In Atkins v. Virginia, the U.S. Supreme Court reverses its 1989 Penry decision and finds that the execution of people who are mentally retarded is “cruel and unusual” punishment, in violation of the Eighth Amendment. The Court cites “evolving standards of decency” as a key basis of its opinion, noting that just two states banned the execution of mentally retarded offenders at the time that Perry was decided, but now, 20 states – and the federal government – ban them.