One controversial element of society’s attitude toward the mentally ill is the insanity defense—the argument that a person accused of a crime is not guilty because, due to mental illness, he cannot control his actions or understand the difference between right and wrong. Although this defense is used in only one percent of criminal cases and successful in only one-quarter of those, opposition to the “not guilty by reason of insanity” verdict has led thirteen states to permit a verdict of “guilty but mentally ill.” Defendants convicted under that verdict are held responsible for their crimes but may be provided mental health care in a prison or hospital.

Supporters of “guilty but mentally ill” argue that this verdict, in addition to acknowledging the defendant’s responsibility, protects society...