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Pop quiz: What is 357 times 289? No pencils allowed. No calculators. Just use your brain.

Got an answer yet? Got it now? How about now? Chances are you still don’t. As you solved the problem one step at a time, you lost track of the numbers. Maybe you tried to start over, lost track again, and eventually gave up in frustration before you could discover that the answer was 103,173. I used a calculator to get that, I confess.

Our mutual failure is absurd. The brain is, in the words of neuroscientist Floyd Bloom, “the most complex structure that exists in the universe.” Its trillions of connections let it carry out all sorts of sophisticated computations in very little time. You can scan a crowded lobby and pick out a familiar face in a fraction of a second, a task that pushes even today’s best computers to their limit. Yet multiplying 357 by 289, a task that demands a puny amount of processing, leaves most of us struggling.

For psychologists, this kind of mental shortcoming is like a crack in a wall. They can insert a scientific crowbar and start to pry open the hidden life of the mind. The fact that we struggle with certain simple tasks speaks volumes about how we are wired. It turns out the evolution of our complex brain has come at a price: Sometimes we end up with a mental traffic jam in there.


One of the first hints of this traffic jam emerged from a 1931 study by psychologist Charles Witt Telford, working out of the University of North Dakota. He had 29 graduate students sit in front of a telegraph key and instructed them to press the key as soon as they heard a sound. Telford played sounds to the students at intervals ranging from half a second to four seconds. He found that the interval between the sounds influenced how long it took for the students to respond. If the interval was one or two seconds, it took the students about a quarter of a second to react. But if Telford reduced the interval to half a second, the students consistently slowed down on their response to sound number two. It took them an extra tenth of a second to press the key.

This result reminded Telford of the way that muscles jerk in response to electric shocks. Muscles need time to recover from one shock before they can respond to the next. If you apply a second shock too soon, nothing happens. Perhaps, he speculated, the brain needs time to reset itself after a pulse of thought before it can carry out another one.

Psychologists have been running variations of Telford’s experiment for the past 80 years, and they all get the same basic result. If we don’t have enough time between two tasks, we slow down on the second one—a lag known as the “psychological refractory period.” The dryness of the term hides its huge importance to our ordinary life. In some situations the psychological refractory period can be a matter of life and death.

Harold Pashler, a psychologist at the University of California, San Diego, ran an experiment in 2006 that shows how crucial this lag can be. He had subjects sit in a driving simulator, complete with gas and brake pedals. As they drove along a virtual road behind another car, the volunteers would hear tones from time to time. They had to call out “one” or “two” depending on the number of tones they heard. Occasionally, the car in front would put on its brakes, and the subjects had to brake as well.

Pashler and his colleagues found that it typically took just under a second for people to respond to the brake lights on the car ahead. But it took longer for them to react if they had responded to a tone within one-third of a second before the lights went on. Pashler found that, on average, the test subjects’ reaction time increased by 0.174 second. That may not seem like a big difference, but if you are driving 65 miles an hour, it translates into an extra 16 feet. That distance can mean the difference between a close call and a high-speed rear-end collision.