Can a brain injury impact criminal behavior?
Can a Brain Injury Cause Someone to Commit A Crime?”
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.
By Sanjay Adhia, M.D.
A brain injury might be a factor in a criminal case:
If someone is / is not competent
How well they make decisions
What they can understand, e.g.,Mens Rea (did the criminal defendant know their actions were wrongful) 
Competency to stand trial
How do brain injuries affect the brain?
All Traumatic Brain Injuries (TBIs), for example, are not the same.
TBIs do not all impact the same part of the brain or in the same way. An injury that impacts skill development on one person might not impact impulsive behavior or moral judgment.
Impulsive behavior, another possible feature of a TBI, may or may not be exculpatory, depending on the relevant jurisdiction and specific details. If it is not exculpatory, it could be a mitigating factor.
The scope of this article is whether a brain injury can impact criminal behavior, which can be relevant in a legal defense. This is not a legal article or definitive description of laws, precedent or legal standards.
Convicted Criminals With a History of Brain Injury
It is worth noting that about half of all incarcerated individuals have a history of brain injury.
If present at the time of the crime, a brain injury could be the mental defect relevant in an Insanity or Criminal Responsibility defense. It could potentially be a factor in both the Cognitive or Volitional prongs.
Cognitive vs. Volitional Prong
The M’Naghten Rule was established to address the Cognitive Prong of an “insanity defense.” From the original case in 1843, “..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” — Cornell Law School Legal Information Institute
The Volitional prong is being unable to conform to the law due to an irresistible impulse. Irresistible Impulse describes the inability, due to mental defect, to resist the commission of a crime.
For example, someone who is experiencing a manic episode who 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. The “Irresistible Impulse” is explained by Cornell LII. 
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’naughten standard meaning only the cognitive prong is allowable as a defense, but the volitional prong is not.
Some states allow both prongs.
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 (NGRI). In Civil Litigation, decision-making, even intentionally fraudulent behavior may be linked to a brain injury—or that 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.”