Supporters of the legislation say Texas has executed six mentally retarded defendants since 1976.

Presently, four states bar the use of the death penalty against mentally retarded defendants.

In addition, mentally retarded defendants most often find it difficult, and sometimes impossible, to participate effectively in their own defense.

During the study, however, the authors found that mentally retarded criminal defendants often suffered from a combination of disorders, such as:

Even when mentally retarded defendants are clearly guilty, it is extremely difficult to determine their level of culpability.

Lawyers who have represented mentally retarded defendants who were executed, as well as the families of those people, said the ruling was bittersweet.

Moreover, their impairments can jeopardize the reliability and fairness of capital proceedings against mentally retarded defendants.

New York is one of 13 states that has banned the execution of mentally retarded defendants.

In the United States, 25 states permit the execution of people with mental retardation, and 34 mentally retarded defendants have been executed since 1976.

Thirteen states now prohibit the execution of mentally retarded defendants, and only a few other countries continue this barbaric practice.