Twelve years later in Hall v. Florida the U.S. Supreme Court narrowed the discretion under which U.S. states can designate an individual convicted of murder as too intellectually incapacitated to be executed.

Samenow assessed Atkins's vocabulary and knowledge of current affairs, and testified that he understood cause and effect, and could use relatively complex words like orchestra and decimal.

The prosecution also presented testimony about Atkins's criminal history which began in early adolescence and included over a dozen prior felony convictions for robbery, larceny and burglary.

[5] Concurring in Penry v. Lynaugh, Justice William Brennan wrote that the proportionality of a punishment depended on the severity of the injury caused and the defendant's moral culpability.

The "relationship between mental retardation and the penological purposes served by the death penalty" justifies a conclusion that executing intellectually disabled people is cruel and unusual punishment that the Eighth Amendment should forbid.

In other words, unless it can be shown that executing the intellectually disabled serves the recognized penological goals of retribution and deterrence, doing so is nothing more than "purposeless and needless imposition of pain and suffering", making the death penalty cruel and unusual in those cases.

[2] The Atkins Court said cognitive and behavioral impairments not only diminished moral culpability for impulsive conduct, they also made it less likely that defendants would be deterred by the death penalty:[7][4] Yet it is the same cognitive and behavioral impairments that make these defendants less morally culpable—for example, the diminished ability to understand and process information, to learn from experience, to engage in logical reasoning, or to control impulses—that also make it less likely that they can process the information of the possibility of execution as a penalty and, as a result, control their conduct based upon that informationAs for retribution, society's interest in seeing that a criminal get his "just deserts" means that the death penalty must be confined to murders that reflect "a consciousness materially more 'depraved' than the average murderer.

They typically make poor witnesses and the presentation of mitigating evidence of intellectual disability can be a "two edged sword that may enhance the likelihood that the aggravating factor of future dangerousness will be found by the jury".

[8][4] In light of the "evolving standards of decency" that the Eighth Amendment demands, the fact that the goals of retribution and deterrence are not served as well in the execution of intellectually disabled people, [5] and the heightened risk that the death penalty will be imposed despite the mitigating circumstances,[4] the Court concluded that the Eighth Amendment forbids the execution of intellectually disabled people, leaving implementation to the states.

The Chief Justice said that "foreign laws, the views of professional and religious organizations, and opinion polls" were not "objective indicia of contemporary values" under the Court's existing precedents.

By creating culpability-based exemptions for death penalty eligibility the Atkins decision will force courts to decide whether other psychiatric disabilities similarly qualify.

[10] Hall was the first case to consider a state-imposed limitation on Atkins-eligibility, holding that a state could not require an IQ score of 70 or below to make an Atkins claim presenting evidence of adaptive impairment.

[12] The seven Briseno factors included questions like "can the person hide facts or lie effectively in his own or others' interests" or if the crime required "forethought, planning, and complex execution of purpose".

They noted the circumstances of the crime including his ability to load and aim a gun, recognize an ATM card, direct the victim to withdraw cash and attempt to hide his involvement in the robbery from police were inconsistent with being "truly mentally retarded".

[17] In January 2008, Circuit Court Judge Prentis Smiley, who was revisiting the matter of whether Atkins was mentally disabled, received allegations of prosecutorial misconduct.

[18] Prosecutors sought writs of mandamus and prohibition in the Virginia Supreme Court on the matter, claiming that Smiley had exceeded his judicial authority with his ruling.