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Can a Brain Injury Impact Criminal Behavior?

By Sanjay Adhia, M.D.

A traumatic brain injury might impact reasoning.

Could it cause someone to commit a crime?


Brain injuries are complex and nuanced, and so is behavior.

In my experience, when there is a possibility a crime was committed by a perpetrator who has a brain injury, a close look by a physician with forensic and brain injury expertise is important.

–Sanjay Adhia, MD

Brain Injury or Disease: Medical-Legal Considerations

Competency Considerations

  • Decision-making (e.g., confusion following a blow to the head or dementia)
  • What the criminal defendant can understand (cognition) about the wrongfulness of their actions, (if proven), aka mens rea [1]
  • Competency to stand trial (CTST)
  • Not guilty by reason of insanity (NGRI) – the “Insanity Defense” [2]

Convicted Criminals with a History of Brain Injury

About half of all incarcerated individuals have a history of brain injury. While correlation is not causation, it always calls for a closer look.

How Brain Injuries Affect the Brain and Behavior

Brain injuries vary in degree and diagnosis. Please read my article Brain Injuries Explained for a more detailed medical discussion.

Traumatic Brain Injury (TBI)

Traumatic brain injury (TBI) is a type of acquired brain injury. It is brain dysfunction caused by an outside force. TBI can be classified by biomechanical forces. Three types of forces are blunt trauma, penetrating injuries, and blast overpressure injury.  

Not all TBIs are the same, nor do they all impact the same part of the brain or in the same way.

Concussions or Mild Traumatic Brain Injuries (mTBI) and Post-Concussive Disorder

The symptoms of a concussion are usually shorter-term in nature with a higher likelihood of recovery. Post-concussive disorder symptoms occur in the first 7-10 days and typically resolve within 3 months, though they can persist for more than a year.

Dementia, Alzheimer’s and Other Neurocognitive Disorders

Dementia, Alzheimer’s and some neurocognitive disorders (NCDs) are conditions that impact behavior, but generally develop slowly.

NCDs can occur directly from TBI, and TBI elevates the risk of developing Alzheimer’s disease and additional TBIs. 

Dementia in boxers or football players, for example, may follow years of repeated concussions or blows to the head.

Chronic traumatic encephalopathy (CTE) is often associated with football players or boxers and is caused by repetitive head injuries.

Brain Injury and the Victim

A victim of a crime might be susceptible to a crime due to a neurocognitive disorder.

For example, a criminal charge of fraud where a neurocognitive disorder is suspected may require an evaluation of the victim’s mental status.

Elder abuse, testamentary and contractual capacity and undue influence cases can be factors in a criminal case, though we tend to see these issues play out in Probate Court. The criminal defendant is the focus of this article.

Brain Injury and Behavior

An injury that impacts the ability to perform activities of daily living (ADLs) in a person might not impact impulsive behavior or moral judgment—relevant to competency and mens rea.[3]

Impulsive Behavior

Impulsive behavior is another possible feature of a TBI. It may be exculpatory, depending on medical details, diagnosis, and interpretation under the law in a particular state. If a medical reason for impulsive behavior is not exculpatory, it could be a mitigating factor in sentencing.

Psychiatric Disorders

Psychiatrically, “irresistible impulse” may be related to impulse-control disorder (ICD), a personality disorder, bipolar disorder, or another psychiatric diagnosis.

Antisocial personality disorder (ASPD), which shares features with psychopathy, is not an admissible defense in every state.

Parsing TBI vs. other psychiatric disorders or another factor means that the crossover between Forensic Psychiatry and Brain Injury Medicine is uniquely helpful in considering impulsive behavior and/or impulsive behavior disorder.

Cognitive vs. Volitional Prong

Courts have addressed whether an insanity defense reflects volition or cognition.

Cognitive Prong

The cognitive prong of an insanity defense is explained:

“…At the time of committing the act, the accused was laboring under such a defect of reason, from disease of the mind, as not to know the nature and quality of the act he was doing or, if he did know it, that he did not know what he was doing was wrong.”[4]

Volitional Prong and the Irresistible Impulse

The volitional prong is being unable to conform to the law due to an irresistible impulse or the inability to resist the commission of a crime due to mental defect, such as a brain injury or a psychiatric disorder. [5][6]

For example, someone who is experiencing a manic episode, which could be a feature of bipolar disorder, knows his act is wrong but can’t control it.

Brain Injury and the Irresistible Impulse

Frontal lobe damage in the brain could be the cause of an irresistible impulse. An irresistible impulse falls under the volitional prong of the M’Naghten Test:

“a jury may find a defendant not guilty by reason of insanity where the defendant was laboring under a mental disease or defect that compelled him to commit the object offense.” – Cornell Law Institute, Insanity Defense[7]

Eden and the Apple

The cognitive prong is the inability to know the wrongfulness of the act while the volitional prong is the inability to resist committing the act, even if the person knows it is wrong.

Both require medical evaluation to determine if a mental defect exists and what is its impact on cognitive or volitional competency.

Texas law follows the M’Naghten standard, meaning only the cognitive prong is allowable as a defense while the volitional prong is not.

Some states allow both prongs, referred to in The ALI Rule, or American Law Institute Model Penal Code rule.[8] A few jurisdictions have the “Durham Rule” aka “Product Test” where the crime results from mental illness. Kansas, Montana, Idaho, Utah, and Alaska do not have a cognitive prong or mens rea defense (2021). [9]

Diminished Capacity

A brain injury can also be a factor in diminished capacity cases.

Typically, only forensic psychiatrists have the training and expertise to examine the nexus between a brain injury and competency to stand trial (CST) or criminal responsibility (not guilty by right of insanity, or NGRI). NGRI is generally associated with criminal cases. 

Diminished capacity can be a factor in civil litigation[10], where decision-making—even intentionally fraudulent behavior or vulnerability to fraud, may be linked to a brain injury—or it may be a claim that proves inaccurate on psychiatric examination.

Many states restrict CST and NGRI exams to forensic psychiatrists or forensic psychologists. Many attorneys are not aware of the local statutory requirements.

Occasionally, it may be helpful to have multiple experts involved in a brain injury case.

Although many forensic psychiatrists have some knowledge of brain injury, very few are board-certified in Brain Injury Medicine.

Case Example: Competency to Stand Trial and a Childhood Brain Injury Case

I worked on a case in which a perpetrator charged with a heinous crime claimed that a brain injury as a child caused him to be incompetent to stand trial.

I opined he was malingering, and though he did have a minor brain injury, it did not render him incompetent to stand trial.  The crime involved rape and social media in a high-profile case in Oregon.  The man was ultimately convicted.  I go into more depth in A Case Study in Malingering.

Sentence Mitigation

Brain injury and the resulting sequalae can be a mitigating factor in criminal cases. In capital mitigation cases involving TBI, a forensic psychiatrist board-certified in Brain Injury Medicine can prove to be valuable.


  1. Mens Rea, Cornell Law School, Legal Information Institute
  2. Insanity Defense, Cornell Law School, Legal Information Institute; see also description of the M’Naghten Rule, credited as the first time an insanity defense was introduce.
  3. Op Cit. Mens Rea, Cornell Law School, Legal Information Institute
  4. Op Cit. Insanity Defense, Cornell Law School, Legal Information Institute, includes history of the M’Naghten case, M’Naghten Rule and Test which first introduced the Insanity Defense in a criminal case.
  5. Irresistible Impulse, Cornell Law School, Legal Information Institute
  6. Op Cit. Insanity Defense, Cornell Law School, Legal Information Institute, also investigates the Volitional and Cognitive Prongs, and the Durham Rule, also called The Product Test: “The Durham rule states “that an accused is not criminally responsible if his unlawful act was the product of mental disease or mental defect.”
  7. Ibid.
  8. Cornell Law Institute Model Penal Code, Insanity Defense
  9. Landess, J. S., & Holoyda, B. J. (2021, March 31). Kahler v. Kansas and the constitutionality of the mens rea approach to insanity. Journal of the American Academy of Psychiatry and the Law Online. https://jaapl.org/content/early/2021/03/31/JAAPL.200086-20
  10. In Civil Litigation precedence and procedural rules are different than criminal cases but competency due to a brain injury can be a very serious question. Undue influence or testamentary capacity cases sometimes feature the question of dementia or other brain disease or a brain injury. Has the injury or disease interfered with decision-making? If a person is the victim of fraud, criminal and civil jurisdictions may overlap or require judicial attention. I defer to attorneys regarding these legal questions.