Unlocking the Chemistry of Crime: Serotonin

This video featuring Adrian Raine provides the perfect introduction to the connection between neuroscience and crime.  The exact scene can be found here.


It exists everywhere; in every city, in every country, every day of the year. In movies like Minority Report, the government has  implemented programs to catch murderers before they act. But in the real world, criminals pounce on their prey daily. When one thinks of crime, evil and devious convicts come to mind. But what if crime can be entirely explained through science? Can what urges a single person to perform an act so disastrous, so devious, so illegal, be revealed through science?

Crime & The Human Brain

This image of the human brain displays the different sections different sections.  The red section is the frontal lobe, the green section is the temporal lobe, the yellow is the occipital lobe, the purple section is the insular cortex, the black section is the brain stem, and the blue section is the cerebellum.

In a recent study, brain scans were administered to individuals with an antisocial personality disorder (a common characteristic of many criminals). This disorder typically causes one to “have no regard for right and wrong…[and] violate the law and the rights of others,” according to the Mayo Clinic, a non-profit medical research organization. The brain scans of those with the disorder were compared to a control group without mental disorders. The results concluded that those suffering from the disorder had an 18% reduction in size of their brain’s middle frontal gyrus and a 9% reduction in the size of the orbital frontal gyrus. Both of these sections are important parts of the brain’s frontal lobe.


This diagram outlines the different sections of the brain.  The frontal gyrus, which was examined in the study above, is highlighted in orange. Braunstein was a convicted sex offender. One can see the smaller frontal lobe, which is also not as activated in Braunstein as in the normal brain. The frontal lobe controls decision making, problem solving, control of purposeful behaviors, consciousness, and emotions.  The frontal lobe is part of the cerebral system which deals with goal related behavior, distinguishing between “good” and “bad” and resulting impulsive decisions.

Another study examining psychopaths versus a control group demonstrated that psychopaths have a thinner outer layer of the cortex in the amygdala and an 18% smaller size of the brain in that region. Adrian Raine, chair of the Department of Criminology reported that the amygdala is the part of the brain responsible for emotion; thus, psychopaths experience inhibited emotional responses.

Beyond psychopaths, convicted criminals tend to demonstrate behavioral differences compared to the average civilian. Raine explained, “There is a neuroscience basis in part to the cause of crime.” Many of the signs indicating one as a criminal can be found in the brain early on, before an individual develops into their psychopathic or criminal tendencies. A study conducted by Nathalie Fontaine found that those who tended to be callous and unemotional as children had a higher risk of becoming psychopaths in adulthood.

But what is it exactly that makes one predisposed to these criminal tendencies?

What is the difference in the chemical makeup of the brain of a criminal?

How does this all tie back into chemistry?

What is Serotonin?

Neurotransmitters are chemicals in the brain that are responsible for sending signals to different cells throughout the nervous system. Serotonin is one of the most abundant transmitters in the dorsal raphe section, helping to send signals between different parts of the brain. Scientists estimate that serotonin influences most of the brain’s 40 million cells, and due to this function, serotonin has a large impact on one’s psychological functions. In particular, serotonin is known to interact with brain cells that regulate mood, appetite, sleep, memory and learning, body temperature, and some social behavior. When serotonin activity or serotonin levels are disrupted, a person’s psychological state can be altered; this kind of phenomenon is often seen in disorders such as depression and schizophrenia.

This image shows how serotonin is synthesized in the dorsal raphe, and moves throughout the brain, focusing in the frontal cortex.         

Serotonin and Crime

The connection between serotonin and criminal activity is quite clear. Criminals have been found to have a smaller frontal cortex and a smaller amygdala. The central amygdala is part of the section of the brain that releases corticosterone. Corticosterone activates the dorsal raphe, which increases serotonin (5-HT). The direct relationship between the amygdala and serotonin can be hindered when the amygdala is reduced in size, as seen in criminals. Criminals produce less serotonin or have serogentonic dysfunction. An abundance of serotonin can also have negative affects on people causing confusion and induced criminal tendencies. On the other hand, non-criminals have a balanced level of serotonin.

The molecular compound of serotonin (5-HT)

Furthermore, impulsive aggression, a characteristic linked with criminals, is explained through the brain being unable to regulate impulsivity. Dysfunctional interactions occur between the serotonin systems in the prefrontal cortex, which is an important mechanism in controlling aggression. This also links serotonin to certain mental disorders such as depression, suicidal behavior, and drug addiction. Serotonin hypofunction (abnormally low function) demonstrates how one can be predisposed to impulsive aggression, and therefore criminal activity. Lower levels of serotonin (5-HT) have been linked with impulsive aggression (Dongju, Kennealy, & Patrick). One whose genes were programmed to produce less serotonin are therefore more susceptible to criminal activity.

The prefrontal cortex controls behavioral aggression, and impairment of this area would increase impulsive aggression. As seen in criminal brain scans, psychopaths are born with reduced frontal cortexes, increasing their aggression. Serotonin also acts as an inhibiting action in the brain. One of its purposes is to inhibit aggression. However, if serotonergic dysfunction occurs, aggressive behaviour will not be limited, therefore inclining one towards criminal activity. No matter the type of crime, lower levels of serotonin have been linked with aggression, showing how science can be broken down to explain crime. All variations of criminal activity can be simplified down to dysfunctional serotonin. One who is simply born with a defect in their brain, impairing their serotonin production, is predisposed to crime.

The red defined part of this image are the Raphe nuclei (the neurons that make up serotonin). Mentioned in this figure are a few of the behaviors that are affected because of the level of serotonin including mood and aggression.

Connections to Chemistry

Serotonin is produced through a process known as the biochemical conversion process. The process begins when two enzymes, tryptophan hydroxylase (Tph1) and tryptophan hydroxylase-2 (Tph2) help to convert tryptophan to 5-hydroxytryptophan. Decarboxylase enzymes then help convert 5-hydroxytryptophan to serotonin.

Enzymes play a crucial role in the production of serotonin. Therefore, the kinetics behind these enzymes are important to the production of serotonin. Enzymes are responsible for catalyzing, or speeding up, chemical reactions by lowering their activation energies. Specifically, enzymes can lower activation energies by providing a template for reactants to come together, stretching the molecules of a substance, providing a proper microenvironment for a reaction or directly participating in a reaction. Enzymes are ultimately responsible for speeding up the production of serotonin. As a result, a lack of these enzymes could cause dangerously low amounts of serotonin, which could stall the communication between neurons and impact a person’s behavior and psychological state. For example, low levels of serotonin can induce depression, anxiety, post-traumatic stress disorder and attention deficit hyperactivity disorder (ADHD). In the end, serotonin enzymes are a major player in human psychiatric disorders. Thus, a dysfunction with the enzyme can upset serotonin levels, and this chemical process can induce the decision to commit a crime.

 Through the enzyme actions and structure of the brain, one can see the clear connection between serotonin and criminal activity. As mentioned, lower levels of serotonin (5-HT) has been linked with impulsive aggression, and high levels of serotonin have been linked to aggression and confusion. Low levels of serotonin are most prevalent during the winter months, explaining why some people suffer from depression during cold and dark winter months.

Meanwhile, the warmer temperatures in summertime result in higher levels of serotonin. A link can be drawn between serotonin production and thermochemistry. At higher temperatures, the rate of a reaction is increased.  At higher temperatures, the production of serotonin is increased, beyond a “normal” limit in most criminals during the summer months.

James Alan Fox carried out an experiment in Columbus, Ohio, between January 1, 2007 and

December 31, 2007. He displayed the link between crime and temperature. the peak of this graph displays that violent crimes were prominent during the summer months. An increase in temperature results in an increase of serotonin which in turn leads to an increase in crime rates.


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