In a 5-3 decision in a capital case the Supreme Court rejected a Texas court’s reliance on a 1992 definition of intellectual disability and the use of a number of factors as indicators of intellectual disability which the Court described an “invention…untied to any acknowledged source.”
In Atkins v. Virginia (1992) the Supreme Court held that executing the intellectually disabled violates the Eighth Amendment’s prohibition against cruel and unusual punishment. The Court tasked states with implementing Atkins.
Generally, to be intellectually disabled for purposes of the death penalty a person must have an IQ of 70 or less (adjusted plus or minus five for the standard error of measurement) and “adaptive deficiencies” (an inability to learn basic skills and adjust behavior to changing circumstances) onset as a minor.