f personal memory systems are so great, why aren't they more widely used? This question is analogous to the old joke about two economists who are walking along when one of them spots a $20 bill. They say: “Look! There's $20 on the ground!” The other replies: “Impossible! If it were really there, someone would have picked it up already.” The analogy is only partial. In fact, Anki seems like a continual supply of $20 bills lying on the ground. And it's reasonable to ask why it's not more widely used. One of the most cited papers in the relevant research literature* is a discussion of why these ideas aren't more widely used in education. Although written in 1988, many of the observations in the paper remain true today. My own personal suspicion is that there are three main factors: In experimental research on memory, people consistently underestimate the gains that come from distributing their study in a manner similar to Anki. Instead, they prefer last-minute cramming, and believe it produces better results, though many studies show it does not. The psychologist Robert Bjork has suggested* the “principle of desirable difficulty”, the idea that memories are maximally strengthened if tested when we're on the verge of forgetting them. This suggests that an efficient memory system will intrinsically be somewhat difficult to use. Human beings have a complex relationship to difficult activities, and often dislike performing them, unless strongly motivated (in which case they may become pleasurable). Systems such as Anki are challenging to use well, and easy to use poorly. It is interesting to consider developing systems which may overcome some or all