MOST people have experienced the feeling, after a taxing mental work-out, that they cannot be bothered to make any more decisions. If they are forced to, they may do so intuitively, rather than by reasoning. Such apathy is often put down to tiredness, but a study published recently in Psychological Science suggests there may be more to it than that. Whether reason or intuition is used may depend simply on the decision-maker's blood-sugar level—which is, itself, affected by the process of reasoning.

E.J. Masicampo and Roy Baumeister of Florida State University discovered this by doing some experiments on that most popular of laboratory animals, the impoverished undergraduate. They asked 121 psychology students who had volunteered for the experiment to watch a silent video of a woman being interviewed that had random words appearing in bold black letters every ten seconds along the perimeter of the video. This was the part of the experiment intended to be mentally taxing. Half of the students were told to focus on the woman, to try to understand what she was saying, and to ignore the words along the perimeter. The other half were given no instructions. Those that had to focus were exerting considerable self-control not to look at the random words.

When the video was over, half of each group was given a glass of lemonade with sugar in it and half was given a glass of lemonade with sugar substitute. Twelve minutes later, when the glucose from the lemonade with sugar in it had had time to enter the students' blood, the researchers administered a decision-making task that was designed to determine if the participant was using intuition or reason to make up his mind.

The students were asked to think about where they wanted to live in the coming year and given three accommodation options that varied both in size and distance from the university campus. Two of the options were good, but in different ways: one was far from the campus, but very large; the other was close to campus, but smaller. The third option was a decoy, similar to one of the good options, but obviously not quite as good. If it was close to campus and small, it was not quite as close as the good close option and slightly smaller. If it was far from campus and large, it was slightly smaller than the good large option and slightly farther away.

A drink to decide

Psychologists have known for a long time that having a decoy option in a decision-making task draws people to choose a reasonable option that is similar to the decoy. Dr Masicampo and Dr Baumeister suspected that students who had been asked to work hard during the video and then been given a drink without any sugar in it would be more likely to rely on intuition when making this decision than those from the other three groups. And that is what happened; 64% of them were swayed by the decoy. Those who had either not had to exert mental energy during the showing of the video or had been given glucose in their lemonade, used reason in their decision-making task and were less likely to be swayed by the decoy.

It is not clear why intuition is independent of glucose. It could be that humans inherited a default nervous system from other mammals that was similar to intuition, and that could make snap decisions about whether to fight or flee regardless of how much glucose was in the body.

Whatever the reason, the upshot seems to be that thinking is, indeed, hard work. And important decisions should not be made on an empty stomach.