It is a common misconception that the link between biology and crime is primarily genetic, yet there is much more to biology than the study of genes. However, our genes do have a profound influence on us, and a great deal of research has been conducted on the genetics of behaviour. As behaviour is highly complex, in almost all cases, any behavioural trait will be influenced by a large number of genes, not just two or three. Therefore, “a gene for crime” or for any complex behaviour cannot exist. Most behaviour is governed by thousands of genes, with each contributing a small amount towards a person exhibiting that behaviour.

Most behaviours are not criminogenic on their own, but under the right circumstances could lead to a person offending. For example, impulsivity is a behaviour that could potentially lead to a criminal event, such as not considering the consequences of stealing a car for a joy ride. Of course, impulsivity could equally result in buying way too many red shoes! How such a behaviour is enacted is greatly influenced by many other factors including socioeconomic status (SES), education, and peers. Therefore, any relationship between genes and behaviour is modulated by myriad genes and the number of genes a person has that influences that behaviour will increase their likelihood of exhibiting that behaviour.

Heritability Studies

A great deal of genetic and environmental research has been conducted using twin and adoption studies. These studies compare the impacts of genes and the environment on behaviour.

Twins are a perfect study group as there are two types of twins: monozygotic (MZ) and dizygotic (DZ). MZ (“identical”) twins began as a single zygote (one egg and one sperm) that, very shortly after fertilisation, divide into two, resulting in two genetically identical babies, aside from small early mutations that may occur (Jonsson et al., 2021). DZ twins result from two zygotes and only share 50% of their genes (actually, we share 99% of our DNA with every other human being, but of the 1% that is different between all people, siblings share 50%). DZ twins in general share 100% of their environment and 50% of their genes, whereas MZ twins, in general, share 100% of both their environment and genes. Comparing behaviours between DZ and MZ twins helps us understand whether environment or genes has a greater influence on a behavioural trait. As both types of twins share the same environment, any differences relate to genetics. A great many twin studies have been conducted globally over the last 100 years and have consistently shown both a heritable component to criminogenic behaviour and an environmental component (e.g., Anderson, 2020a; Kendler et al., 2015).

One problem with twin studies is the assumption that each set of twins share the same environment, but MZ twins, who look identical, may share more of their environment than DZ twins, who look less similar and may be different sexes. Such factors would impact the results (Burt & Simons, 2014). Adoption studies offer a much more powerful method of separating the effects of genes and the environment by comparing adopted children with their adopted and biological families. In such situations, biological parents can only contribute biological effects, and adoptive parents can only contribute environmental effects on the behaviour of adopted children, as the studies focus on children adopted by non-relatives, neatly separating biological and environmental effects. Many large studies conducted worldwide have shown that a child is much more likely to offend if their biological rather than adoptive parents were offenders, and even more likely if both are offenders (Mednick et al., 1987). These findings show both a heritable relationship and the impact of the environment.

Gene X Environment Interactions (GxE)

People with different genetic backgrounds may react differently to the same environment. We know many risk factors influence the likelihood of committing a crime—such as child abuse, low SES, and peer pressure, but most people who experience these environmental factors do not turn to crime and may be exemplary members of society. Likewise, privileged, wealthy people with supportive peers and abuse-free childhoods may still commit many crimes. We now understand that persons with certain genetic backgrounds are more sensitive to specific environmental triggers than others (Mullineaux & DiLalla, 2015). Someone without a predisposition for criminal behaviour may never offend, irrespective of an adverse environment, and a person with a predisposition for criminal behaviour may never offend if they do not experience adversity. Adversity may be any form of hardship, which includes trauma, physical, sexual or emotional abuse, starvation, or any form of severe suffering. For example, as we will discuss later, males with a certain form of a gene for a neurotransmitter or chemical messenger have a higher predisposition for aggressive behaviour only if they are severely physically abused as a child. If they are not abused, that is, they are never exposed to this trigger, they are no more likely to be aggressive than any other male (Caspi et al., 2002). A predisposition together with an adverse environment increases risk but still does not guarantee a criminal outcome because gradients in each either increase or decrease risk (Gajos et al., 2016). Several models predict these variations, such as the diathesis stress model, which suggests that a genotype has a number of different alleles or different gene variants, and each adds a tiny bit of risk (Bersted & DiLalla, 2016). If the person is exposed to a bad environment, then they are very likely to be antisocial, but if they are exposed to a good environment, they may not show any antisocial behaviour at all. So, this model predicts that the basic causes of antisocial behaviour are triggers in the environment interacting with the person’s genotype (Boardman et al., 2014). For example, it has been shown that children with certain risk factors are at greater risk of antisocial behaviour if they experience parental conflict (Feinberg et al., 2007). These findings help identify not only environmental triggers but protective factors that can ameliorate or even eliminate risk.

Epigenetics

The genome is a person’s complete set of genetic instructions or blueprint (the DNA sequence), and it is controlled by the epigenome, an array of chemicals that tell the genome which genes should be turned on (expressed) and which should be turned off. The epigenome can also change in response to experiences, altering the way a gene is expressed—that is, what the gene actually does—without changing the DNA sequence. Therefore, a person’s genome remains the same, but its functions may change in response to experiences (DeLisi & Vaughn, 2015). For example, astronauts Scott and Mark Kelly are MZ twins, but only Scott spent a year in space. See the NASA twins study revealing that space flight can cause genetic changes. Studies of their DNA before and after the space travel showed that, although their DNA remained identical, stressors experienced on the flight had changed Scott’s DNA expression (Garrett-Bakelman et al., 2019).

Interestingly, although only the expression of the genes changes and not the DNA sequence, these epigenetic changes can be passed on to the next generation, so they are heritable (National Human Genome Research Institute, 2016). This very exciting new area is only just being explored, primarily as it relates to healthcare, but some work has been done on criminogenic behaviour that helps explain GxE interactions. The epigenome is changed by the environment to allow the body to respond; changes may occur in neural development or in neurotransmitter or hormonal function, which could impact behaviour. Studies on rodents show that maternal care could result in gene expression changes in the first week of life, with increased maternal care resulting in calmer offspring that exhibit less stress to new environments than those with low maternal care (Weaver et al., 2004). When rat pups were abused for 30 minutes a day during their first week of life, the brain changes lasted a lifetime, resulting in rats that abused their own offspring (Roth & Sweatt, 2011). In both studies, the changes could be reversed with medication.

Many studies on children have shown that early life adversity and parenting decisions have an epigenetic effect on a child’s developing brain that can impact their future behaviour, mental abilities, reaction to stress, and resilience to further adversity, making them less able to cope, and such changes can be transgenerational (DeLisi & Vaughn, 2015). This is a very new understanding and means that the experiences of your parents can epigenetically affect their DNA, which will impact the way your genes and even your children’s genes will be expressed. In other words, a person’s ancestor’s experiences can genetically impact later generations. Studies show that this epigenetic effect can increase antisocial behaviour and callous unemotional aggression, reduce empathy, increase depression and reduce normal stress responses, resulting in a lack of fear of danger or consequences (DeLisi & Vaughn, 2015; Rutter, 2012).

When the communist government of Romania fell in 1989, the world was horrified to see images of hundreds of thousands of children abandoned and warehoused in appalling conditions, without the most basic necessities of life, and no human contact except abuse. This lack of basic care and human contact together with extreme deprivation and institutionalisation meant many of these children exhibited cognitive problems. What 100,000+ Children Taught Us About Neglect in Early Childhood describes some of these issues. Imaging studies showed that these children had less total grey and white matter in the brain and an enlarged amygdala—a part of the brain responsible for dealing with emotions (Mehta et al., 2009). These findings may be a result of developing epigenetic coping mechanisms and reduced responses to extreme institutionalisation. See Romania’s Abandoned Children about the impacts of institutionalisation. In other studies of abused children, epigenetic changes increased their likelihood of developing post-traumatic stress disorder in response to adversity experienced in their adult lives (Mehta et al., 2013). Kayla Bourque is a Vancouver example of a high-risk violent offender with a history of animal abuse who was adopted from a Romanian orphanage. See B.C. animal killer called ‘psychopathic’ for more on Kayla Bourque.

It has long been accepted that experiencing an abusive childhood increases risk for later offending. These studies not only show the environmental impacts of such abuse but now also a major biological impact on a child’s developing brain, making them more susceptible to later environmental triggers, potentially resulting in antisocial behaviour, an inability to deal with stressors, as well as a lack of parenting skills. Moreover, these changes can be transgenerational.

Residential Schools

Epigenetic studies help us understand why atrocities such as residential schools not only had major and long-lasting impacts on the Indigenous children who were abused, but also how the effect of this abuse is magnified as it is perpetuated biologically through the next generations, as discussed in Can Trauma Be Inherited?. Also, when child abuse causes a child to shut down, become socially withdrawn, become unreactive to normal pleasurable experiences and have reduced responses, it has long-term effects on their own relationships with their children, as a Residential school survivor explains the impact on her family.

These intergenerational impacts, although considered here in a scientific context, clearly illustrate the connections between the many aspects of knowledge, including the cognitive, spiritual, emotional and physical elements, which are all parts of Indigenous epistemologies (Doetzel, 2018; Simpson, 2011; Smith, 2012). In many cases, the disruption of these interconnections has also prevented the passage of traditional, ancestral knowledge to subsequent generations (Monchalin, 2016).

Social Implications