Can states eliminate the insanity defense?

That brings us to the new case, Kahler v. Kansas. The state of Kansas has effectively eliminated the insanity defense. “Mental disease or defect is a defense to a prosecution under any statute,” Kansas law says, only if it negates “the mental state require[ment] as an element of the offense charged.” Kan. Stat. Ann. § 22-3220 (2009). In other words, a defendant is always free to show that his insanity explains his innocence of the crime charged. But that’s the only way insanity can be used. It’s not an independent defense.

Here’s a hypothetical to see the difference. Imagine a person suffering from mental illness pushes a victim off a bridge. The victim falls to his death. The pusher is then charged with intentionally killing another human being. At trial, the defendant cannot argue to the jury that he is not guilty by reason of insanity because his mental illness impaired his ability to identify right from wrong or to conform his conduct to the law. For example, he can’t argue that he heard voices telling him to kill the victim that he couldn’t ignore.

Instead, the defendant can only argue to the jury that his mental illness is the explanation for why he lacked the intent to kill and is therefore not guilty of an intentional killing. For example, he could argue that his mental disease meant that he wrongly believed his victim could fly: Because he didn’t realize that pushing the victim off the bridge could harm him, he didn’t cause the victim’s death intentionally.