@EDHDAD So in this case it seems to have given the school an advantage in recruiting your son. It seems to me the low yields at the schools that both have EA and are roughly 25-50 in USNWR suggests that it doesn’t work for them with most kids. This is also the response to the “demonstrated interest does seem to matter” - there is a pretty good correlation with the importance of demonstrated interest and low yields. This tells me adcoms aren’t very good at divining actual interest.
Clearly there is some perceived advantage for the university or they wouldn’t do it. Do they think their overall yields would be even lower without it? On the other hand, Tufts seems to have gone with ED1 and ED2, so there should probably be a new term for yield protection in early admissions.