Neurochemistry is not often covered in criminology texts as it is highly specialised, yet some of the most exciting discoveries within biosocial criminology result from our rapidly increasing understanding of brain chemistry and its interactions with the environment. The brain controls all behaviour, and we are beginning to understand how imbalances in certain brain chemicals can affect health and behaviour. Neurotransmitters are chemicals involved in communication between nerve cells. Neurotransmitters are synthesised by the catalysis of an amino acid precursor. Once released, they bind to a receptor cell to transmit a message, then break down into their metabolites. Each of these stages is under genetic control and involves many genes. Imbalances and interactions between these neurotransmitters and our environment can have profound effects on our behaviour.

Serotonin

Low levels of serotonin, a behavioural inhibitor, have been linked to impulsive-aggressive behaviour in extensive research conducted globally over six decades. A great deal of evidence shows that serotonin plays a role in impulsive-aggressive behaviour, which makes sense because serotonin regulates the stress response in the brain, meaning that it impacts the way we handle and react to stress, in general having a calming effect.

Many studies have shown that dysfunction at any stage of the serotonergic system can reduce levels of serotonin, and alleles have been linked to increased levels of suicide (Antypa et al., 2013). For example, one such gene has two alleles referred to as S and L. We all have two alleles for every gene, so a person could have SS, SL or LL. The S allele makes a person more sensitive to stress, and the L allele makes them more resistant. In a seminal, long-term study of almost 900 people, Caspi and colleagues looked at 21- to 26-year-olds who had suffered multiple stressors (Caspi et al., 2003). They found that almost half of those with SS developed depression, and 11% attempted suicide as opposed to only 17% of those with LL who developed depression and 4% who attempted suicide (Caspi et al., 2003). This early study has been repeated many times around the world and again highlights the link between a person’s genetic background and their environment. Numerous studies have also linked low serotonin levels with aggressive behaviour (Brown et al., 1979; Glick, 2015), psychopathy (Glenn, 2011), impulsivity (Linnoila et al., 1983), and an inability to learn from negative experiences (Helmbold et al., 2015).

Overall, the studies on serotonin indicate that any sort of dysfunction in the serotonergic system has an adverse effect on behaviour. However, in many cases, we can reverse the effects. Neurotransmitters are synthesised from building blocks or precursors of basic amino acids, and the precursor for serotonin is tryptophan. This amino acid can be obtained through diet. Even people with perfectly normal serotonin systems may get edgy or upset if their tryptophan levels drop. In contrast, increasing a person’s tryptophan levels often results in a feeling of peace and general well-being. One source of tryptophan is turkey, and this may account for the overall general feeling of benevolence often felt after eating a large turkey dinner. Therefore, such chemical imbalances may be successfully treated, often with something as simple as a change in diet.

Dopamine

Dopamine is often considered a pleasure or reward neurotransmitter as it produces many positive feelings in response to the normal pleasures in life, such as love, sex, and food. Many drugs target the dopamine system, causing a rapid increase in dopamine levels, resulting in euphoria (Volkow et al., 2003). In some cases, increases in dopamine can increase aggression, particularly in people with deficits in the normal reward system who do not gain pleasure from normal pleasurable activities and need greater stimulation (Blum et al., 2015). This means that such people are not happy with the usual simple pleasures like sex, chocolate or the company of friends, for example, so they need to push for greater excitement. A number of alleles within the dopamine system have been linked to increased risk for anti-social behaviour (Blum et al., 2015; Young et al., 2002; Yun et al., 2015), but remember again that each allele only contributes a very small increased risk.

Monoamine Oxidase (MAO)

MAO is an enzyme responsible for breaking down several neurotransmitters, including serotonin and dopamine, so although not a neurotransmitter itself, it still impacts neurotransmitter levels. Many studies have linked low MAO levels to a variety of antisocial behaviours, including aggression (Antypa et al., 2013); however, perhaps the most interesting studies on MAO are those that show a distinct GxE interaction. The two MAO enzymes, A and B, are the result of two different genes on the X chromosome, making them sex-linked. Sex-linked traits are mostly expressed by males, as females have two X chromosomes. If one X has a deleterious allele of a gene, it is usually masked by the presence of a normal X. Males, however, only have a single X and a Y chromosome, so any genes on the X chromosome will be expressed.

The MAOA gene has several different alleles, with some resulting in normal levels of MAOA (MAOA-H) and others producing low levels (MAOA-L). In one of the first studies to find a GxE interaction, Caspi and colleagues considered a cohort of 442 male adults, identifying 154 who had been abused as children, 33 severely (Caspi et al., 2002). Each was evaluated for MAOA-H or MAOA-L and assessed for antisocial behaviour based on convictions for violent crime or exhibited antisocial behaviour. Only 12% of the entire cohort had MAOA-L, yet they accounted for 44% of violent crime convictions. A significant relationship was found between MAOA-L, child abuse and criminal convictions. Of the severely abused, 85% exhibited antisocial behaviour. Men with the normal MAOA-H alleles rarely exhibited violent behaviour, even if they had been severely abused as children (Caspi et al., 2002). It is important to understand that the risk of violent behaviour only occurred when the genotype was combined with child abuse (Caspi et al., 2002). This is fascinating as people have often looked at a violent criminal and said, “Well, what did you expect, when he had such a violent upbringing?” Immediately, however, someone else will say, “My best friend was brutally abused as a child, and he’s not at all violent.” This seminal work was the first to demonstrate at least one pathway to explain why an abusive childhood may lead some to violence and not others, and it shows protective factors—a safe, stable home life in childhood can ameliorate such risk factors.