Brain Says Guilty! Neural Imaging May Nab Criminals

judge's gavel
Brain scans might one day decide a suspect's guilt or innocence in court.

NEW YORK — Someday soon, judgments of guilt or innocence in a courtroom might be determined from a brain scan, scientists say.

Technologies for imaging the brain have advanced rapidly, to the point where it's possible to infer, for example, what object a person has stolen based on that person's neural activity. But how reliable is the science, and should it determine criminal fate? A panel of scientists and legal experts discussed these issues Saturday (June 1) at the World Science Festival, an annual celebration and exploration of science held here.

The panel discussion was based on an upcoming PBS documentary called "Brains on Trial with Alan Alda," expected to air in September, and moderated by Alda himself. [Watch a Replay of 'Brains on Trial' Discussion]

Guilt in the brain

Functional magnetic resonance imaging (fMRI) is one promising technique for determining a person's guilt. The technique detects changes in blood flow that highlight which parts of the brain are active.

In a clip from the documentary, Alda participates in an fMRI experiment. He is told to "steal" an object — either a ring or a watch — from a drawer, without telling the researcher what he took. Alda undergoes an MRI scan where he is instructed to lie about what he stole. From that scan, the researcher correctly determines what Alda stole, because when he lied, the activity in part of his brain changed and gave him away.

Using brain scans, scientists can detect when a person is lying with 70 to 90 percent accuracy, said panelist Anthony Wagner, a psychologist and neuroscientist at Stanford University in Palo Alto, Calif.

In other studies, a subject being scanned need only look at an image (of a crime scene, for instance), and their brain will light up in a certain way if the image is familiar, Wagner said. The problem with using fMRI scans as evidence of guilt is that the brain may show similar activity patterns if the subject simply imagines committing the crime.

Scanning the brain of an accused person also brings up concerns about the right to privacy. "What kinds of constitutional or other legal protections might a person have, which would preclude police from being able to give a suspect a brain scan?" said panelist Nita Farahany, a law professor at Duke University in Durham, N.C. "There's nothing clear-cut that would protect us against that kind of thing if we are a legitimate suspect of a crime," Farahany said.

Assuming brain scans are used, there's always the chance people will find ways to beat the system. Farahany herself participated in one of Wagner's experiments that used fMRI scans to assess a person's familiarity with an image. Wagner told Farahany to deliberately try to confuse the system, just as a criminal might do to conceal guilt. Farahany was fairly successful, and Wagner was only able to detect her mental familiarity with the images with 70 percent accuracy. 

Still, brain scans might be an improvement over eyewitness testimony, which is notoriously fallible. When determining truth and lies from a brain scan, Farahany said, "this is dangerous information to admit [into a courtroom] if it's wrong." However, she said, the technology is more reliable than some of the evidence used today, and its accuracy is improving faster than people think.

Neuroscience in sentencing

Advances in neuroscience are also providing insight into the brains of criminals and groups at risk for committing a crime.

The justice system in the United States considers people ages 18 and above adults, and criminals are tried as such. But research shows the average adolescent's brain continues to develop well into their 20s. In particular, the prefrontal cortex, the part of the brain involved in impulse control, doesn't mature until a person reaches about age 25.

Psychopaths also display brain differences compared with non-psychopaths, studies show. The limbic system, the brain's emotional center, is less dense in people with a diagnosis of psychopathy.

These kinds of insights have led criminals to claim brain weaknesses in defense of their actions. In other words, "Don't blame me, blame my brain." Neuroscientists are continually learning about what makes people criminals. But how society will use this knowledge remains to be seen.

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Tanya Lewis
Staff Writer
Tanya was a staff writer for Live Science from 2013 to 2015, covering a wide array of topics, ranging from neuroscience to robotics to strange/cute animals. She received a graduate certificate in science communication from the University of California, Santa Cruz, and a bachelor of science in biomedical engineering from Brown University. She has previously written for Science News, Wired, The Santa Cruz Sentinel, the radio show Big Picture Science and other places. Tanya has lived on a tropical island, witnessed volcanic eruptions and flown in zero gravity (without losing her lunch!). To find out what her latest project is, you can visit her website.