The human mind is a remarkable device. Nevertheless, it is not without limits. Recently, a growing body of research has focused on a particular mental limitation, which has to do with our ability to use a mental trait known as executive function. When you focus on a specific task for an extended period of time or choose to eat a salad instead of a piece of cake, you are flexing your executive function muscles. Both thought processes require conscious effort-you have to resist the temptation to let your mind wander or to indulge in the sweet dessert. It turns out, however, that use of executive function—a talent we all rely on throughout the day—draws upon a single resource of limited capacity in the brain. When this resource is exhausted by one activity, our mental capacity may be severely hindered in another, seemingly unrelated activity. (See here and here.)



Imagine, for a moment, that you are facing a very difficult decision about which of two job offers to accept. One position offers good pay and job security, but is pretty mundane, whereas the other job is really interesting and offers reasonable pay, but has questionable job security. Clearly you can go about resolving this dilemma in many ways. Few people, however, would say that your decision should be affected or influenced by whether or not you resisted the urge to eat cookies prior to contemplating the job offers. A decade of psychology research suggests otherwise. Unrelated activities that tax the executive function have important lingering effects, and may disrupt your ability to make such an important decision. In other words, you might choose the wrong job because you didn't eat a cookie.



Taxing Tasks



But what types of actions exhaust executive function and affect subsequent decision-making? Until recently, researchers focused on activities that involved the exertion of self-control or the regulation of attention. For instance, it's long been recognized that strenuous cognitive tasks—such as taking the SAT—can make it harder to focus later on. But recent results suggests that these taxing mental activities may be much broader in scope-and may even involve the very common activity of making choices itself. In a series of experiments and field studies, University of Minnesota psychologist Kathleen Vohs and colleagues repeatedly demonstrate that the mere act of making a selection may deplete executive resources. For example, in one study the researchers found that participants who made more choices in a mall were less likely to persist and do well in solving simple algebra problems. In another task in the same study, students who had to mark preferences about the courses they would take to satisfy their degree requirements were much more likely to procrastinate on preparing for an important test. Instead of studying, these "tired" minds engaged in distracting leisure activities.



Why is making a determination so taxing? Evidence implicates two important components: commitment and tradeoff resolution. The first is predicated on the notion that committing to a given course requires switching from a state of deliberation to one of implementation. In other words, you have to make a transition from thinking about options to actually following through on a decision. This switch, according to Vohs, requires executive resources. In a parallel investigation, Yale University professor Nathan Novemsky and his colleagues suggest that the mere act of resolving tradeoffs may be depleting. For example, in one study, the scientists show that people who had to rate the attractiveness of different options were much less depleted than those who had to actually make choices between the very same options.



Choosy about Choices



These findings have important real world implications. If making choices depletes executive resources, then "downstream" decisions might be affected adversely when we are forced to choose with a fatigued brain. Indeed, University of Maryland psychologist Anastasiya Pocheptsova and colleagues found exactly this effect: individuals who had to regulate their attention—which requires executive control—made significantly different choices than people who did not. These different choices follow a very specific pattern: they become reliant on more a more simplistic, and often inferior, thought process, and can thus fall prey to perceptual decoys. For example, in one experiment participants who were asked to ignore interesting subtitles in an otherwise boring film clip were much more likely to choose an option that stood next to a clearly inferior "decoy"—an option that was similar to one of the good choices, but was obviously not quite as good—than participants who watched the same clip but were not asked to ignore anything. Presumably, trying to control one's attention and to ignore an interesting cue exhausted the limited resource of the executive functions, making it significantly more difficult to ignore the existence of the otherwise irrelevant inferior decoy. Subjects with overtaxed brains made worse decisions.



These experimental insights suggest that the brain works like a muscle: when depleted, it becomes less effective. Furthermore, we should take this knowledge into account when making decisions. If we've just spent lots of time focusing on a particular task, exercising self-control or even if we've just made lots of seemingly minor choices, then we probably shouldn't try to make a major decision. These deleterious carryover effects from a tired brain may have a strong shaping effect on our lives.

Mind Matters is edited by Jonah Lehrer, the science writer behind the blog The Frontal Cortex and the book Proust was a Neuroscientist.