The Science of Memory

Many students have stayed up most of the night before an exam, passed it the next morning, and discovered a week later that almost nothing remained. The experience is so common that it can seem like a simple fact of life, but it is better understood as a consequence of how memory works. Psychologists usually describe remembering as three linked processes, called encoding, storage and retrieval, and cramming undermines every one of them.

Encoding is the process of turning an experience into a form the brain can keep. It depends heavily on attention: information that never receives focused attention is rarely encoded at all, which is why a reader can finish a page and recall none of it. Encoding is also stronger when new information is connected to meaning. In a well-known experiment published in 1975, people later recognized far more of the words they had judged for whether they fit sensibly into a sentence than the words they had judged only for whether they were printed in capital letters. A tired student rushing through notes at two in the morning tends to process material shallowly, noticing its surface rather than its meaning.

Storage is the keeping of encoded information over time. The brain does not save a memory the way a computer saves a file. New memories are fragile at first and become stable only gradually, through a process called consolidation. A curved structure deep in the brain, the hippocampus, is essential for forming new long-term memories, and studies suggest that much consolidation takes place during sleep, when recently formed memories appear to be replayed and strengthened. An all-night study session therefore sacrifices exactly the sleep that would have helped secure what was studied.

Retrieval is the process of bringing stored information back to mind, and it turns out to be more than a simple readout. Each successful act of recall makes a memory easier to recall later. Flash cards, practice questions and closing the book to summarize a chapter from memory all put this principle to work. In a 2006 study, students who reread a passage did slightly better on a test given five minutes later than students who had practiced recalling it, but one week later the students who had practiced recall remembered considerably more. Rereading creates a comfortable feeling of familiarity, which is easily mistaken for knowledge; cramming is largely rereading.

Timing matters as well. In the 1880s, the German psychologist Hermann Ebbinghaus memorized lists of nonsense syllables and tested himself at intervals, finding that forgetting was rapid at first and then slowed. He also found that repetitions spread over several days fixed a list more firmly than the same number crowded into a single sitting, a pattern now called the spacing effect. Reviewing after a gap forces the brain to work a little harder to recall the material, and that effort seems to be part of what makes the memory last. The practical lesson is not that students must study longer, but that they should arrange the same hours differently: spread over several days, spent recalling rather than rereading, and followed by sleep.