Glucose, Self-Control and the Prefrontal Cortex

Cognitive Daily has a typically great review of some recent research connecting blood glucose levels and self-control:

Matthew Gailliot, along with Baumeister and six other researchers, asked 103 psychology students to fast for three hours before watching a video [the video required subjects to ignore salient stimuli, much like the stroop task]. Half the students were told to ignore the words, while the rest weren't required to exercise any self-control. Blood glucose levels were measured before and after this task. The students exercising self-control had significantly lower glucose levels after watching the movie, while the other students did not. In another experiment, students performed the Stroop task after watching the movie. The students who had to resist reading the words performed significantly worse on the Stroop task; their lower blood glucose levels after watching the movie and avoiding reading seemed to impair performance.

The scientists also did a neat experiment with Splenda. But you'll have to check out Munger for that.

This research is yet another reminder that the brain is severely bounded, a machine subject to all sorts of shortcomings. The history of Western thought is so full of paeans to the virtues of human rationality (see, for instance, Plato, Descartes, modern economics, etc.) that we've neglected to fully consider its limitations. The prefrontal cortex, it turns out, is easy to hoodwink. Forget to feed it for a few hours, or feed it Splenda instead of sugar, and you'll lose a significant amount of your rationality and self-control.

Consider this experiment. You're sitting in a bare room, with just a table and a chair. A scientist in a white lab coat walks in and says that he's conducting a study of long-term memory. (He's lying: this is actually a study of self-control.) The scientist gives you a seven digit number to remember, and asks you to walk down the hall to the room where your memory will be tested. On the way to the testing room, you pass by a refreshment table for subjects taking part in the experiment. You are given a choice between a decadent slice of German chocolate cake or a bowl of fruit salad. What food do you choose?

Now let's replay the experiment. You are sitting in the same room. The same scientist gives you the same dishonest explanation. The only difference is that instead of being asked to remember a seven digit number, you are only given two numbers, a far easier mental task. You then walk down the hall, and are given the same choice between cake and fruit.

You probably don't think the number of digits will affect your choice. If you choose the chocolate cake, it is because you wanted cake. But you'd be wrong. When the results from the two different memory groups were tallied up, the scientists observed a striking shift in behavior. Fifty-nine percent of people trying to remember seven digits chose the cake, compared to only 37 percent of the two digit subjects. Distracting the brain with a challenging memory task made people much more likely to give in to temptation, so that they chose the calorically-dense dessert. Their self-control was overwhelmed by five extra numbers.

Why did the two groups behave so differently? According to the Stanford scientists who designed the experiment, the effort required to memorize seven digits drew cognitive resources away from the part of the brain that normally controls our emotional urges. Because working memory and self-control share a common cortical sourceâ¯the prefrontal cortexâ¯a mind trying to remember lots of information is less able to exert control over its impulses. The substrate of reason is so limited that a few extra digits can become an extreme handicap.


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Perhaps those given the more difficult task chose the cake (and its refined sugar) because they need the immediate blood-glucose boost to assist with the task?

Yeah, I was wondering what other factors might explain the difference. I thought of blood-glucose levels, too.

Another explanation might be that subjects, anticipating poor performance at the more difficult task, seek comfort food as consolation. Did they do any sort of post-experiment questioning to see what reasons people gave.

I'd also agree with the suggestion that the more difficult task resulted in a greater desire for a quick glucose fix.

Also, I'm not entirely sure that a comparison of the percentages (59 vs. 37) alone, fully supports the loss-of self-control-due-to-a-demanding task hypothesis. If the participants were just randomly choosing a refreshment, 50% of them should be observed to choose the cake and 50% the fruit salad. The data shows that for the 7 number condition, 59% chose the cake; and for the 2 number condition 37% chose the cake. So, about 9% of participants are choosing the cake at a rate above chance in the 7 number condition; and 13% are choosing the cake at a rate below chance in the 2 number condition.

The 22% difference between the two conditions thus seems to be the result of two circumstances: (1) the participants in the 7 number task are choosing the cake at a rate 9% greater than chance; and (2) the participants in the 2 number task are choosing the cake at a rate 13% lower than chance in the 2 number condition.

This seems to suggest that in a baseline condition (where there are no task demands), the participants are most likely to have a preference for the fruit salad; which seems questionable.

By Tony Jeremiah (not verified) on 10 Mar 2008 #permalink

Yes, i completely support Tony Jeremiahs opinion. We should never forget to calculate the chance level, which in this experiment is 50% and thus the results are not striking.
Moreover, why eating chocolate cake instead of fruit saladis equal to lack of self control?
Did the researches made a control group, who just faced the choice? They need it, because its results should be the anchor for the evaluation of the preference.
Anyway - interesting experiment, not presented clear enough.