WE HUMAN LIKE to think that we have much more self-discipline than other animals. We know how to set goals—losing 25 pounds, starting our own businesses—and then we resist temptations and slog through difficulties to achieve them. We are far from perfect at this talent, but in most of our minds there is no question that our powerful self-control is one of the things that sets us apart from more lowly beasts.

Scientists have long argued that delaying gratification requires a sense of “self.” Having a personal identity allows us to compare who we are today, at this very moment, with who we want to be—an idealized self. Such aspirations are thought to foster the kind of behavior that leads to self-improvement. But new research suggests a more primitive source of our powers of self-discipline. It appears that, lofty as our goals may be, we rely on the same basic biological mechanism for self-discipline as our four-legged best friends.

Sit. Now Stay.
Experimental psychologist Holly Miller and her colleagues at the University of Kentucky knew from previous research that in people, self-control relies on the brain’s “executive” powers, which coordinate planning and action. It is further known that this kind of effortful cognitive processing requires energy in the form of glucose, the simple sugar that serves as the body’s fuel. Studies show that depletion of the brain’s glucose supply compromises self-discipline. For instance, passing up a tempting happy-hour drink after work may make it tougher to forgo your favorite television show later on that evening to exercise. Of course, all mental activities require energy, but self-control seems to be one process that is especially compromised when the energy starts running out. But is this a uniquely human phenomenon?

To find out, Miller recruited a group of dogs ranging in age from 10 months to more than 10 years old. Some were purebreds, such as Australian shepherds and vizslas; others were mutts. All the dogs were familiar with a toy called a Tug-a-Jug, which is basically a clear cylinder with treats inside; dogs can easily manipulate the Tug-a-Jug to get a tasty payoff. In the experiment, some of the dogs were ordered by their owners to “sit” and  then “stay” for 10 minutes. That’s a long time to sit still; it was meant to exhaust the animals mentally and thus to deplete their fuel reserves. The other dogs, the controls, merely waited in a cage for 10 minutes.

Then all the dogs were given the familiar Tug-a-Jug, except that it had been altered so that it was now impossible to get the treats out. The hungry dogs could see and hear the treats—but they could not get at them. The idea was to see if the previous demand for self-discipline made the dogs less, well, dogged in working for the treats. And it did, unmistakably. Compared with the dogs that had simply been caged, those that had willed themselves to stay still for 10 minutes gave up much more quickly—after less than a minute, as opposed to more than two minutes of effort from the controls. In other words, it seemed as though exerting self-discipline had used up much of the dogs’ blood sugar supply—weakening their brain’s executive powers and diminishing the animals’ ability to exert goal-
directed effort.

Sugar-Powered Discipline
Executive powers? In old Shep? These findings suggest that self-control may not be a crowning psychological achievement of human evolution and indeed may have nothing to do with self-awareness. It may simply be biology—and beastly biology at that. These are humbling results, so the scientists decided to double-check them in a different way. In a second experiment, they recruited another group of dogs, this time made up of Shetland sheepdogs and border collies. As before, some of the dogs sat and stayed for 10 minutes, whereas the others were caged. But this time half of the obedient dogs got a sugar drink following the exercise, whereas the others got an artificially sweetened drink. Miller wanted to see if she could restore the dogs’ executive powers by refueling their brains.

And that is exactly what happened. As reported in the April issue of the journal Psychological Science, the dogs that exerted self-control and then got replenished with sugar performed just like the dogs that had not been exhausted to begin with. They persisted with the Tug-a-Jug, even though it was frustrating and demanding to do so. The depleted dogs that had not received the sugar drink gave up much more quickly. In short, all the dogs acted the way that humans do in similar situations requiring restraint and goal-directed activity.

So perhaps humans are not unique—at least not in this regard. It appears that the hallmark sense of human identity—our selfhood—is not a prerequisite for self-discipline. Whatever it is that makes us go to the gym and save for college is fueled by the same brain mechanisms that enable our hounds to sacrifice their own impulses and obey.