The introduction of a brain scan in a legal case was once enough to generate local headlines. No more. Hundreds of legal opinions every year have begun to invoke the science of mind and brain to bolster legal arguments—references not only to brain scans but to a range of studies that show that areas such as the amygdala or the anterior cingulate cortex are implicated in this response or that. The legal establishment, in short, has begun a love affair with all things brain.

Nita Farahany, a professor of law at Duke University, laid out the extent of this infatuation at the 2013 meeting of the Society for Neuroscience. Helped by a team of 20 law students and undergraduates, her research sifted through a massive pool of data to find more than 1,500 judicial opinions from 2005 to 2012 in which an appellate judge mentioned neurological or behavioral genetics evidence that had been used as part of a defense in a criminal case. “The biggest claim people are making is: Please decrease my punishment because I was more impulsive than the next person, I was more likely to be aggressive than the next person, I had less control than the next person,” Farahany said at a press conference.

Most neuroscientists think that studying brain scans may elicit overarching insights into the roots of violence, but individual scans lack the specificity to be used as evidence. “I believe that our behavior is a production of activity in our brain circuits,” psychiatrist Steven Hyman of the Broad Institute in Cambridge, Mass., told a session at the American Association for the Advancement of Science's annual meeting earlier last year. “But I would never tell a parole board to decide whether to release somebody or hold on to somebody based on their brain scan as an individual, because I can't tell what are the causal factors in that individual.”

It does not seem to matter, though, what academic experts believe about the advisability of brain scans at trial. Farahany found that most cases where neuroscience evidence was introduced resulted in an unfavorable outcome for the defendant, but not all. A bizarre twist has turned up in some cases in which a defendant overturned a decision that went the wrong way by accusing his counsel of failing to look into whether he had some kind of brain abnormality—ineffective counsel typically being an impossibly difficult claim. “If you were asleep as a defense counsel the entire time during the trial, if you were dead during the trial or if you failed to investigate a brain abnormality, you can be found responsible for ineffective assistance of counsel,” Farahany said. “That's a surprising trio.”

There is more to come. The arrival of brain science in the courtroom is “challenging fundamental concepts of responsibility and punishment,” Farahany said. “Should we hold people responsible for their actions once we understand concepts of impulsivity?”

Brain science also has implications for the fate of a convicted offender. “This is a country largely focused on retributivism as a basis for punishment,” she continued. “Is that a legitimate justification for punishment, or do we need to rethink what we do and instead focus more on rehabilitation?” Whichever way things go, jurors and judges are going to be hearing a lot more about amygdalae and orbitofrontal cortices.