Can a brain injury impact criminal behavior?
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In my experience, there is the possibility that where a crime was committed and the perpetrator has a brain injury, a close look is important. They might be associated in some way and evaluation by a qualified physician with forensic training is essential as brain injuries are complex and nuanced, and so is behavior.
Brain Injury or Disease: Medical-Legal 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. 
Competency to stand trial (CTST)
Not Guilty by Reason of Insanity (NGRI) – the “Insanity Defense” 
Convicted Criminals with a History of Brain Injury
About half of all incarcerated individuals have a history of brain injury. Correlation is not causation, but always calls for a closer look.
How Brain Injuries Affect the Brain and Behavior
Brain Injuries vary in degree and diagnosis. For a more detailed medical discussion, I encourage you to read my article on this site Brain Injuries Explained.
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 the biomechanical forces. The three types of forces are blunt trauma, penetrating injuries and from blast overpressure injury.” Adhia, S. Brain Injuries Explained
All Traumatic Brain Injuries (TBIs) are not the same. TBIs do not 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 a full(er) recovery. Post-Concussive Disorder symptoms occur in the first 7-10 days and 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 are “diseases” and also impact behavior, but generally develop slowly. Neurocognitive Disorders (NCDs) can occur directly from TBI. It is worth noting that 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 blows to the head or concussions. 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. A criminal charge of fraud in such cases usually requires 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 ability to perform Activities of Daily Living (ADLs) in one person, for example, might not impact impulsive behavior or moral judgment—relevant to competency and Mens Rea.
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 Sate. If a medical reason for impulsive behavior is not exculpatory, it could be a mitigating factor in sentencing.
Psychiatric Disorders and Impulsiveness
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.
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.”
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. 
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.
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.”
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, but the volitional prong is not.
Some states allow both prongs which is referred to The ALI Rule, or American Law Institute Model Penal Code rule. A few jurisdictions have the “Durham” aka Product Test  where the crime results from mental illness. There a few states do not have an Insanity Defense.
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 more commonly referred to as NGRI). NGRI is generally associated with criminal cases.
Diminished Capacity can be a factor in Civil Litigation , 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 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, social media and a high-profile case in Oregon. The man was ultimately convicted. I go into more depth in “A Case Study in Malingering.”
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.
 Mens Rea, Cornell Law School, Legal Information Institute
 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.
 Op Cit. Mens Rea, Cornell Law School, Legal Information Institute
 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.
 Irresistible Impulse, Cornell Law School, Legal Information Institute
 Op Cit. Insanity Defense, Cornell Law School, Legal Information Institute, also investigates the Volitional and Cognitive Prongs
 Op Cit. Cornell explains 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.“”
 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.