22.15 THIS IS HOW WE DO IT: Does human judgment depend on blood sugar?

A balanced diet is essential for health—from simply surviving to maximizing physiological functioning. But what about for rationality, judgment, and decision making? To what extent is brain functioning influenced by our diet and nutrition?

These are important questions. But it can be challenging to approach such complex, subtle processes from an experimental perspective while retaining real-world validity.

Three researchers recently reported results from a clever study designed to do just this, evaluating some of the factors that may influence cognitive reasoning. Their approach is an elegant reminder that rigorous scientific thinking and investigation doesn’t necessarily involve a clear “control group” versus “experimental group” comparison.

The researchers examined real-world data on judicial rulings, to address the question: “Does the outcome of legal cases depend solely on laws and facts?”

Specifically, they set out to determine whether “extraneous factors can sway highly consequential decisions of expert decision makers.” Here’s what they did.

They evaluated all the decisions made by judges presiding over two parole boards in Israel, over the course of 50 days during a 10-month period. This included 1,112 judicial rulings. On any given day, the exact composition of the parole boards varied, but always included a judge, a criminologist, and a social worker. Serving four major prisons, these two parole boards process approximately 40% of all parole requests in Israel.

When a prisoner appeared before a parole board, the judge was given the case file, which included information on (1) the number of previous incarcerations, (2) the seriousness of the crime, and (3) the number of months already served in prison. Parole boards did not have any case information prior to the parole hearing.

Each day, each parole board heard 14–35 cases (mean = 22.6) in succession. The cases were presented in a random order, and the judge deliberated for an average of 6 minutes per case. The judge then ruled to either “accept” or “reject” each parole request, and the time of the decision was recorded. The default decision was to reject: 64.2% of requests were denied.

The routine of the parole board hearings is very regimented (and is recorded by a clerk). The judges had a late-morning snack (starting between 9:49 and 10:27 A.M. and lasting an average of 38.5 minutes) and a lunch break (starting between 12:46 and 2:10 P.M. and lasting an average of 57.4 minutes), so each day’s hearings were divided into three decision sessions.

The researchers were interested in the food breaks because many published research reports present evidence that mental fatigue can interfere with brain functioning. For example, one study reported that car buyers, after being asked to make numerous consecutive decisions about which options they preferred, became more likely to simply accept the default option in subsequent choices. And we know that consumption of food increases blood glucose levels, which can replenish mental resources.

Presenting their results in the Proceedings of the National Academy of Sciences, the researchers plotted the proportion of favorable decisions as a function of the time of day the case was heard, as shown below.

They found a clear and dramatic pattern: a parole request was much more likely to be granted at the beginning of the day or just after a food break—indicated by a dotted line in the graph. As each session wore on, the probability of a favorable ruling decreased, until it was close to 0%. Following a food break, the probability of a favorable ruling returned to about 65%. The first decision of each session is circled in the graph.

Because the order in which cases were presented was random, if the cases were being decided based solely on law and the facts of the case, the probability of a favorable decision should have been the same regardless of what time of day the case was heard.

The researchers concluded that legally irrelevant information may influence how a judge rules in such cases. More specifically, they suggested that mentally fatigued, hungry judges fall back on the easier default position of denying a request for parole. Engaging in cognitive reasoning depletes blood sugar, making additional mental effort difficult and unpleasant, while ingesting food restores one’s decision-making abilities. (To further support their conclusions that favorable decisions required greater mental effort, the researchers noted that those decisions took 41% longer and the written verdicts were almost twice as long.)

In their analyses, the researchers tested several additional hypotheses. They found that prisoners without previous incarceration were more likely to be granted parole. But the likelihood of getting parole was not influenced by the severity of the crime, the amount of time already served, sex, or ethnicity.

Viewed more broadly—as an investigation of the impact of nutritional state on decision making—these results may have numerous important implications. Can you see their relevance for doctors? Or stockbrokers? Or university admissions committees?