The US supreme court on Thursday threw out a challenge by the state of Alabama to a judicial finding that a death row inmate convicted of a 1997 murder is intellectually disabled and thus ineligible under the US constitution for the death penalty.

In this highly unusual move, and in a single-sentence, unsigned order, the court dismissed Alabama’s petition for review in Hamm v Smith without deciding it, effectively undoing its earlier decision to take up an appeal by state officials to the method used by a lower court to determine that Joseph Clifton Smith was intellectually disabled and therefore could not be executed.

At issue was how to assess multiple IQ scores that fall above and below the cutoff for execution, and how far courts should go in assessing additional evidence of intellectual capacity beyond IQ. Had the court sided with Alabama, it could have greenlit a path to a significant increase in the number of people with intellectual disabilities – a group overrepresented on death row – who are executed.

After the court heard arguments in December, the majority ruled on Thursday on procedural grounds that it should not have accepted the case of Smith, an Alabama man who was convicted of capital murder, and would not weigh in, thus leaving in place the lower court ruling blocking his execution.

Liberal justices Sonia Sotomayor and Ketanji Brown Jackson concurred in the decision to dismiss the case, while conservative Justices Clarence Thomas and Samuel Alito dissented. Chief Justice John Roberts and fellow conservative justice Neil Gorsuch joined Alito’s dissent in part.

Sotomayor wrote in a concurring opinion that the lower court’s determination “that Smith has significantly subaverage intellectual functioning and is intellectually disabled” was “correct, or at least, very plausible”.

“The court is not equipped to provide any meaningful guidance on how courts should assess multiple IQ scores,” Sotomayor went on. “That is because the differences between methods used to assess multiple IQ scores raise complicated questions on which even experts may disagree.

“If a conflict among the states or lower courts emerges and a case properly presents the issue, it may be appropriate for this court to weigh in with more specific guidance about the permissible method or methods by which courts must analyze such scores,” she added.

Because of a major 2002 supreme court precedent that executing an intellectually disabled person violates the constitution’s eighth amendment ban on “cruel and unusual punishment”, Alabama could not execute Smith. But that landmark ruling left it to the states to set standards for eligibility.

Supreme court rulings in 2014 and 2017 allowed courts to consider IQ score ranges that are close to 70 along with other evidence of intellectual disability, such as testimony of “adaptive deficits”.

Smith’s five IQ scores, which measure learning, reasoning and problem-solving, range from 78 to 72 – all around the bottom fifth percentile of the population. In Alabama, someone is ineligible for execution if they have an IQ at or below 70and can demonstrate the existence of “significant deficits” in everyday skills that occurred before the age of 18. Many other states have similar standards.

A federal judge in the 11th circuit noted that Smith’s lowest score could in fact be as low as 69, given the standard error of measurement, and allowed him to present additional evidence of his mental capacity. The judge found that Smith had significant deficits in his adaptive behaviors, and manifestation of these problems from an early age – in Smith’s social and interpersonal skills, independent living and schooling.

Smith’s school records showed that he was classified as “educable mentally retarded” in seventh grade – an outdated term to refer to someone who had a mild intellectual disability. The court also considered other factors including “his failure to maintain a bank account and difficulties purchasing groceries”.

The court found that “[a] person with an IQ score above 70 may have such severe adaptive behavior problems … that the person’s actual functioning is comparable to that of individuals with a lower IQ score” – and thus Smith could not be sentenced to death. An appeals court upheld that ruling, contending that the decision was made using a holistic approach.

Alabama disagreed with the lower court’s decision on grounds that it placed too much weight on his lowest IQ score, which only placed Smith below the cutoff for execution when considering the margin of error. Alabama twice appealed to the US supreme court, arguing that more weight should have been placed on the cumulative IQ scores that placed him above the cutoff. The state was backed by Trump administration, which lifted a moratorium on the federal death penalty, in the case.

Reuters contributed reporting