The spacing effect was reported by a German psychologist Hermann Ebbinghaus in 1885. He observed that we tend to remember things more effectively, if we spread reviews out over time, instead of studying multiple times in one session. Since the 1930s, there have been a number of proposals for utilizing the spacing effect to improve learning, in what has come to be called spaced repetition.

One example was in 1972, when a German scientist called Sebastian Leitner popularized a method of spaced repetition with paper flashcards.


Background - Anki Manual