The Mentally Retarded and the Death Penalty

I'm opposed to the death penalty, so I should be cheering the Supreme Court decision years ago to ban execution of the mentally retarded. While I agree with the order the Court has given, I have serious problems with the justification given for the decision and the logical implications of the case.

First is the methodology. The majority opinion in this case, written by Justice John Paul Stevens, cites a growing "national consensus" as reasoning for the need to ban execution of the mentally retarded. Basically, the majority feels that because 30 states do not execute the mentally retarded, it constitutes "cruel and unusual punishment." Yes, the Constitution is a living document, and yes, the death penalty constitutes cruel and unusual punishment. But these two items cannot combine in a judicial decision which bases itself on the work of state legislatures. The cornerstone of the American political system is separation of powers, and it is constitutionally dubious for the Supreme Court, a presumably extralegislative body, to base decisions on those of legislatures. Americans don't elect Supreme Court justices (at least not directly) and Supreme Court decisions should not be based on the American vox populi.

My second point regards the logical implications of the case. How do we set a mark of mental retardation at an IQ of 70? Is someone with an IQ of 71 more qualified for the death penalty than someone who has an IQ of 69? What about disparities in multiple IQ tests? And what's to prevent a borderline defendant from purposely bombing an IQ test in order to escape death? This doesn't seem as if it can be properly applied. And that's problematic.