Between the courtroom antics of lawyers, witnesses, and jurors, reason doesn’t always prevail in our legal system. But judges are trained to be impartial, consistent, and rational, and make deliberate decisions based on the case in front of them, right? Actually no, according to a new study in PNAS, which shows that judges are subject to the same whims and lapses in judgment as the rest of us.

The authors examined over 1,000 parole decisions made by eight judges in Israel over a 10-month period. In each parole request, a prisoner appeared in front of a judge, and the judge could either accept or deny the request. The judges heard between 14 and 35 of these cases per day, separated into three distinct sessions. The first session ran from the beginning of the day until a mid-morning snack break, the second lasted from the snack break until a late lunch, and the third lasted from lunch until the end of the day.

Overall, judges were much more likely to accept prisoners’ requests for parole at the beginning of the day than the at end. Moreover, a prisoner’s chances of receiving parole more than doubled if his case was heard at the beginning of one of the three sessions, rather than later on in the session. More specifically, it was the number of rulings that a judge made, rather than the time elapsed in a session, that significantly affected later decisions. Every single judge in the sample followed this pattern.

Several attributes of the case, such as the severity of the crime, the months served by the prisoner, the prisoner’s prior number of incarcerations, and the prisoner’s demographics were taken into account in this analysis. While the judging patterns held up even when these variables were controlled for, not all these attributes tended to influence the rulings. Surprisingly, the severity of the crime and the amount of prison time served did not affect whether or not the prisoner was granted parole. Overall, the judges denied parole to 64.2 percent of the prisoners.

As a case study, one of the judges started in the morning by granting parole to about 65 percent of the prisoners; that percentage dropped to near zero by the end of the first session, then rebounded to about 65 percent after the snack break. The same pattern repeated in the second and third sessions.

The researchers suggest that as the number of rulings in a session increase, the judges become mentally fatigued. Once their mental resources are depleted, the judges are more likely to simplify their decisions. Ruling in favor of the status quo—denying parole—is the "easier" decision, the authors argue, since these rulings take generally take less time and require shorter written verdicts. After taking a break, their faculties are restored, and they are more likely to make "harder" decisions and grant parole requests again.

Since the order of cases is random and the judge decides when to take breaks, it seems reasonable that some type of fatigue is the best explanation for this pattern. The study didn’t test the effect of a meal versus the effect of a break, so it isn’t clear whether eating or just taking a time out causes this effect. However, what is clear from this research is that completely extraneous variables—such as when during the day a case is heard—can greatly influence a legal outcome.

PNAS, 2010. DOI: 10.1073/pnas.1018033108 (About DOIs).