Do you really forget half of what you learn within days?

Roughly, according to the classic forgetting curve, but the evidence is thinner than the famous graph suggests. The Ebbinghaus forgetting curve measures how memory declines over time when there is no attempt to retain it, and a typical version of the graph purports to show that people halve their memory of newly learned knowledge within days or weeks unless they consciously review it. It rests on a limited, incomplete study that Hermann Ebbinghaus ran on himself from 1880 to 1885, memorizing nonsense syllables like "WID" and "ZOF" and testing himself after various time periods.

That study was still a landmark: he was the first to run a series of well-designed experiments on forgetting, and one of the first to use artificial stimuli in experimental psychology. He hypothesized that the speed of forgetting depends on how difficult and meaningful the material is, how it is represented, and physiological factors such as stress and sleep. In 2015, an attempt to replicate the curve, again with a single subject, produced results similar to his original data.

Ebbinghaus also described what slows the decline. Each repetition increases the optimum interval before the next one is needed, from days at first to years later; information is easier to recall when built on things already known; and the curve flattened with every repetition. Evidence suggests that waiting 10 to 20 percent of the time until the information will be needed is the optimum moment for a single review. Simple equations, though, have not been found to fit the available data well, so the exact shape of forgetting is still debated.