Psychologists have been quite interested in understanding why willpower works so poorly. Essentially, your action system has two components: a Go System and a Stop System. The Go System is the one that supports action. It helps you engage in lots of activities that allow you to achieve your goals. It also supports the many habits that allow you to get through your day effectively without having to think about the many actions you take on a daily basis.

The Stop System (what we call willpower) applies the brakes to actions that are begun by the Go System, but conflict with some other newer goal. is a great example. You have built up lots of eating habits over the years. When you decide to go on a diet, you are adding a new goal that conflicts with many of those eating habits. You need your Stop System to prevent you from eating foots that would get in the way of your diet.

Unfortunately, the Stop System is not as efficient as the Go System. Lots of things can interfere with it. When you are , the Stop System does not work so well. Drugs and can prevent the Stop System from working. In addition, lots of research on suggests that when you use the Stop System a lot, it begins to work less effectively. So, a long day of having to control yourself at work can get in the way of your diet is likely to be in peril.

There is an ongoing debate in the research community about why these ego depletion effects happen. An intriguing set of studies by Matthew Gailliot and Roy Baumeister in a 2007 issue of and Social Psychology Review suggested that ego depletion may reflect actual energy levels within your body. There were two kinds of data that led them to this conclusion. First, they found that having to use the Stop System repeatedly led to decreases in blood glucose (sugar) levels (as measured with the kinds of glucometers that diabetics use regularly). Second, they found that after making people use their Stop System, a high-glucose drink kicked the Stop System back into gear.

A number of other researchers questioned these findings, however. A new set of studies by Daniel Molden, Chin Ming-Hui, Abigail Scholer, Brian Meier, Eric Noreen, Paul D’Agostino, and Valerie Martin in the October, 2012 issue of Psychological Science strongly suggests that failures of willpower reflect aspects of the motivational system, and not low levels of energy.

First, they repeated the physiological studies of blood glucose levels with more sensitive measuring equipment. They taxed the Stop System using a task that has been used in previous studies. They asked participants to look at a passage of text and cross out every letter e. This is not hard to do. Some participants then did the same task with a second passage. Another group had to cross out every e that was not next to another vowel. This latter task is harder and requires a lot of use of the Stop System. They measured people’s blood glucose before and after doing this task. Finally, they gave people a seven-letter word and asked them to generate as many smaller words as they could from the letters.

Consistent with lots of previous work, people who just did the more difficult letter-crossing task spent less time generating words than people who did the easier task. That is the typical ego depletion effect. However, the measure of blood glucose levels showed no difference between the groups in their glucose levels. That is, the more difficult self-control task did not consume more glucose than the easier task.

Next, the researchers explored the effect of drinking a high-sugar drink. They pointed out that drinking a sugary drink could have many effects on a person. Obviously, one is that it raises the level of glucose in the blood. However, the taste of the glucose is pleasant, and that might make people happy. Most importantly, the presence of glucose in the mouth might activate reward centers in the brain that could influence people’s to perform a task.

They explored this possibility in several other studies. In one study, they had people do the same letter-crossing task I just described. Then, people swished a sweet drink in their mouth and spit it out. For some people, that sweet drink had an artificial sweetener in it (which does not activate reward centers in the brain), while for others the drink had glucose in it. After that, people were asked to squeeze a hand grip for as long as they could. Squeezing a hand grip has been used as a measure of persistence in other studies.

The group that swished the drink with artificial sweetener showed the usual ego depletion effect. Those who did the hard letter-crossing task held the grip for less time than those who did the easy letter-crossing task. The group that swished the glucose drink in their mouth showed no ego-depletion effect. They were able to hold the hand grip for about the same amount of time regardless of which letter-crossing task they did. The researchers did a few other studies showing similar effects of glucose on more cognitive tasks.

So, what does all this mean?

Failures of willpower reflect the operation of the motivational system and not just a lack of energy. How will this help you engage your Stop System? If you find yourself in a difficult spot, try to find a way to give yourself a little reward. When you stand in front of that buffet table filled with desserts, seek out a friend and have a fun conversation. That rewarding conversation will help revive the Stop System and keep you on your diet.

This set of studies also shows how the scientific process works. The initial studies had some intriguing results. Whenever scientists find something new, others come along to explore the topic more fully. Even though the initial explanation (that the Stop System requires energy) does not seem to be right, we have learned a lot about motivation from this initial set of studies.

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