Making a Killer: The Science of Crime

Introduction

As public interest in true crime grows, fueled by Netflix specials, podcasts, and YouTube channels, many have become more intrigued by the reasons behind these infamous crimes. Personally, throughout quarantine I've been sucked in as well. After listening to 98 two-hour long episodes of My Favorite Murder and watching too many episodes of forensic files, it's safe to say I'm obsessed. Like many others, I've found myself asking: "What makes a killer?" Unfortunately, the answer to that question isn't so simple. There is no personality quiz that will spit out a yes or no answer, no brain scan that will tell you whether or not you'll become a murderer. The specific conditions that cause this extreme violence are rare, even though they seem so common because of the sheer amount of attention that is given to these crimes. The "making" of a killer is not black and white. Instead, it is a complex combination of genetics, environment, and free will.

The "Serial Killer" Gene

Brain scans showing the MAOA gene's effect on frontal lobe function source

The idea of a "serial killer" gene has interested the public for years, appearing in movies and TV shows, most notably The CW's Riverdale. Unfortunately, Riverdale's portrayal of this real life gene is less than accurate. MAOA is a gene commonly found in many psychopathic killers that causes low levels of the enzyme monoamine oxidase. Officially titled "the warrior gene" as to not label anyone with the gene as a killer, the gene affects brain activity, resulting in a hyperactive amygdala and a underactive vmPFC.  The amygdala helps to regulate emotions and encode memories while the vmPFC regulates and inhibits reactions to emotions. The gene often results in heightened aggression and violence. Additionally, the gene is X-linked, contributing to the trend of more men than women committing violent crimes.

Image explaining the results of an MAOA mutation source

The important thing to understand is that having the MAOA gene doesn't mean that a person will exhibit violent or aggressive tendencies. If this was true, the world would be overrun with killers and fall into chaos. In fact, researchers have discovered that approximately 30% of the male population has the mutated MAOA gene. Neuroscientist Jim Fallon, a professor at UC Berkeley, has devoted over 20 years of his life to investigating the effects of this gene and what it is that actually creates a psychopathic killer. He argues that specific conditions must be met in order for a person with the MAOA gene to have violent tendencies. A combination of genetics, environment and often some sort of brain trauma act as a catalyst in early childhood, and without these factors, many with the MAOA gene will lead normal lives. The MAOA gene isn't a surefire way of telling if a person is going to have violent tendencies, but instead is used as an explanation after someone has committed a violent crime. When someone like Albert Fish, or Charles Manson commits a crime, scientists look at their genetics and their childhood to look for an explanation as to why. MAOA is often a part of that explanation. 

A graph correlating environment to MAOA activity source

Fallon himself is an incredible example of how having the MAOA gene doesn't make a killer. After studying the gene and its effects for over two decades, he came to a startling discovery. His mother revealed to him that he actually comes from a long line of murderers, one of whom was the infamous Lizzie Borden, another was Thomas Cornell, (yes, like the University) who murdered his brother in one of the first cases of matricide in America. Although it is not confirmed whether or not Borden or Cornell had the MAOA gene, it made Fallon curious about his own potential for murder. He soon had an answer: he too carries the MAOA gene. 

Jim Fallon giving a TED talk about his discoveries

Other Genetic Factors

The MAOA gene isn't the only gene that can cause these violent tendencies, even though it is the most well known and the most well researched. In addition to the MAOA gene, both the CDH13 gene and the HTR2B gene have been linked to the actions of killers and other violent criminals. The CDH13 gene has been included in many studies that look at the affects of genetics in crime. A Swedish research group investigated the DNA of over one thousand Finnish prisoners and came to the conclusion that "two genes are mainly responsible for aggressive behavior." They discovered that every participants DNA contained either the MAOA gene or the CDH13 gene, and occasionally both. The CDH13 is known for its connection with ADHD, bipolar disorder, and schizophrenia, and is responsible for the connections between neurons, specifically aiding with cell signalling. This gene mutation causes a lack of transmembrane and cytoplasmic domains, however the specific mechanism that contributes to the aforementioned conditions remains unknown. 

Additionally, the processing of serotonin has been investigated in its connection to aggression. In 1984, a study looked specifically at genes that were involved in serotonin pathways, HTR2B being the most notable example. They found a stop codon in the HTR2B gene that was associated with both substance abuse and committing "impulsive crimes" like arson and homicide. However, without further research into how this specific gene mutation affects serotonin pathways, it remains unknown exactly how the HTR2B gene results in this higher risk for "impulsive crimes".

Organizer of important studies, made by the Berkeley Science Journal source

Looking to the Future

As more and more connections are made between genetics and crime, an important question needs to be asked: How will genetic evidence affect the justice system? The answer is unclear. Although these genetic factors can cause an increased risk of violent crime, it isn't always the explanation and many are concerned that genetic evidence could negatively impact trials if evidence of the MAOA gene, or other "serial killer" genes, were presented in court. In 2010, an Italian judge made history after allowing the defense in a murder case to conduct "genetic susceptibility testing" to see if the defendant carried any of the genes associated with aggression. This decision caused uproar in the legal community after the findings of this testing caused the sentence of the individual to be reduced from 9 to 8 years. Many question how the introduction of genetic evidence will affect juries. Will they give lighter sentences because the defendant couldn't help themselves, or will they give harsher sentences because they believe the defendant will never change?

Conclusion

There is no definitive answer to what makes a killer, instead there is a number of factors that can contribute to violent tendencies. There is no surefire way to determine if someone will become a killer, and no way to be sure that they will never become one. As genetic and environmental factors are investigated by pioneers like Jim Fallon we may come closer to discovering more about this topic, but until then it will be a subject of public speculation, popping up in various crime novels, podcasts and TV shows. The science of crime is sensitive, and discovery takes time, but as this science continues to develop, it will certainly change the way we deal with crime and even the justice system itself.