In the case of Kahler v. Kansas, the US Supreme Court upheld a decision by the state of Kansas to abolish the insanity defense in criminal trials. The petitioner, James Kraig Kahler, was convicted for murdering four family members and sought to overturn his conviction on grounds that he was not allowed to use an insanity defense due to changes in Kansas law. He argued this violated his Eighth Amendment rights (prohibiting cruel and unusual punishment) and Fourteenth Amendment rights (guaranteeing due process). However, in a 6-3 ruling delivered by Justice Elena Kagan, it was determined that states have discretion over how they define legal insanity; thus abolishing traditional 'insanity defense' does not violate constitutional rights. Instead of using mental illness as an excuse for crime under M’Naghten rule (where defendants are considered legally insane if they couldn't understand their actions were wrong), Kansas allows evidence of mental health issues only to argue that defendants did not intend to commit crimes - known as mens rea approach.

In the dissenting opinion for Kahler v. Kansas, Justice Stephen Breyer argued that the majority's decision was a significant departure from historical legal principles and practices regarding insanity defenses. He contended that by allowing states to abolish traditional insanity defenses, the court had effectively permitted punishment of individuals who lacked basic moral understanding of their actions due to severe mental illness at the time they committed their crimes. This, he said, contradicted centuries-old common law principles which held such individuals were not criminally responsible for their actions because they could not understand right from wrong or control their behavior accordingly. Furthermore, Breyer pointed out that this ruling conflicted with modern standards of decency and international human rights norms which recognize these fundamental principles about criminal responsibility in cases involving serious mental illness.