Games that use attention can help improve fluid memory.

Psychologists have long regarded intelligence as coming in two flavors: crystallized intelligence, the treasure trove of stored-up information and how-to knowledge (the sort of thing tested on “Jeopardy!” or put to use when you ride a bicycle); and fluid intelligence. Crystallized intelligence grows as you age; fluid intelligence has long been known to peak in early adulthood, around college age, and then to decline gradually. And unlike physical conditioning, which can transform 98-pound weaklings into hunks, fluid intelligence has always been considered impervious to training.

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Over the past three decades, theorists and researchers alike have made significant headway in understanding how working memory functions. They have developed a variety of sensitive tests to measure it and determine its relationship to fluid intelligence. Then, in 2008, Jaeggi turned one of these tests of working memory into a training task for building it up, in the same way that push-ups can be used both as a measure of physical fitness and as a strength-building task. “We see attention and working memory as the cardiovascular function of the brain,” Jaeggi says.“If you train your attention and working memory, you increase your basic cognitive skills that help you for many different complex tasks.”

Jaeggi’s study has been widely influential. Since its publication, others have achieved results similar to Jaeggi’s not only in elementary-school children but also in preschoolers, college students and the elderly. The training tasks generally require only 15 to 25 minutes of work per day, five days a week, and have been found to improve scores on tests of fluid intelligence in as little as four weeks. Follow-up studies linking that improvement to real-world gains in schooling and job performance are just getting under way. But already, people with disorders including attention-deficit hyperactivity disorder (A.D.H.D.) and traumatic brain injury have seen benefits from training. Gains can persist for up to eight months after treatment.