Justices Brennan and Marshall, arguing to eliminate the death penalty for retarded defendants, considered mental retardation as such a deficit in reasoning ability, impulse control, and moral development as to limit culpability whatever the degree of impairment. The defendant's forensic evaluation should include a determination of his moral understanding of the crime with which he is charged. An answer that shows poor understanding of its involuntary nature. The jury gasped at such an explanation. Strange as it may seem, some criminal defendants do not understand why society wants to punish them.
Moral understanding is tied to mental development.
For example, when a very young child breaks a glass, he is often punished irrespective of intent. The child does not distinguish between an accident and an intended consequence. Reciprocity is fundamental to understanding the Golden Rule. However, retarded defendants almost never possess such awareness. While many non-retarded defendants probably lack this awareness as well, at least they are intellectually capable of acquiring it.