That’s not really responsive to my point, @sbballer. I’m suggesting that the reason that the SCEA admissions rate at HYP is ~4x the RD rate is that the SCEA pool at these schools is very strong (much stronger than the RD pool, on average), at least half of the applicants HYP consider to be the most desirable (and available to them) apply SCEA, and HYP admit them then, without much fear of losing them. HYP don’t need to give anyone a boost to apply SCEA, because they aren’t nearly as worried about yield protection or fin aid budgets as the ED schools. Accordingly, I don’t think they do.
The principal consideration for HYP, if I’m right, is how much room they want to leave for people in the RD round (in which many of the best applicants participate only to a limited extent, if at all, because they already got into their preferred school early), while being aware that they’re somewhat less likely to land the ones they admit because they haven’t indicated a preference and may have other options within this peer group of schools. It’s more about class planning than yield or fin aid management.
Regarding recent history, H &P dropped early admissions for a few years, then reinstated it because Y (and S, I believe) didn’t play and got too many of H & P’s coveted admits early. As long as all these schools are behaving the same way, the equilibrium holds.
In the abstract, the math is that an average applicant is ~4x as likely to get in if they apply SCEA, but I don’t believe applying SCEA helps any individual applicant to anything like that extent. If you apply early and H wants you, they’ll take you. If they wouldn’t want you in the RD round, they’re very unlikely to want you SCEA. This is in contrast to the ED schools, which give some preference to ED applicants to induce them to commit early and provide these schools with the yield protection, selectivity benefit and fin aid management they need.
By the way, I would guess that there’s also very little difference in the likelihood of admission SCEA or RD to S, for the same reason. I think if you’re going to get in, leaving aside special cases like legacies who need to demonstrate interest by applying early, you’ll get in, in whichever round you apply.