PENRY v. LYNAUGH, 1989
"In 1988, when Congress enacted legislation reinstating the federal death penalty, it expressly provided that a "sentence of death shall not be carried out upon a person who is mentally retarded."In 1989, Maryland enacted a similar prohibition. It was in that year that we decided Penry, and concluded that those two state enactments, "even when added to the 14 States that have rejected capital punishment completely, do not provide sufficient evidence at present of a national consensus.'"
-Justice John Paul Stevens, Opinion of the Court, Atkins v. Virginia, referring to Penry v. Lynaugh
"In sum, mental retardation is a factor that may well lessen a defendant's culpability for a capital offense. But we cannot conclude today that the Eighth Amendment precludes the execution of any mentally retarded person of Penry's ability convicted of a capital offense simply by virtue of his or her mental retardation alone."
-Justice Sandra Day O'Connor, Opinion of the Court.