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How can brain damage be used to create reasonable doubt?

In 1990, a 40-year-old schoolteacher suddenly began using prostitutes and making inappropriate advances to his stepdaughter. The courts ordered him to go to Sexaholics Anonymous or go to prison. He failed the program because he kept asking people for sex. The hospital found a large brain tumor in his frontal lobe, and it was removed. The deviant sexual behavior stopped after removal of the tumor.

The famous case in 1848 of Phineas Gage also demonstrated how personality can be affected by injury. Mr. Gage’s frontal lobe was severely damaged in an explosion when he was working on the railroad. He had dramatic personality and behavioral changes after the injury, becoming irritable and amoral and exhibiting symptoms of antisocial personality disorder becoming involved in fights and drunken brawls.

Patients with frontal lobe damage can be disinhibited and unable to control their conduct. They may know the difference between right and wrong but can’t act upon their knowledge. They need to be evaluated to present a defense of insanity based on impaired impulse control as a result of their brain injury. Many of these defendants may not think to disclose or forget to disclose their injury to their attorney. Therefore, if the attorney suspects anything after questioning their client, especially if the client lacks insight into their behavior, they should investigate the person’s medical history and the possibility that their frontal lobe might be impaired. Therefore, doing your homework and knowing the symptoms of frontal lobe dysfunction may be beneficial in preparing your case. (Redding, 2006)

Effect of Brain Damage on Personality

The frontal lobe of the brain controls personality. Therefore, if the frontal lobe is damaged in any way, a person’s personality may change as well…and sometimes not for the best. Occasionally, the changes are severe enough to result in criminal behavior.

The frontal lobe disorders that can cause personality changes are listed below:

A) Focal lesions:

Huntington’s disease

Infection (abscess, toxoplasmosis)


Pediatric autoimmune neuropsychiatric disorders associated with streptococcal infections.


Tourette’s syndrome

B)  Diffuse or distributed processes:


Alzheimer’s disease

Corticobasal ganglionic degeneration

Frontotemporal dementia

HIV encephalopathy

Huntington’s disease

Prion diseases

Vascular dementia


Parkinson’s disease

Post concussive syndrome

Progressive supranuclear palsy

Traumatic brain injury or closed head injury

When the circuits in the brain are disrupted by injury or lesion the personality can change in varying degrees depending on the severity of the injury. Emotion, behavior, and cognition converge in personality and when any of these three are impaired there can be a change in personality. There are psychiatric instruments that can be used to measure the degree of personality change, but none can determine a patient’s ability to function at home or in society. These changes can manifest in multiple ways and cannot always be determined by the specific location of the injury within the frontal lobe.

There is a “dysregulated physiologic response to emotional stimuli,” which is an inability to control and manage emotional responses to their environment, often seen in patients who have lesions in their frontal lobe. This is notable in those with prefrontal cortex damage. This can also be known as a “frontal lobe personality” or “acquired psychopathy.” The characteristics exhibited can include:

People who have dementia or Alzheimer’s disease can also display personality changes if the frontal lobe is affected. These changes can be rather marked and frequently socially undesirable such as apathy, disinhibition, euphoria, obsessive-compulsive behaviors, and hoarding. And, depending on the location of the damage, their insight regarding these changes can vary.

When a person experiences a traumatic brain injury or closed head injury the shear forces on the white matter of the brain can disconnect cortical regions from one another and recreate any of the frontal-subcortical syndromes mentioned above. The personality can also change after a concussion; known as post-concussion disorder or persistent post-concussion syndrome. These changes can manifest as social or sexual inappropriateness, irritability, aggression after little provocation, anxiety, depression, apathy, and emotional lability.

Epilepsy with seizures that occur within the frontal lobe is usually associated with personality disorders that are defined within the DSM-V under cluster B (dramatic/emotional) or cluster C (anxious/fearful).


Other factors associated with epilepsy can also contribute to these issues such as auras, social sequelae, and medication.

Timing of a frontal lobe lesion can also change a person’s personality by hindering the acquisition of social and moral reasoning. For example, a two-year old’s response to punishment and interventions to alter behaviors can be affected and thus cause behavioral issues because they cannot reason out the connection between the action and the consequence. In contrast, elderly patients with dementia may be able to reason this out but not react appropriately.

There are various instruments used by medical professionals to measure or assess personality. However, they have not proven to be useful for predicting performance. A good example of this is that a person with diabetes insipidus (dysregulation of how the body handles fluids) is told not to drink water and is able to remember that he isn’t supposed to drink the water but drinks it anyway because he can’t control his actions. And tests such as the Hare Psychopathy checklist are used to measure the risk of violent behavior in criminals who have antisocial personality disorder. This test takes into consideration that a psychopath can cognitively manipulate people with no regard for moral standards. Therefore, this test cannot accurately be used in a person with a similar disorder caused by damage to the frontal lobe. (Chow, 2000)

Works Cited

Chow, T. W. (2000, Oct ). Personality in Frontal Lobe Disorders. Current Psychology Reports, 2(5), 446-451. doi:

Redding, R. E. (2006). The Brain-Disordered Defendant: Neuroscience and Legal Insanity in the Twenty-First Century. American University Law Review, 56(1), 1-79. Retrieved from


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