The brain is the seat of all behaviour, so obviously damage to the brain is likely to impact behaviour, including, potentially, antisocial behaviour. By design, the brain physically changes with experience and is another complicated mix of genetics and environment.

Head Injuries

Most behaviours are controlled by complex connections within the brain and not by one part alone. However, the frontal lobe, found from the forehead region above the eyes to the midway of the skull, is heavily involved in inhibiting inappropriate behaviour, aggression and impulsivity. This area is at the front of the head so it is more likely to be injured in an accident or assault. People with a frontal lobe injury often lose their social graces, self-control, and patience, and may experience personality changes, develop anxiety or depression, demand instant gratification, or have poor planning skills (Lane et al., 2017). In 1848, Phineas Gage was a railroad construction supervisor when an accident drove a metal bar through his cheek and up through the top of his head, destroying his frontal lobe. Miraculously he survived, but his kind, polite, gentle personality was gone, and he became violent, irritable, and irresponsible. His friends said he was “no longer Gage” (Damasio et al., 1994, p1102). His memory and ability to do his job, however, had not changed. Similar changes have been seen in modern patients with frontal lobe damage (Damasio et al., 1994), and we are only just beginning to understand the impacts of repeated concussions in professional athletes, chronic traumatic encephalopathy and the relationship to antisocial behaviour (Stern et al., 2011).

The brain is not fully developed at birth but continues to develop for many years, influenced by experience. Most brain development is completed around the mid-twenties, which is important as the law considers an accused to be an “adult” and entirely responsible for their actions by the age of eighteen or even younger, yet the brain is still developing at this age. In particular, the prefrontal region, the last to mature, is especially important in developing social behaviour, appropriate responses, cognition, abstract thought, and inhibiting inappropriate behaviour (Arain et al., 2013). Damage to the developing brain is, therefore, particularly harmful, as children and youth have not yet developed the inner control mechanisms or socially acceptable behaviour of adulthood (Anderson, 2020b).

Child abuse commonly results in brain injury as even a single blow can cause damage. Even a mild traumatic brain injury (TBI) can have major effects, particularly on social and emotional maturity and reduced cognition, which can lead to poor social interactions and peer relationships and long-term risk (Mychasiuk et al., 2014). Child abuse, therefore, not only creates a dysfunctional social environment but may result in lasting behavioural damage.

Many studies have shown that incarcerated youth have much higher rates of TBI than non-incarcerated youth (Gordon et al., 2017; Hughes et al., 2015), recidivism is increased in youth with TBIs (Williams et al., 2010), and in most cases the TBI predates the offence (Lewis et al., 1988). Not only can such an injury result in a youth being more likely to commit a crime, but TBI-related cognitive impairment, language deficits, comprehension, and social skills mean that the youth is also at a greater disadvantage when trying to navigate the highly complex legal system, which requires excellent communication skills and a clear understanding of all proceedings, rights, and legal advice, putting them at even greater risk (Wszalek & Turkstra, 2015).

TBI in adults has also been frequently linked with violence and crime, with some TBI sufferers exhibiting a loss of control, temper, increased physical and verbal violence, agitation and frustration (Bannon et al., 2015). In a meta-analysis of studies of incarcerated men and women in US prisons, 60% had suffered at least one TBI (Shiroma et al., 2010), with one study reporting 88% (Diamond et al., 2007). In a meta-analysis of studies from the US, the UK and Australia, prevalence ranged from 9.6 to 100%, with an average of 46% (Durand et al., 2017). In Canada, the prevalence of TBIs was over 50% in incarcerated men and 38% in incarcerated women, with many reporting multiple TBIs (Colantonio et al., 2014). Higher recidivism rates are also seen in adult offenders with TBIs as well as an earlier age of first offence, faster re-arrests, greater violence, increased sentences, and infractions while incarcerated (Ramos et al., 2018; Ray & Richardson, 2017).

Brain Disorders

Brain damage can also result from disease or toxins. Stroke, brain tumours, meningitis, alcoholism, and cannabis use can all damage or cause changes in the brain (e.g., Arain et al., 2013; Burns & Swerdlow, 2003). For example, a happily married, middle-aged schoolteacher suddenly developed an obsession with child pornography, was inappropriately sexual with children, and eventually sexually assaulted his own young stepdaughter. He was convicted and ordered to enter a treatment program from which he was expelled due to inappropriate sexual behaviour towards other patients and the instructor. His inability to complete treatment resulted in an order to serve time, but before being sent to prison he complained of violent headaches and said he was scared he might rape someone. An MRI scan showed a large tumour in the orbito-frontal region of the frontal lobe of his brain, the area responsible for judgement, appropriate social behaviour and self-control (Burns & Swerdlow, 2003). The tumour was removed, and his behaviour returned to normal. He successfully completed treatment and returned home to his wife and stepdaughter. Sometime later, he began secretly collecting child pornography again. A scan revealed that the tumour had returned; it was removed, and his behaviour returned to normal. This case indicates a major ethical dilemma—was he responsible for his actions? It seems clear that his criminal behaviour was directly related to a medical condition over which he had no control. However, he later stated that “somewhere deep, deep, deep in the back of my head, there was a little voice saying ‘you shouldn’t do this’” (Glenn & Raine, 2014, p.58). This indicates that even at his worst, he was still able to form mens rea or the ability to understand that his actions were criminal.