To prove insanity, the defense must establish that a mental illness prevented the defendant from understanding that his actions were wrong at the time of the offense.
And they must do it with "clear and convincing evidence." In federal courts, jurors are instructed that the term refers to evidence that makes it "highly probable" that the person was insane when she committed the crime. Jurors are permitted to consider the defendant's mental condition before and after the crime, as well as witness testimony and the opinions of the psychiatric experts, which they may accept or reject as they see fit, in reaching this decision [source: Legal Information Institute].
Regardless of the specific terms used to describe the evidence, the defense must establish a clear connection between the defendant's mental illness and the actual commission of a crime. Just because a person suffers from episodes of psychosis, doesn't mean that the psychosis caused her to rob a liquor store. Maybe she was just thirsty [source: Singer-Vine].
The insanity question often comes down to expert opinion. Mental health professionals typically interview a defendant and people who know her -- family, friends, co-workers -- to determine insanity. They also review other information like the arrest warrant and charging documents, the person's medical and criminal history and any statements made on social media sites like Facebook and Twitter. In addition, experts rely on specialized tests designed to catch defendants embellishing their impairments by claiming that they experience exaggerated symptoms or those that are not typically associated with a specific disorder [source: Singer-Vine].
The results of these reviews often weigh against the accused. A 2004 study of more than 5,000 sanity evaluations conducted by forensic evaluators in Virginia showed that defendants were deemed insane in just 15 percent of the cases [sources: Warren, Singer-Vine].
If all the legal jargon, rules and statistics haven't driven you to the brink of insanity, read on for more information about mental health and criminal justice.