John McCluskey killed a vacationing couple in eastern New Mexico in 2010, set their camper trailer on fire with their bodies inside, and took off with their truck. In sentencing hearings held after his conviction, McCluskey's lawyers argued that he should be spared the death penalty because abnormalities in his brain had made him impulsive and unable to control his behavior. Last week, a jury declared it had been unable to reach the unanimous decision required to sentence him to death.

It's not known if the brain scans and other scientific evidence played a role in McCluskey escaping the death penalty. And it's not the first time such evidence has been introduced when the death penalty was on the line. In fact, neuroscience is making increasingly regular courtroom appearances.

"It's amazing the extent to which judges, attorneys, and juries are taking this in stride," said Owen Jones, a legal scholar at Vanderbilt University who observed a few hours of testimony in McCluskey's case. "Just a few generations ago, this was beyond the realm of science fiction," Jones said. But now, "you watch the jurors and they reflect no outward manifestation of what an extraordinary thing it is to look inside another person's brain."

>'It's amazing the extent to which judges, attorneys, and juries are taking this in stride.'

Nita Farahany, a bioethicist at Duke University has been tracking the rise of legal cases involving neuroscience evidence in the U.S. The number of judicial opinions mentioning neuroscience evidence tripled between 2005 and 2011, from roughly 100 to more than 300. "It's more prevalent than my numbers show," Farahany said. That's because most cases involving neuroscience evidence do not result in a written judicial opinion, and those that don't are exceedingly difficult to find.

Common uses of neuroscience evidence, Farahany has found, include establishing whether a defendant is competent to stand trial, and mitigation during sentencing, along the lines of the evidence presented in McCluskey's trial. Another common use, and perhaps an even more telling one, is to establish ineffective assistance of counsel (in other words, to establish that an attorney isn't doing a good enough job for a client). For example, Farahany says, a defendant might try to convince a judge to order a new trail or dismiss a case altogether because his attorney failed to investigate a neuroscience-based claim regarding his mental state.

There was a modest dip in the number of cases involving neuroscience in 2012, which Farahany attributes to fewer capital cases overall that year. Capital cases, ones in which the death penalty is on the table, make up a large proportion of the cases in her analysis. She's just starting to look at the data for 2013, but so far she says it appears the numbers will be at least as high as those for 2012. She presented her findings at a recent meeting and will describe them in more detail in a paper in press at Nature.

Evidence presented in individual cases is just one way in which neuroscience is influencing the legal system. Brain science is playing a role in legal and policy decisions in other ways too. Sometimes that's a good thing. But it comes with the risk of getting too far out ahead of the science and basing real-world decisions on research that's too preliminary or otherwise unready for prime time. Below we've rounded up several recent developments that illustrate what's happening – for better and worse – and hint at what the future may hold.

Head injuries in sports ———————–

In August, the National Football League agreed to pay $765 million to settle a lawsuit brought by more than 4,500 former players and their families, alleging the league hid what it knew about the dangers of concussions suffered in the course of play. (Many think the league got off cheap.) Emerging evidence linking a history of concussions to a type of brain degeneration called chronic traumatic encephalopathy had a huge role in bringing about the settlement and persuading the league to institute rule changes and educational programs for players on the risks of head injuries, says Francis Shen, who studies the intersection of law and neuroscience at the University of Minnesota.

And the ripples caused by this evidence of brain damage keep spreading through the sports world. In November, 10 former National Hockey League players filed a class action lawsuit against the league, claiming the NHL knew about the long-term risks of head trauma but did little to protect players. A bill submitted in New York state this year would raise the eligibility age for playing tackle football to 14 from 11. The bill faces long odds, but even so it marks a dramatic turnaround, Shen says.

"We wouldn't have seen a bill like that even a few years ago," he said. "You have to wonder if we're watching the slow death of football."

No lie, we're out —————–

Until recently there were two companies offering lie detection services based on fMRI scans of brain activity: California-based No-Lie MRI, and Cephos, a company based in Massachusetts. But earlier this year Cephos decided to get out of the brain scan lie detection game – at least for now.

"I firmly believe that fMRI for lie detection works and is the most accurate means to determine truth from deception," Cephos founder Steven Laken told WIRED. But Laken says he underestimated the resistance he'd face from the judicial system. Cephos played a role in a 2010 trial in Tennessee in which a doctor accused of defrauding Medicare and insurance companies enlisted their services to try to establish his innocence. But after hearing from experts on both sides, the presiding judge decided that the fMRI evidence lacked scientific reliability and should not be admitted as evidence.

For the time being, Laken says Cephos will focus on forensic genetics and biotech consulting.

Mefloquine ———-

The case of Robert Bales earlier this year renewed longstanding worries about a malaria drug called mefloquine (brand name Lariam). Bales was a staff sergeant in Afghanistan when he went on a rampage, killing 16 people – mostly women and children – in two nearby villages. The nightmarish crime prompted speculation that mefloquine, a drug with a dubious history and long list of psychiatric side effects, may have played a role.

As Bales was about to be sentenced, the FDA added a black box to the drug's label warning of neurological and psychiatric side effects that can last for years after a person stops taking it. Bales's defense chose not to raise mefloquine as a possible mitigating factor during his sentencing, but questions about the drug won't go away anytime soon.

Mental illness redefined ————————

In May, psychiatrists issued a new edition of their profession's main diagnostic guidebook, the DSM. The DSM5, the first new edition in 13 years, has been controversial from the beginning. Its founders hoped to devise a scheme for diagnosing mental illness that's rooted in neuroscience and genetics. That turned out to be premature, and what resulted is a book that makes hardly anyone happy.

For the legal system, which uses DSM critieria for everything from resolving disputes about which kids are eligible for special education programs to deciding whether a defendant is mentally competent to stand trial, all those new definitions are going to take some getting used to. "Even though DSM says it's not for forensic purposes, it's used all the time for forensic purposes," said Shen. "The legal implications are quite enormous."

Cold water for 'seductive allure' ———————————

As neuroscience has infiltrated the courtroom, some researchers have worried that juries will be unduly influenced by all the pretty images and science-y talk. The notion that neuroscience evidence may have a particularly seductive allure arose mainly from two studies, both published in 2008. They found that images of brain scans or text that evokes brain science can make arguments about people's behavior (for example, that watching TV makes people better at arithmetic) seem more convincing than they would otherwise.

These studies have been cited hundreds of times, but more recent research – including several larger and more carefully controlled studies – doesn't back up their conclusions. Several papers published this review this literature and argue that there's nothing very special about images of the brain. (The title of one paper even warns of "The seductive allure of 'seductive allure'"). The new consensus? Laypeople may be influenced by expert testimony about the brain, but the images themselves add little if anything else.

Bioprediction ————-

Earlier this year, a team of neuroscientists reported a remarkable finding: They'd used fMRI scans to predict which inmates at two state prisons were most likely to commit a crime after they got out of prison. Those who showed a particular pattern of brain activity during a computer test of impulsivity were almost twice as likely to reoffend within 4 years.

At the time, the researchers were careful to say the method was nowhere near ready for real-world use, and other scientists pointed out a bunch of potential caveats. One neuroscientist followed up with a blog post, in which he reanalyzed some of the team's data and came to the conclusion that the predictive value of the brain scans was considerably less than what had been reported.

Even so, using brain scans to predict anything about human behavior is still pretty remarkable.

The work, and the reaction to it, raises an important issue: What scientists consider convincing and what the legal system considers good enough are two entirely different things.

"Scientists are trying to rigorously pursue an accumulation of truths that add up to a store of reliable knowledge," said Jones. "People in the legal system are trying to administer the fairest justice they can with the time, money, and human resources available." The time element is especially key, Jones says. "A judge can't say, let's keep this guy in jail while we do some more experiments."

Scientists may have the luxury of waiting for statistical significance, and peer review, and replication, but the legal system is often more concerned with whether a new type of evidence is better than the alternative, which is often 12 jurors squinting at a witness trying to figure out if he's telling the truth. So don't be surprised if neuroscience keeps showing up in court – sometimes in ways that make good sense, and sometimes in ways that make scientists squirm.