On May 27, 2014 the U.S. Supreme Court held (5-4) in Hall v. Florida that states must generally consider more than IQ scores in determining intellectual disability as an exemption from the death penalty. As Justice Kennedy wrote for the majority, "Intellectual disability is a condition, not a number."
However, Justice Kennedy and the concurring Justices went further in describing the importance of the Eighth Amendment, the risks of the death penalty, and our responsibility as a leader in the civilized world :
"The Eighth Amendment’s protection of dignity reflects the Nation we have been, the Nation we are, and the Nation we aspire to be. This is to affirm that the Nation’s constant, unyielding purpose must be to transmit the Constitution so that its precepts and guarantees retain their meaning and force."
And later concluding :
"The death penalty is the gravest sentence our society may impose. Persons facing that most severe sanction must have a fair opportunity to show that the Constitution prohibits their execution. Florida’s law contravenes our Nation’s commitment to dignity and its duty to teach human decency as the mark of a civilized world. The States are laboratories for experimentation, but those experiments may not deny the basic dignity the Constitu¬tion protects."
Source : Death Penalty Information Center : http://www.deathpenaltyinfo.org/