Skip to main content

Verified by Psychology Today

This Is How We Make Our Worst Decisions

New research on the roots and impact of brain drain.

In the interest of preserving your mental energy, let’s start this article with the eventual takeaway:

Don’t make important decisions when your brain is on empty.

You’ve heard this advice before, and it sounds reasonable enough, but does the science really support it? Increasingly the answer is: Yes. More and more it seems that our brains do hit an energy-drain threshold beyond which impulsiveness increases and sound decision-making suffers.

Source: pathdoc/Shutterstock

The latest evidence comes from a small study by researchers at the French Institute of Health and Research (INSERM). The team put a group of volunteers through six hours of challenging memory tasks. At various points, the volunteers were faced with choosing between receiving a small amount of cash now or a larger amount later.

As the hours of memory challenges ticked by, the volunteers were increasingly more susceptible to taking the easy money. The researchers then compared that group of participants with a group of volunteers who did easy memory tasks or spent the time reading and playing video games. For the most part individuals in the second group didn’t break down and choose the smaller amounts of cash up front.

The researchers also scanned the volunteers’ brains via fMRI and found that people doing the hardest memory tasks showed decreased activity in a brain area called the middle frontal gyrus, which earlier studies have shown is closely involved in decision-making. When you view this study in light of previous research, it seems the dynamic at play is a form of brain fatigue: Brain areas crucial to decision-making run low on energy and succumb to fatigue, not unlike muscles after a difficult workout.

The brain is an energy hog that uses 15 to 20 percent of the body’s circulating blood glucose each day. Particular brain areas use more or less of this energy depending on our neural workload. The latest study suggests that doing hard memory work requires a glut of energy and leaves less available for making good decisions. And when that happens, the cerebral gears start slipping and impulsiveness kicks in.

These findings build on those of another study from earlier this year suggesting that cognitive fatigue is a major factor in student performance on standardized tests. Over the course of a regular day, students’ mental resources get drained. The study showed that for every hour later in the day, test scores decreased by 0.9 percent.

Bottom line: Guard your mental energy and defer important decisions for when you have more on tap.

The study was published in the Proceedings of the National Academy of Sciences.