The problem is that we have misjudged the depth of what we know:

We are duped by a misperception of “fluency,” believing that because facts or formulas or arguments are easy to remember right now, they will remain that way tomorrow or the next day. This fluency illusion is so strong that, once we feel we have some topic or assignment down, we assume that further study won’t strengthen our memory of the material. We move on, forgetting that we forget.

Often our study “aids” simply create fluency illusions — including, yes, highlighting — as do chapter outlines provided by a teacher or a textbook. Such fluency misperceptions are automatic; they form subconsciously and render us extremely poor judges of what we need to restudy or practice again. “We know that if you study something twice, in spaced sessions, it’s harder to process the material the second time, and so people think it’s counterproductive,” Nate Kornell, a psychologist at Williams College, said. “But the opposite is true: You learn more, even though it feels harder. Fluency is playing a trick on judgment.”

The best way to overcome this illusion is testing, which also happens to be an effective study technique in its own right. This is not exactly a recent discovery; people have understood it since the dawn of formal education, probably longer. In 1620, the philosopher Francis Bacon wrote, “If you read a piece of text through twenty times, you will not learn it by heart so easily as if you read it ten times while attempting to recite it from time to time and consulting the text when your memory fails.”