Eighteen US states have abolished the death penalty. Three states – Maine, Michigan and Wisconsin – have completely banned the death penalty since the mid-nineteenth century. Fifteen states abolished the death penalty at various points throughout the twentieth and twenty-first centuries.

The remaining 32 states, however, allow the death penalty for certain crimes, but prohibit the punishment from being used on specific population subsets, such as children or persons with intellectual disabilities. When the US Supreme Court ruled that the execution of persons with intellectual disabilities did not violate the US Constitution’s Eighth Amendment in the 1989 case, Penry v. Lynaugh, only two states that allowed the death penalty, Georgia and Maryland, had state laws specifically prohibiting the execution of person with mental disabilities.

Following the Penry decision, 16 states outlawed the death penalty for persons with intellectual disabilities. Of those states, six required that a qualified examiner, such as a court-appointed state-licensed psychiatrist or psychologist, evaluate the defendant for intellectual disability. As a result of the Penry decision, the US Congress passed the Violent Crime Control and Law Enforcement Act of 1994, known as the Federal Crime Bill. The law prohibits the federal government from executing a “mentally retarded” person.

After the Supreme Court ruled that executing persons with intellectual disabilities violated the Eighth Amendment in the 2002 decision, Atkins v. Virginia, eight additional states changed their statutes to comply with the decision, bringing the total number of states with statutory bans for death penalty altogether or for defendants with intellectual disabilities to 38.

Many states that amended their death penalty statutes following the Penryor Atkins decisions adopted the term “significantly subaverage general intellectual functioning” to define a determination of mental disability in a defendant. Arkansas code states that there is a rebuttable presumption of mental retardation when the defendant has an IQ of 65 or below. Delaware, Idaho, Kentucky, Nebraska, North Carolina, South Dakota, Tennessee and Washington all define the term as a person with an IQ of 70 or below. Colorado state law specifically declines to name a numerical IQ level to define intellectual disability. For perspective, the average IQ score in the US is 100.

In addition, 18 of the 32 states that allow the death penalty statutorily require that the intellectually disabled defendant have exhibited signs of the disability prior to age 18 or during the defendant’s “developmental period.” Two states require that the defendant exhibit signs of intellectual disability before age 22. Defendants that exhibit signs of intellectual disability after the age of 18 or 22, or after they have completed their developmental period, are not considered mentally disabled and can be subject to the death penalty if convicted. Nebraska is the only state that currently has the death penalty but does not have an age limit for when signs of intellectual disability must manifest themselves.

Four states that adopted either post-Penry or post-Atkins laws prohibiting the execution of a person with intellectual disability have subsequently abolished the death penalty altogether. New York has had a moratorium on the death penalty since 2007, when the New York State Court of Appeals declared the practice in violation of the state Constitution. New Mexico, Illinois and Connecticut abolished the practice by legislative action in 2009, 2011 and 2012, respectively. Moreover, Maryland – one of the first two states to prohibit the execution of persons with intellectual disabilities – abolished the death penalty by legislative action in 2013.