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We tend to assume that learning things easily is the same as learning them well. In school, teachers are pleased when children grasp a concept or a skill in one lesson, and so, of course, are children. The trouble is, when learning is too easy, we may not actually be learning much at all.

We know Abraham Lincoln, for example, as an autodidact who made himself erudite in literature, history, and the law. But if you had been at school with him, you probably wouldn’t have marked him out as a future lawyer, let alone a future president. A cousin remembers him as “somewhat dull … not a brilliant boy, but worked his way by toil.” Lincoln himself remarked that “I am slow to learn and slow to forget that which I have learned. My mind is like a piece of steel, very hard to scratch anything on it but almost impossible after you get it there to rub it out.”

Lincoln’s phrase — “slow to learn and slow to forget” — actually describes a universal truth about the way our brains absorb information. In the early 1990s, a cognitive scientist at the University of California named Robert Bjork landed upon an insight that changed the way psychologists think about learning. In its simplest form, it is this: We learn better when we ﬁnd learning difﬁcult.

In a series of carefully designed experiments, Bjork showed that when people learn something rapidly, they often learn it superﬁcially; that is, they are more liable to forget it in the long term. They are also less likely to integrate the new information with what they already know, which means the new knowledge is less “transferable” — that is, applicable to other problems. In one experiment, students were asked to study a passage of text with the aim of remembering it. Before reading it, one group was given an outline that summarized the information in the same order as the text, while another group was given an outline that put the same information in a different order. Participants in the ﬁrst group appeared to learn the text better—they scored higher on a test of recall. But when the two groups were set creative problem-solving tasks related to the text — the kind of tasks that required a deeper understanding of its content—it was the second group who came out on top. The extra difﬁculty faced by the second group in comprehending the text made it more difﬁcult to recall, but increased their understanding of what it was about. That meant they were better equipped to transfer their knowledge to the creative problems.

Similar findings abound: Psychologists at Princeton and Indiana University found that students remembered reading material better when it was printed in an ugly, difﬁcult-to-read font. And scientists from the University of Amsterdam gave people anagram puzzles to solve, while, as an obstacle to concentration, a series of random numbers were read out. Compared with those in a control group who performed the same task without this distraction, these subjects displayed greater cognitive agility — they were more likely to take leaps of association and make unusual connections. These researchers also found that when people are forced to cope with unexpected obstacles, they react by increasing their “perceptual scope” — taking a mental step back to see the bigger picture. In real life, this happens when you ﬁnd your journey to work blocked by a construction site, requiring you to mentally map the city in order to reach your destination.

Bjork coined the phrase “desirable difﬁculties” to describe the counterintuitive notion that we learn better when the learning is hard. His work has inﬂuenced thinking on education; he recommends, for instance, spacing teaching sessions further apart so that students have to make more effort to recall what they learned last time. Difﬁculty is desirable because it forces our brains to work harder at encoding and integrating the inputs that it has coming in. It makes us think, to put it plainly, and the harder we think, the better we remember. The same applies to anything we can get better at. Skills, in other words, come from struggle.

Excerpted from Curious: The Desire to Know and Why Your Future Depends On It by Ian Leslie. Available from Basic Books, a member of The Perseus Books Group. Copyright © 2014.