The ‘Double-Edged Sword’ of the Insanity Defense

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Illustration by Stan Dominguez via Flickr

Is neurological impairment sufficient evidence to relieve an accused person of criminal liability for his or her acts?

In many cases—especially those involving the death penalty—evidence showing that neurological damage influenced criminal behavior, can be a “double-edged sword,” used by prosecutors as well as defense attorneys, according to a Vanderbilt Law School study.

The study, recently published in the Journal of Law & the Biosciences, argues that neuroscientific evidence has at best limited value when it is used in trials to determine whether or not an individual is guilty—but instead could be more useful as a tool for the prevention of rehabilitation and re-offending.

“Even on the assumption that the data presented are accurate, much commonly proferred neuroscientific evidence is immaterial or only weakly material, not only at trial, but also at sentencing,” wrote the study author, Christopher Slobogin, the Milton Underwood Professor of Law at Vanderbilt University Law School.

Slobogin identified five types of neuroscience evidence that have been used to define “criminal liability and criminal punishment.”

  • evidence of abnormality—evidence of neurological impairment;
  • evidence that neurological impairment is common in criminals and those who behave in antisocial manner (Cause-and-effect research);
  • evidence showing that neurological impairment predisposed the defendant’s criminal behavior;
  • psycho-neurological testing results that show the defendant’s behavioral impairments are legally relevant; and
  • evidence showing that the defendant’s impairments are similar to impairments that the law has recognized as exculpatory or mitigating.

Slobogin cites, as an example, the trial of Andrea Yates, a Texas woman convicted of capital murder after she drowned her five children in 2001. In such cases, The question of “intent” to kill could be mitigated by a “mens rea” defense that showed extreme mental or emotional disturbance or a psychosis. On appeal, Yates was found not guilty by reason of insanity and committed to a mental hospital.

Nevertheless, aside from situations where there is “already strong evidence of automatic behavior, lack of intent, or severe cognitive or volitional impairment,” not many states allow these defenses to be introduced in criminal courts, the study pointed out.

“It should be apparent that in most cases neuroscience is not likely to be useful in proving these defenses, even if the court allows it to be introduced,” the study said. “In the typical criminal case…the defendant acts voluntarily, with intent, and with an understanding of the distinctions society makes between right and wrong.”

The “only viable defense” in a typical neuroscience case is either a claim of emotional or mental disturbance or an insanity claim based on “volitional impairment” —a defense recognized by few states.

Capital sentencing cases are the exception. Since the Supreme Court recommends admitting any evidence related to the offender’s character, neuroscience is often used in death penalty cases, the study said.

Slobogin notes, however, that bringing in neuroscience as a defense even in capitol punishment cases can be a “double-edged sword,” since prosecutors can use it to argue that a person’s “dangerousness” is an argument in favor of the death penalty.

The study said this issue could be removed if prosecution references to a person’s dangerousness were banned, or if a requirement were introduced to instruct the jury “that the only alternative to the death penalty is life without parole, alerting them to the fact no one outside of prison will be endangered by the defendant if the jury rejects the death penalty.”

The death penalty could also be formally removed from the sentencing choices open to the jury if evidence of neurological damage is presented during the sentencing phase of the trial.

However, Slobogin, also says he is sympathetic to arguments for a “more radical” legal reform that would transform the justice system into a “vehicle for addressing the causes of criminal behavior” rather than simply establishing blame or guilt.

“Prevention of reoffending would be a significant goal of such a (reformed) system, but so would rehabilitation,” he wrote.

Under such a change, the role of neuroscience would be critical, he argued.

“Neuroscience experts would focus not on blameworthiness or desert, but on risk and treatment.”

This summary was prepared by TCR intern Julia Pagnamenta. The full study can be downloaded here. Readers’ comments are welcome.

4 thoughts on “The ‘Double-Edged Sword’ of the Insanity Defense

  1. McNaghten’s Rule will outeun Neuroscience for generations to come unless it is reformed because it is based on profound ignorance.

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