emotions are held in check by another, an evolutionarily modern structure in the brain, the prefrontal cortex.

This structure, with the help of some adjacent areas, is the seat of judgment, planning, abstract thinking. It inhibits inappropriate or impulsive behavior and is engaged in constant self-monitoring. So in typical Yin/Yang fashion, the outcome of our behavior must then be the product of the interaction between the amygdala and the prefrontal cortex.


What does neuroimaging show?

Adrian Raine, a neurobiologist at the University of Pennsylvania, studied the PET scans of 41 convicts incarcerated for murder and compared them with those of 41 controls. The murderers’ brains showed what appeared to be a significant reduction in the development of the prefrontal cortex, “the executive function” of the brain, compared with the control group.

Kip Kinkel
Kip Kinkel | via Criminal Minds

Kip Kinkel, a 15-year-old Thurston High School student, was suspended pending an expulsion hearing for being in possession of a loaded, stolen handgun. On May 21, 1998, he murdered his parents and then went on a rampage at his school in Springfield, Oregon leaving two students dead and 25 others wounded. He is serving a 111-year sentence without the possibility of parole.

His brain scan showed a completely “silent” prefrontal lobe; he had nothing to check and balance the raging anger emanating from his amygdala. So is this the answer? It’s not so simple.

Other studies have shown smaller amygdala in violent criminals. The emerging picture is that these individuals probably have a structurally and functionally deficient limbic system, which includes both the amygdala and the prefrontal cortex.


Can you tell a murderer by his brain scan?

It’s a tempting prospect, isn’t it? Except that two neurobiologists who study violent behavior say “don’t even go there.” And they should know; each of them has the brain scan of a violent criminal! One is Adrian Raine, himself. In a remarkable interview with The Guardian, he said as much. “When you have a brain scan that looks like a serial killer’s, it does give you pause,” he said.

The other is John Fallon, a neuroscientist at the University of California-Irvine who has studied the brains of psychopaths for the past 30 years. Fallon made a startling discovery about his ancestry. His direct great grandfather, Thomas Cornell, was hanged in 1667 for murdering his mother. That line of Cornells produced seven other alleged murderers, including Lizzy Borden. “Cousin Lizzy,” as Fallon wryly calls her in an interview with NPR, was accused (and controversially acquitted) of killing her father and stepmother with an ax in Fall River, Mass., in 1882. So naturally, he PET-scanned his own brain. The result was like any murderer’s, his prefrontal cortex was inactive!


We are not slaves to our genes, we can control our brain

The Minnesota study of twins and other genetic studies all converge on the finding that genetics accounts for 50- 60% of psychopathy. The other half is contributed by environmental factors. What are those factors? They are mostly developmental. Alcohol consumption and drug abuse during pregnancy, child neglect, child abuse, head trauma, family violence—the usual suspects.

Do we have to get into the tired argument of nature vs. nurture? Not anymore. Environmental factors, including the ones predisposing to psychopathy, have been shown to chemically alter genes and brain structures.

This is a new field of genetics, called epigenetics. It is not nature “vs.” nurture anymore. Rather, it is nature “and” nurture. Both are equally important.

So, did we finally close the loop? Can genetics, neurobiology, the environment, and epigenetics explain the phenomenon of violence?

Not quite. Many societies have members with genes gone awry, with malfunctioning brain circuits, with males suffering from raging hormones, with children raised in violent homes. But, sad to say, we have the dubious distinction of being the champions of gun violence in the civilized world. So how are we exceptional?


What about our violent culture?

For whatever historical, political or economic reasons, we are the most violent society among all industrial countries. Our gun-related homicides were 3.55 and 6.7 suicides per 100,000 in 2013. For comparison, United Kingdom’s rates were 0.05 and 0.17 respectively.

Events like the shootings at Columbine High School in Colorado shocked us for a few days, but then it wore off. A short while later the Sandy Hook elementary school massacre again shocked us for a few days. But even the horror of a disturbed young man mowing down kindergartners with a semi-automatic .223 Bushmaster rifle also wore off after a while.

sandy hook elementary shooting
Police evacuate children from the Sandy Hook Elementary School | via CNN

The corrosive effect on our culture is incalculable. Subconsciously, we incorporated every downward spiral as the “new normal,” or as Senator Daniel Patrick Moynihan memorably puts it, we define deviance down.

By all measures, such levels of violence would qualify as an epidemic. And, like any epidemic, it is hard to contain. We are now infecting other cultures with our disease as illustrated in this recent headline in the NYTIsraeli Terrorists, Born in the U.S.A.

So now that we understand the full range of factors feeding into the phenomenon of violence, surely we are in a position to tackle the problem using the best science our country is capable of, right?


Can we get to the root of the problem?

The shocking answer is that we can’t! Not that the problem is so intractable as to defy investigation, or that our scientists are incompetent. We have some of the best science on the planet, but some of the worst politicians.

For nearly 20 years, Congress has pushed the Centers for Disease Control and Prevention (CDC) to steer clear of firearms violence research. They even threatened to defund the CDC, the premier epidemiological research organization in the world, if one dollar goes into gun-violence research.

The result is predictable. Review of the literature on violence shows a lack of rigorous studies, and most of the literature is anecdotal and speculative.

You can put it squarely at the feet of the National Rifle Association (NRA) and their political lackeys. All the PET scans, fMRI studies, psychological profiles, and political platitudes will not shed more light on the problem. We need a lot more epidemiological research, and maybe more than a bit of outrage.

If we fail to address this head-on, we are destined to have the pattern keep on repeating itself. It is in our hands, as Pogo said, “We have met the enemy, and it is us.”

Dov Michaeli, MD, PhD
Dov Michaeli, MD, PhD loves to write about the brain and human behavior as well as translate complicated basic science concepts into entertainment for the rest of us. He was a professor at the University of California San Francisco before leaving to enter the world of biotech. He served as the Chief Medical Officer of biotech companies, including Aphton Corporation. He also founded and served as the CEO of Madah Medica, an early stage biotech company developing products to improve post-surgical pain control. He is now retired and enjoys working out, following the stock market, travelling the world, and, of course, writing for TDWI.


  1. When there pieces of $hit are caught, tried, and CONVICTED, what aren’t THEY simply gotten rid of?Why do liberal cry-babies want to stick them in prison to the tune of some$30,000.00 to $50,000.00 a year? JUST EXTERMINTE these evil creatures!!!

  2. Correction: James Holmes was NOT a serial killer. He’s a mass murderer by all means. 12 fatalities and 72 injured in one single event. He outlined his considering serial killing as an option for carrying out his murder plan in a notebook. He ruled it out as too risky and high chance of getting caught early on.


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