Morgan v. Louisiana was a United States Supreme Court case that dealt with the issue of whether a state could constitutionally impose a death sentence on a defendant who was mentally retarded. The case was brought by a man named Robert Morgan, who had been convicted of murder in Louisiana and sentenced to death. The Supreme Court held that the Eighth Amendment's prohibition against cruel and unusual punishment barred the execution of a mentally retarded person. The Court reasoned that the execution of a mentally retarded person was not a deterrent to crime, and that such a person was less culpable for his or her actions than a person of normal intelligence. The Court also noted that the execution of a mentally retarded person would be a violation of the evolving standards of decency that mark the progress of a maturing society. The Court's decision in Morgan v. Louisiana was a landmark ruling that established a precedent for the protection of the rights of the mentally retarded. The decision has been cited in numerous subsequent cases involving the rights of the mentally retarded, and has been used to support the argument that the execution of a mentally retarded person is unconstitutional.

In Morgan v. Louisiana, the Supreme Court was asked to decide whether a state statute that allowed for juvenile offenders to be tried as adults violated the Fourteenth Amendment's Equal Protection Clause. The majority opinion held that it did not violate this clause because juveniles were still afforded due process of law and equal protection under the law in their trials. However, Justice Field dissented from this decision on two grounds: firstly, he argued that allowing juveniles to be tried as adults without any special consideration or protections violated both substantive and procedural due process; secondly, he believed that such a practice would lead to unequal treatment of similarly situated individuals based solely on age. He concluded by stating his belief that all persons should receive equal protection under the law regardless of age or other factors.