Most criminal offenses require the state to prove that the defendant knowingly or intended to commit the crime. But what if the crime is committed by someone who is mentally ill or developmentally disabled?

For example, in the recent Illinois case, People v Mayo, the defendant was charged with aggravated criminal sexual abuse. The defendant, however, had suffered a brain injury in infancy and presently had the mental capacity of a 3-year-old and an IQ of 48. Therefore, could the defendant truly form the required intent to commit the crime?

In such cases, Illinois law provides certain procedures. First, the court must determine whether the defendant is mentally fit to stand trial. In the overwhelming majority of cases, the court will determine that the defendant is fit, and the case moves to trial. If the defendant is not fit, the court holds a discharge hearing.

A discharge hearing determines whether a defendant should be acquitted, but not whether a defendant is guilty. If the defendant cannot be acquitted because there is enough evidence of guilt, the court may find the defendant “not not guilty.” The defendant is then subject to one to five years of treatment, depending on the offense. If the defendant is still unfit after the treatment ends, the court may involuntarily commit the defendant for further treatment. The commitment, however, cannot exceed the maximum sentence prescribed under the original criminal charge.

If a defendant is fit to stand trial, the defendant may seek a verdict of “not guilty by reason of insanity.” Under these circumstances, a defendant must show that as a result of mental disease or defect, he or she lacked substantial capacity to appreciate the criminality of his or her conduct. The court must first enter a verdict of guilty, then find the defendant not guilty by reason of insanity. (See our related post The Insanity Defense in Illinois.

If you have a friend or relative charged with a crime who suffers from a mental disability or illness, contact an experienced attorney immediately. An attorney can help present evidence of your loved one’s difficulties in their most favorable light. Often, a mental capacity defense will require expert testimony. An attorney can help select and prepare these experts.

If you have questions about this or another related Illinois criminal or traffic matter, please contact Matt Keenan at 847-568-0160 or email

(Besides Skokie, Matt Keenan also serves the communities of Arlington Heights, Chicago, Deerfield, Des Plaines, Evanston, Glenview, Morton Grove, Mount Prospect, Niles, Northbrook, Park Ridge, Rolling Meadows, Wilmette and Winnetka.)

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