When people try to sneak something past a security checkpoint, their brains show different activity patterns depending on whether they are certain there is contraband in their suitcase or just know there's a chance of it, according to a new study. 

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Did you knowingly commit a crime? Brain scans could tell

The number of years someone spends behind bars can hinge on whether they were clearly aware that they were committing a crime. But how is a judge or jury to know for sure? A new study suggests brain scans can distinguish between hardcore criminal intent and simple reckless behavior, but the approach is far from being ready for the courtroom.

The study is unusual because it looks directly at the brains of people while they are engaged in illicit activity, says Liane Young, a Boston College psychologist who was not involved in the work. Earlier research, including work by her, has instead generally looked at the brains of people only observing immoral activity.

Researchers led by Read Montague, a neuroscientist at Virginia Tech Carilion Research Insitute in Roanoke and at University College London,  used functional magnetic resonance imaging (fMRI), which can measure brain activity based on blood flow. They analyzed the brains of 40 people—a mix of men and women mostly in their 20s and 30s—as they went through scenarios that simulated trying to smuggle something through a security checkpoint. In some cases, the people knew for certain they had contraband in a suitcase. In other cases, they chose from between two and five suitcases, with only one containing contraband (and thus they weren’t sure they were carrying contraband). The risk of getting caught also varied based on how many of the 10 security checkpoints had a guard stationed there.

The results showed distinctive patterns of brain activity for when the person knew for certain the suitcase had contraband and when they only knew there was a chance of it, the team reports today in the Proceedings of the National Academy of Sciences. But there was an unexpected twist. Those differing brain patterns only showed up when people were first shown how many security checkpoints were guarded, and then offered the suitcases. In that case, a computer analysis of the fMRI images correctly classified people as knowing or reckless between 71% and 80% of the time.

Montague isn’t sure why the results hinged on the order in which the scenario unfolded. It’s possible there is some threshold for the amount of information the brain needed before a distinctive thought process connected to recklessness kicked in, he says. In the laboratory, he says, perhaps the information about both the risk of grabbing a “hot” suitcase and of encountering a security guard “had to be all on board before we could see this difference.”

“I see this as a proof of principle that raises more questions than it answers,” says Montague, who notes that his team repeated the experiment several times with the same results. The lawyers working on the study recoiled at the idea that the results depended on the sequencing of the events, because people were engaged in risky behavior no matter the order. “I’m a scientist, so I was like, ‘This is the most interesting part of what we’ve found. We don’t know what to do with this,’” Montague says.

But another researcher who has used brain imaging to study criminal behavior questioned whether the differences found in the study really revealed a meaningful difference between the two mental states. Ruben Gur, a psychologist and director of the Brain Behavior Laboratory at the University of Pennsylvania’s medical school, says that with just 40 participants, a study may not be able to account for all the variables at play in brain scans. “To me, it proves that when you apply a statistical method on small samples you come up with interesting results, and some that make less sense.” 

Study author Gideon Yaffe, a philosopher at Yale Law School, notes there are some limitations of the research. It’s not known whether the different brain patterns are confined to this very specific scenario, or would apply more broadly to other situations outside the lab, he says. And he cautions that it’s a very big leap from there to visions of “evil intent detectors” scanning people’s brains. “It was so hard to do this modest thing that it feels very, very far away to be able to do something as grand as that.”

A more relevant scenario might be looking at whether the brains of people with certain conditions, such as drug addiction, process risk differently. That research, Yaffe says, might be introduced in court as people wrestle with how much someone really knew as they committed a crime.