A backlash after the verdict pushed Congress and legislatures in many states to change their laws. Some states required that defense attorneys, not prosecutors, prove insanity. Others, like Utah, removed the insanity plea for defendants who argued that they saw nothing wrong in their actions.

The first type of mental health defense in Utah requires an attorney to prove that a client was mentally ill at the time of the crime and, based on delusions, believed the crime was justified.

So if someone believed in a deluded state that he was in a war firing at enemy soldiers or protecting someone else from harm he would be legally justified in using deadly force. The hard part is proving the defendant's deluded state was real.

"It's not that uncommon that the notice is given that the (attorney) intends to run this kind of defense," Horton said. "(But) not many of them actually end up in contested trial."

The second defense requires an attorney to prove that a client didn't fully realize she was killing someone. An example: a mentally ill woman who thought she was squeezing a grapefruit but was actually strangling a child.

During trial, an attorney would likely have mental health experts testify about the person's behavior and mind-set at the time of the crime.

A conviction after the first type of defense, a special mitigation defense — allowed only for capital murder, murder or attempted murder cases — would reduce the criminal charge one degree — from a first-degree felony to a second-degree felony, etc.

A conviction with the second defense would find an individual guilty and mentally ill. The person's sentence wouldn't be lessened but would allow for mental health treatment.

"Practically 97 percent of (people) who were mentally ill at the time (of the crime) qualify for the guilty and mentally ill," Horton said, citing rough statistics from personal experience. "It doesn't reduce the crime, it doesn't take away possibility of punishment. It's first and foremost a plea of guilt."

While not a reduction of the crime or the level of the subsequent punishment, the plea just helps an individual get mental-health treatment, Horton said.

Insanity laws elsewhere

In California, unlike Utah, defendants have the option to plead "guilty, but not guilty by reason of insanity," which means they will spend their entire sentence in the state mental hospital rather than prison, if convicted.

Also unlike Utah, California still allows an individual an insanity plea if he didn't understand or believe that his behavior was criminal. The attorney must also prove that the individual was unable to conform his behavior to the law.

The case of Dan White in San Francisco also dramatically affected how insanity defenses are now conducted.