John McCluskey escaped from an Arizona prison in July, 2010. A few days later, he and two accomplices – one of whom was both his cousin and fiancee – carjacked Linda and Gary Haas, a vacationing Oklahoma couple in their 60s. McCluskey shot the Haases inside the camping trailer they were towing behind their truck, and set the trailer on fire with their bodies still inside. McCluskey was convicted for the carjacking and two murders in federal court on Oct. 7.

Yesterday the jury charged with deciding his sentence announced that it had been unable to come to a unanimous decision on the death penalty. That means he'll get life without parole.

Perhaps it's little wonder the jury couldn't agree – they'd been given a lot to consider. McCluskey's defense team had tried to convince them that he has several brain defects that, combined with other factors, contributed to his crimes and should be considered mitigating circumstances. The defense presented the results of several types of brain scans and various psychological tests, as well as testimony from neurologists and other experts.

Evidence of John McCluskey's brain abnormalities presented by his defense team. Blue areas represent larger deviations from normal. PACER

"What they found, in a nutshell, was a few structural abnormalities in his brain," said Kent Kiehl, a neuroscientist at the Mind Research Network and University of New Mexico who was not directly involved in the case but observed several days of testimony. The defense did their best to argue that McCluskey's brain abnormalities made him more impulsive and altered his emotional processing, Kiehl says. The prosecution did their best to tear these arguments apart.

In the sentencing phase of the trial, McCluskey's lawyers argued that, as a result of his brain abnormalities – as well as a slew of other unfortunate circumstances ranging from a breech birth, to abuse as a child, to drug and alcohol addiction – he was incapable of "a level of intent sufficient to allow consideration of the death penalty." Essentially, they argued that his acts were impulsive, that he would have been incapable of planning such things.

So what were these brain abnormalities? Not all of the evidence is currently available, but the account below draws on court documents and Kiehl's recollection.

The defense presented evidence of damage to the cerebellum, a region at the back of the brain best known for its role in balance and coordinating movement. But the cerebellum has other roles as well, and the defense argued that McCluskey's damage, likely caused by a stroke, was indicative of something called cerebellar cognitive affective syndrome, which can cause problems with planning and controlling behavior.

"I'd never heard of this before, but it's in neurology texts," said Kiehl.

MRI scans presented by the defense suggested that McCluskey's superior frontal lobes are unusually small in relation to his overall brain volume. According to court documents, Penn neuropsychologist Ruben Gur offered this explanation of what that might mean when asked by the defense: "And these additional anatomic abnormalities that we see on MRI in the frontal lobe would – would also relate to deficits in the – in putting your action in context, in the ability to control your behavior, especially when under emotional stress or arousal."

A report submitted to the court by Gur, and provided to WIRED by Kiehl, suggests abnormalities in brain activity as well as anatomy. PET scans, for instance, found hyperactivity in McCluskey's frontal lobes and amygdala, a region involved in regulating emotion. "A damaged amygdala will misinterpret danger signals and when excited, will issue false alarms that require intact frontal components of the limbic system for modulation," Gur wrote in the report. The frontal lobes normally provide a brake on the amygdala, Gur explains, but this brake seems to be broken in McCluskey, thereby allowing the "primitive emotional impulses emanating from the amygdala" to go unchecked.

Gur's report also mentions abnormal activity in the parietal lobe and corpus callosum, the thick band of fibers connecting the two halves of the brain, and suggests that these abnormalities too could cause impairments of reasoning.

Many, many more abnormalities go undiscussed in the report. All in all, the PET scans found 10 areas that were less active than normal in McCluskey's brain and 17 areas that were hyperactive, each one of which presumably could exert some positive or negative influence on his behavior.

The prosecution didn't take all this lying down, Kiehl says. They argued that, brain scans aside, McCluskey showed clear evidence of being able to plan and control his behavior and exercise intent. "The prosecution argued that he didn't lack volition, he ran a drug circle in prison, he had over 50 clients, he was basically able to organize, procure, and deliver. They found his accounting books after he escaped," Kiehl said. "He worked out and exercised for 6 months so he'd be able to run five miles through the desert after he escaped."

Kiehl says prosecutors questioned Gur on cross-examination about what regions of the brain would be involved in planning and executing those types of behavior. "Well, the frontal lobes, the parietal lobes...," Kiehl recalls Gur saying. In other words, some of the same areas that were supposedly impaired in McCluskey.

"It was a really great cross-examination about how just because he has these 'holes' in his head, he's still able to plan and do careful accounting and all these other things," Kiehl said.

At this point, it's not clear whether the brain scans and other scientific evidence played a role in McCluskey receiving the lighter of the two sentences for which he was eligible. But there's reason to think it might have.

In a 2009 case in which Kiehl testified for the defense, lawyers introduced fMRI scans of brain activity in an attempt to spare convicted murderer Brian Dugan from execution. The jury didn't buy it, but they deliberated for 10 hours before handing down a death sentence. "This guy was guilty of raping and killing little girls," Dugan's defense lawyer told Science magazine at the time. "Without the brain imaging stuff the jury would have been back in an hour."

Then in 2010, a Miami jury rejected the death sentence for Grady Nelson, who stabbed his wife to death and raped her developmentally-disabled 11-year-old daughter, after hearing evidence that Nelson had abnormal brain activity. Afterwards, some jurors said they'd been swayed by the brain recordings presented by the defense. "It turned my decision all the way around," said one.