When finding unconstitutional the execution of defendants who were mentally retarded at the time of their crime in Atkins v. Virginia (2002), the US Supreme Court left the States to decide on procedures for deciding a defendant's mental retardation. This has and will lead to substantial variation, and will include juries being responsible for these verdicts. Two studies are presented that test procedural, evidentiary, and attitudinal effects on mock juror verdicts as to a capital defendant's mental retardation. Both studies show significant effects of procedural variables. Making the retardation and death decision at the same trial phase changed jurors' interpretation of evidence, including severity of mental problems. Jurors were insensitive to differences in the burden of proof on mental retardation verdicts, although demanding proof beyond a reasonable doubt may make jurors more sensitive to retardation evidence when deciding on a death sentence. Areas for future research are outlined.