In one study, she tested the patience of more than 230 hungry student participants by deliberately but discreetly crashing their computers when they were in the middle of a tedious task. That frustration alone was not enough to get a rise out of the hungry students. Ms. MacCormack had divided the subjects into two groups, one that focused on their emotional state as they worked on the computer and another that did not. Only the individuals in the second group, presumably less self-aware of their growing agitation, showed clear signs of stress and annoyance when the computer crashed.

“Being hungry clearly does change our affect, our emotional state,” Ms. MacCormack said. “But this evidence suggests that it does not automatically lead to being angry or more selfish.”

A recent series of experiments, led by Jan Hausser of Justus-Liebig-University in Germany and Nadira Faber of the University of Oxford, reached a similar conclusion. The researchers found that acutely hungry college students were as generous toward strangers as sated ones were. In one experiment, 51 students who had not eaten in 14 hours participated in so-called cooperation games, which digitally simulated joint investment decisions and rated the selfishness of those decisions. The students were famished, by their own reports, and measures of their blood glucose levels confirmed as much.

But they were just as generous in the cooperation tests as 51 peers who had arrived at the lab well fed , the study found. The researchers then tested the results in the real world. They recruited more than 600 participants at a cafeteria, half before they had eaten and the other half after lunch. Each participant was given either money (10 euros) or food (packets of dried fruit and nuts) and soon after was approached by a stranger, discreetly working with the research team, with a request: “Could you spare some change, or something to eat? I’m hungry.”

The famished students shared just as much of their money or food as the students who had just eaten, the study revealed. “We found that this big sensation of hunger did not affect how prosocial and cooperative these students were,” said Paul A.M. Van Lange, a psychologist at VU Amsterdam and an author of the study.