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Netflix’s Killer Inside is the latest documentary to pose the question, “what role does traumatic brain injury play in criminal activity.”

Following the story of Aaron Hernandez, it looks at the potential link between the prevalence of concussions and CTE in the NFL, and violent crimes. The idea of CTE having possible ties to violent crime is no new phenomenon. The 2016 film, Concussion was based on Dr. Bennet Omalu, who poses that chronic traumatic encephalopathy (CTE) may be responsible for O.J. Simpson’s links to crime.


Aaron Hernandez was a tight end for the New England Patriots. He had run-ins with the law both before and throughout his NFL career. A suspect in a shooting in 2007, a double homicide in 2012, and a shooting in 2013, Hernandez was not convicted of murder until after the murder of Odin Lloyd.

In 2013, Hernandez was charged with first-degree murder, as well as five other gun-related charges. Upon his conviction in 2015, he was sentenced to life in prison without the possibility of parole.

In 2017, Aaron Hernandez was found hanging from his bedsheets and was pronounced dead at the hospital. Hernandez had consumed K2 (synthetic cannabis) within 30 hours of his death, a drug known to cause psychosis. While CTE could have potentially lead to his suicide, the drugs could also have been a key factor.

After his death, Hernandez’s family donated his brain to a CTE Research Center. The results showed clear signs that Hernandez had severe CTE damage, likely as a result of his high-impact football career. Although it’s unclear whether the brain trauma, drugs, or both were responsible for his suicide, it is highly-suspected that CTE played a role in his aggressive behaviour and violent crimes.

This adds one more name to the list of NFL players with CTE who have been convicted of violent crimes. Studies have found that there is a clear link between violent behaviour and CTE. With concussion and CTE research in the spotlight, they may play a role in criminal defence.


There is a clear link between traumatic brain injury and violent criminal charges, yet there remains a lot of unknowns. Concussions have really only become a major focus of study within the last decade. As a result, how much brain injuries affect behaviour is still a question mark.

Even with an identifiable relationship between aggression and CTE, it’s only a small portion of people suffering CTE who have exhibited these behaviours. Boston University research suggests that as much as 99% of NFL players may suffer from chronic traumatic encephalopathy, making aggressive behaviour more of an outlier.

An additional challenge is that, so far, there is no known way to diagnose CTE while the patient is alive. It is only identifiable after an autopsy, as the doctors need to study slices of brain tissue.

Once a way to accurately diagnose this brain trauma while alive is discovered, this could have an impact on the use of CTE as a criminal defence. For the time being, doctors can only make a guess based on symptoms, behaviour, and history.


Having a TBI will not prevent someone from going to jail. In prison populations, studies have found high instances of people who have, or have had, a traumatic brain injury. As we learn more about how these injuries affect behaviour, it is possible they will one day play a larger role in criminal defence.

While traumatic brain injury is not a criminal defence, it is possible for it to be a mitigating factor. Although a TBI is not enough to be found not criminally responsible, it is something judges may consider in sentencing.


If you or a loved one are facing criminal charges in Toronto, a criminal defence lawyer is your best option. They will help you understand your charges, your options, and the possible outcomes.

William Jaksa is a Toronto criminal defence lawyer with over a decade of experience, and expertise in criminal law. Contact William Jaksa today for a consultation.

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