Blue. Try to say that word aloud and you might find yourself straining against the impulse to say the name of the color in which it is written. This cognitive interference is called the Stroop Effect, and it is most pronounced in those with self-control problems. While some people are much better at resisting impulses, even the most tightly controlled individuals sometimes succumb to a temptation. A study conducted by Dr. Roy Baumeister, published in the December 2007 issue of Current Directions in Psychological Science, helps explain why self-control sometimes crumbles.

As the Stroop task has been employed in countless psychological experiments, many researchers have noticed that subjects' performances degrade with repeated attempts. This is against expectations, because the commonly encountered Practice Effect usually ensures that subjects do better and better as they become more familiar with a task. In March of 2007, researchers from the University of Kentucky established that not only does self-control tire, but that this slipping can be detected with physiological measurements. They exposed subjects to a tantalizing platter of sweets and a platter of healthy food, and let them choose. Those who chose the health food had a tougher time controlling themselves on a second task. The sweet-snatchers also showed elevated heart rate variability, with the highest variability corresponding to the greatest decline in control on the second task.

Following up on this work, Dr. Baumeister gave subjects a string of Stroop tasks, taking blood-samples at regular intervals. He found that glucose levels steadily declined along with performance. To prove that glucose was responsible for the degeneration of self-control, subjects were divided up into two groups, one given an artificially sweetened drink and the other given a sugar drink. The group given sugar recovered its self-control and began to finish Stroop tasks with the initial speed and accuracy. It is a curious coincidence that the University of Kentucky used sweets to tempt its subjects. In the light of Dr. Baumeister's work. it seems likely that those who abstained from sweets in that prior study deprived themselves of the sugar needed to control themselves later.

It is far from clear that glucose is the sole fuel for our self-control mechanisms, but this latest experiment does strongly suggest that it is a crucial component of that fuel. This finding may help researchers looking for ways to treat impulse control disorders, and it may even lead to insight into the daily battles that the average person wages every day against the lure of cookies and internet distractions. For now, we can remember, that when we are better energized, we may be better able to exercise self-control.