The results again challenged the energy model. As reported in a forthcoming issue of the journal Psychological Science, those who rinsed with the artificially sweetened drink were much less persistent -- consistent with the idea that self-control is mentally depleting. However, rinsing with the sugary solution appeared to restore the volunteers' lost willpower -- significantly more than rinsing with the artificially sweetened drink. The rinse is crucial here, and a departure from the original lab work: In the earlier experiments that led to the energy model, the volunteers had to actually ingest the sugar to get mentally replenished. But this study showed that merely rinsing with the sugary mouthwash had the same effect, restoring self-discipline. What's more, it had this effect immediately. The experiment allowed no time to metabolize the sugar and make it into brain fuel.
So what's happening here? If mental exertion is not depleting blood sugar, but is compromising subsequent self-discipline, then what's the mechanism? And what's restoring self-control, if not metabolized carbs? The scientists believe the mechanism is motivation. They believe that the mouth "senses" the carbohydrates in the mouthwash, and this sensation signals -- likely through the brain's dopamine system -- the possibility that a reward is coming. Sensing that an energy boost is coming, the brain is motivated to put in extra effort. In short, the sugar motivates -- rather than fuels -- willpower.
The scientists ran two different versions of the rinsing experiment. One demonstrated the effect of the mouthwash on physical persistence; the other on cognitive persistence. But one important question remained unanswered: Is it possible that even rinsing one's mouth with sugar might boost blood glucose -- drawing out the body's supplies? If so, this would revise -- but support -- the energy model of self-control. To address this, the scientists directly tested the effect of carbohydrate rinsing on blood glucose levels. They had a group of volunteers rinse repeatedly with a carbohydrate solution that was much stronger than the usual rinse -- to make the standard of proof as rigorous as possible. Others drank the same concentrated solution. The results gave further support to the new motivational model of self-control. Blood glucose levels jumped in those who drank the sugary drink, but didn't budge for those who rinsed.