The supreme court will soon rule on Hamm v Smith, an Alabama death penalty case that could significantly increase the number of people with intellectual disability who are executed. In this case, Alabama is fighting to execute a man named Joseph Smith. Smith’s five IQ scores – 72, 74, 74, 75 and 78 – all fall around the bottom fifth percentile of the population.

Based on these IQ tests, which measure learning, reasoning and problem-solving, and Smith’s adaptive behaviors, which include the social and practical skills that Smith uses to navigate everyday life, a federal court determined that Smith is intellectually disabled. Because the supreme court held in its landmark 2002 Atkins ruling that executing anyone with an intellectual disability violates the constitution, Alabama cannot execute Smith.

But Alabama disagreed with this decision, even though empirical standards put the IQ threshold for intellectual disability between 70 and 75. Yes, Alabama wants to execute Smith. But the case could also create a new, dangerous protocol: when a capital defendant has taken multiple IQ tests, any score above 70 could close the door an intellectual disability claim.

Such a process contradicts the clinical consensus that intellectual disability is a holistic diagnosis and can never be based on just one IQ score. Alabama’s argument is wrong, but not surprising. Since the Atkins decision, some states have looked for ways to continue executing people with intellectual impairments. When Florida tried to create a strict IQ cutoff of 70 before considering an intellectual disability claim, the supreme court said no. When Texas implemented a determination process that relied more on the fictional character Lennie from Of Mice and Men than on clinical diagnostics, the supreme court said no again.

On paper, the supreme court has consistently protected a vulnerable group of people from execution and affirmed that states must rely on “current clinical standards” when determining who fits into that group. But as a reporter and former capital defense investigator, I’ve worked directly with many people on death row who fall at the cusp of intellectual disability and still face execution. If the majority-conservative supreme court goes against precedent and agrees with Alabama in Hamm, our already unreliable protections for people with intellectual disability will only collapse further. Where a professional psychologist would diagnose intellectual disability, a stringent, non-clinical analysis like Alabama’s proposal would greenlight a path to the execution chamber. Any state could follow Alabama’s example. The court’s decision in Hamm will determine whether the cruel, unconstitutional execution of people with intellectual disability becomes even more prevalent.

Those with intellectual disability are overrepresented on death row. They are more likely to give false confessions, and less able to assist their legal teams. And they are still being killed. In just a few days, on 14 May, Texas plans to execute Edward Busby Jr, even though both the defense and the prosecution agreed he should not be killed.

Upon investigating, I learned that Busby’s prior execution date was halted for the courts to consider whether he was intellectually disabled. During those proceedings, the expert for the defense found him intellectually disabled. The prosecution’s expert agreed. But instead of entering the consensus order to remove Busby from death row, the trial judge found that “Busby’s intellectual functioning is not so subaverage that he is exempt from the death penalty”. He will now be executed in a matter of days.

I am horrified, but not surprised, by Busby’s predicament. It captures just how misunderstood intellectual disability still is. Judges, attorneys, and juries continue to make decisions as if this impairment is easy to see and to rule out. But the reality is far more complicated.

In my work as an investigator, I watched some clients refuse assessments for intellectual disability. The very impairment that could save their lives also stopped them from understanding the importance of testing. They feared embarrassment and further vulnerability in prison. I would bring paperwork to visits with other clients, who glanced at it before giving it to another trusted inmate to read the documents on their behalf. And I saw, over and over, how race and access to resources made life-altering differences in who received supportive interventions, and who did not.

The problem is not that it is difficult to apply straightforward, consistent clinical standards in a legal context. The problem is that it is difficult to reconcile horrible crimes with the reality that many who commit them also have serious, consequential limitations. Hamm v Smith is ultimately about this punitive urge, with some states wanting the ability to impose the harshest possible retribution without meaningfully considering individual culpability and individual humanity.

Accepting that punishment should be proportional to culpability does not excuse terrible harm. Despite what Donald Trump and the Department of Justice would have us believe, most people on death row are not ruthless masterminds. Often, who we are actually executing are people like Smith and Busby: those who have always needed significant support, but pass as just functional enough to slip through the cracks. If Texas kills Busby on 14 May, we will do it again. And if the supreme court sides against Smith in Hamm, we will revert even further in how we fail the vulnerable among us.

• Sophia Laurenzi is a journalist and writer who tells human stories about inhuman systems