By Zoë Corbyn of Nature magazine

A prisoner's chance of parole depends on when the judge hearing the case last took a break, say researchers who have studied decisions in Israeli courts. As judges tire and get hungry, they slip towards the easy option of denying parole, say the researchers.

The bias could apply in any situation in which people make sequential decisions, such as doctor's consultations, university admissions or grant-review panels.

Jonathan Levav of Columbia Business School in New York and his colleagues analyzed 1,112 parole hearings for inmates of four Israeli prisons, made by eight judges over a ten-month period.

Judges' days were divided into three sessions broken by two meal breaks -- a morning snack and lunch. Judges decided when to break, but had no control over the ordering of cases, which was determined by when a prisoner's attorney arrived.

At the beginning of a session, a prisoner had a 65% chance of being paroled, the authors found. This declined to almost zero by the end of a session, and leaped back to 65% after a break.

The severity of the crime, the time served in prison, any previous incarcerations, and the availability of rehabilitation programs were not enough to explain the effect on the probability of parole, and the nationality or sex of a prisoner made no difference. The findings are published this week in the Proceedings of the National Academy of Sciences.

"The work shows the consequences of mental fatigue on really important decisions even among excellent decision-makers," says Levav. "It is really troubling and quite jarring -- it looks like the law isn't exactly the law."

Easy way out

Lavav thinks that decision-makers gravitate towards the easier, default option as they tire. In a judge's case, this would be to deny parole.

Whether the effect is caused by a lack of food, rest or both remains unclear. And the study draws no conclusions about the quality of the decisions made. "We can't say without a shadow of a doubt that there is a causal link here, but the data are extremely suggestive of one," says Levav.

The bias could "happen anywhere where there is sequential decision-making and some kind of status quo or default that allows people to simplify those decisions", Levav says.

"Even if the effect is half the size it appears to be it is still enormously important," says David Schkade, an expert in judges' decision-making at the University of California, San Diego. The methods used to reveal the bias are sound, he adds: "They have done the things you have to do to run it to ground."

"It is hardly the first empirical demonstration of 'extra-evidentiary bias' in judges' decisions, but it is a particularly striking one because the biasing factor is seemingly innocuous and so patently irrelevant to the case at hand," says Robert MacCoun, who studies law and public policy at the University of California, Berkeley.

But Jeffrey Rachlinski, who studies judges' decision-making at Cornell University Law School in Ithaca, New York, thinks that the finding is too neat to be plausible. He suspects that the judges might have in fact ordered their cases without the researchers' realizing it. "I would like to see something similar in another group of judges before I really believe it is a finding of general importance," he says.

But Lavav says the secretary of the proceedings confirmed that the order of cases was arbitrary, and that judges did not list timing as one of the factors influencing their decisions. He now plans to look at doctors' sequential treatment of patients.

This article is reproduced with permission from the magazine Nature. The article was first published on April 11, 2011.