Hermann Ebbinghaus published his forgetting curve in 1885, and nearly every spaced-repetition app on the market today still schedules reviews as if it were the only curve that exists. Cognitive scientists have known for decades this isn't quite right — forgetting rates vary enormously depending on what kind of memory is being formed, and most consumer apps don't distinguish between them at all.

Not all forgetting is the same shape

Rote factual recall — a vocabulary word, a date, a formula — does decay in something close to Ebbinghaus's classic exponential curve. But conceptual understanding, the kind that lets you apply an idea in a new context rather than just recite it, decays far more slowly and resists the same review-interval math almost entirely.

  • Fact recall genuinely benefits from tight, algorithmic spaced intervals.
  • Procedural skill (like solving a type of equation) needs varied practice contexts, not just repetition timing.
  • Conceptual understanding often needs elaboration — explaining the idea in your own words — more than repetition at all.

Where the popular apps flatten this

Most flashcard-style apps apply one scheduling algorithm to everything a user adds, whether it's a vocabulary word or a nuanced argument from a philosophy course. The result, according to researchers we spoke with, is that students often feel like they're "reviewing" material they can technically recognize but can't actually use — a gap the algorithm has no way to detect, because recognition and application look identical from the outside of a flashcard interface.

An app that only measures whether you remembered the card is optimizing for a very narrow definition of learning. It's a good definition for a phone number. It's a bad one for a mental model.— Cognitive scientist interviewed for this piece

What to actually do about it

For rote facts, keep using spaced repetition apps as designed — the algorithm genuinely works for that narrow case. For anything conceptual, researchers recommend supplementing with retrieval practice that forces explanation rather than recognition: closing the book and writing out the idea from memory, or teaching it to someone else, rather than flipping a card.