The world of law as practiced in the real world is far removed from that usually discussed by law professors and philosophers or shown on television and in movies. In idealized or fictional cases the law always operates formally and may seem to pursue some abstract quest for justice. In the everyday practice of law, however, things work differently—it is all about cobbling together the most compelling and convincing story possible either for or against a defendant. Attorneys will shoehorn into their arguments any information they can find that might further their ends. The legal system therefore efficiently uses every scrap of relevant data that might pertain to a case—including findings such as brain scans that might not normally be formally admissible during trials.

The reality is that few criminal cases—only about 3 percent of federal ones and a bit more than 4 percent at the state level—ever go to trial. Normally, after someone is charged with a crime, the prosecution and the defense attorneys engage in confidential plea bargaining away from the courtroom. The plea negotiations are usually conducted informally (although in intense cases, such as those that might involve the death penalty, they can take on the appearance of a condensed sentencing hearing).

During these plea bargaining discussions, the defense presents any available information that might reduce the severity of the charges or the potential penalty against the defendant. To save a client from the death penalty, for instance, the defense might bring up brain-imaging data that suggests the defendant has a certain neurological or psychiatric condition that should reduce his or her culpability. The defense might suggest that the brain scans would at least cast doubt on the prosecution's ability to get a jury to return a sentence of death.

Of course, the prosecutor is free to reject this information and "let the jury decide." But in practice, if this informally presented neurological data seems persuasive, the prosecution may often agree to reduce the charges or to press for a lesser sentence—say, life imprisonment rather than death.

Why would prosecutors agree to such a change when brain scans are currently so rarely admissible as evidence in formal trials? They may not want to take a chance on what a judge will allow or what juries will believe. The more important reason, however, may be that negotiating an outcome without a trial usually makes both sides happy. Prosecutors might make a public show of disapproving of plea bargain deals but the fact is that trials are expensive; avoiding unnecessary ones conserves resources for the overburdened justice system.

This practice of acknowledging the neurological evidence during informal proceedings is not restricted to murder cases. In all cases where a client with a demonstrable neurologic or psychiatric issue faces a harsh penalty, negotiating a plea of not guilty by reason of insanity or a lesser charge is in the interest of justice.

The legal processes and standards governing the standards for evidence in actual trials powerfully influence the vast majority of cases subject to plea bargaining. Nevertheless, in the less formally bound proceedings of the court system brain scans and other neurological evidence can already have a significant impact. Because the content of plea bargain negotiations is never revealed, we cannot easily assess how profound their effect already is.