What goes on in the brain of the groom who says “I do,” then has an affair? Or the friend who pledges to repay a loan but never does? Breaking a promise is a complex neurobiological event, a new study shows—and a brain scan may be able to predict those who are making false promises before they break their word.

Using functional MRI, scientists at the University of Zurich in Switzerland scanned the brains of subjects playing an investment game. Subjects assigned to be “investors” had to decide whether to pledge to share their money with other players who were “trustees.” This arrangement boosted the amount of money in the pot, but it also could result in a loss to the investor if the trustee chose not to share. Nearly all the subjects said they would give to the trustee—but in the end, not everyone kept this promise.

Based on the fMRI scans, the researchers were able to predict whether the players would break their promise before they actually had the chance to do so in the game. Promise breakers had more activity in certain brain regions, including the prefrontal cortex, an indication that planning and self-control were involved in suppressing an honest response, and the amygdala, perhaps a sign of conflicting and aversive emotions such as guilt and fear.

If the predictive ability of these scans is borne out in future studies, someday the technique could be of use to the justice system. “Brain imaging might be able to help psychologists or psychiatrists decide whether a criminal offender can be released or whether the risk of relapse is too high,” says lead author Thomas Baumgartner, who emphasizes that such scans would supplement assessments by health professionals, not replace them.