If James Holmes ends up pleading not guilty by reason of insanity in the Aurora theater massacre, prosecutors will be fighting an uphill battle—because in Colorado, unlike most other states, the burden of proving the defendant is sane lies with the prosecutors. In around 40 other states as well as federal court, it's up to the defense to prove its client is insane. "It's burden of proof on steroids," one former federal prosecutor tells the AP. "It's totally subjective. It's not like proving somebody pulled the trigger. That's objective."
An insanity defense basically argues that the defendant should be absolved of the crime because he couldn't tell right from wrong at the time. State psychiatrists complete a court-ordered evaluation of the defendant, and defense attorneys or prosecutors can request additional state evaluations. It's rare for a state doctor to determine a defendant in a criminal case is insane; even so, Colorado prosecutors face an additional difficulty because they can't have their own experts examine the defendant. And if Holmes is found sane and is convicted, his lawyers can still argue he is mentally ill in an effort to avoid the death penalty.