We learn more from our mistakes than from our successes, the old cliché says—and now scientists know why. Researchers at the University of Exeter in England discovered a brain mechanism that alerts us to situations in which we previously went wrong.

In the study, students playing physicians had to diagnose a fictitious disease based on images from equally fictitious blood samples. When participants saw images that had previously led them to an erroneous diagnosis, warning signals in the brain appeared only a tenth of a second later—much more quickly than did signals triggered by images that had resulted in a correct diagnosis. Earlier studies had confirmed that slipups do indeed result in better learning, but this one is the first to show the brain's specific reaction to a prior blunder.

This early-warning signal may be invaluable in situations ranging from the dangerous to the mundane. A child who touches a hot stovetop learns the hard way not to do it again—when she sees a glowing burner in the future, her brain will alert her to avoid the painful decision she made the last time.