Reviving this old thread: based on recent information and a lot of thinking about the subject, I think it works something like this. The admissions office has a pretty clear maximum number that they’ll take EA (~750 - ~800, at least until the new colleges open). I think they keep this to just under 40% of the total number of acceptances over the cycle, for policy reasons (they want the number admitted RD to be meaningfully larger). Some number of SCEA candidates are eliminated on first reading as unqualified. They go through the committee rounds and admit the candidates they really don’t want to lose to comparable schools or have to take for some reason (e.g., recruited athletes, strong URMs, development cases, kids with some unique talent, cockeyed geniuses, etc.), and when they hit the maximum number, they’re done with the acceptances. Those who got far enough through the committee rounds automatically get deferred. Some of the acceptances may be legacies, and many of the other legacies get automatically deferred (if they’re good enough to stand a chance in the regular pool, as many probably are, or if their parents are involved/generous). They then look at who’s left and size the deferral pool such that it has an anticipated admit rate comparable to the RD pool. Anyone left over is rejected.