FILE PHOTO OF TEXAS DEATH ROW CELL IN LIVINGSTON TEXAS, 2002-06-20 (photo)

The United States Supreme Court ruled by a 6-3 vote on June 20, 2002 that executing mentally retarded murderers violates the constitutional ban on cruel and unusual punishment in one of its most important death penalty rulings in years. There are no reliable numbers on how many of the more than 3,700 death-row inmates across the nation are mentally retarded. Death row guard Sgt. James Ludwig stands outside of a death row cell in Livingston, Texas which holds 435 condemned prisoners, the largest in the U.S., in this November 21, 2001 file photo. REUTERS/File Photo/Richard Carson WP/CM/GAC