UPDATE: Moving to front from last year.
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Here is the official APA statement:
Students are under no obligation to respond to offers of financial support prior to April 15; earlier deadlines for acceptance of such offers violate the intent of this Resolution. In those instances in which a student accepts an offer before April 15, and subsequently desires to withdraw that acceptance, the student may submit in writing a resignation of the appointment at any time through April 15. However, an acceptance given or left in force after April 15 commits the student not to accept another offer without first obtaining a written release from the institution to which a commitment has been made. Similarly, an offer by an institution after April 15 is conditional on presentation by the student of the written release from any previously accepted offer. It is further agreed by the institutions and organizations subscribing to the above Resolution that a copy of this Resolution should accompany every scholarship, fellowship, traineeship, and assistantship offer.
It goes without saying, I hope, that if a student is able to decide before April 15 that is often very helpful both to the Department and to students on wait-lists for financial aid.
Best of luck to everyone making decisions on where to study.
UPDATE: A philosopher from Britain writes:
Just seen the APA resolution you posted. Interesting, of course, that UK and Australasian departments are not part of the APA and the APA has never approached us, as far as I know, to see if we wish to be bound by this arrangement. Therefore we are not. Candidates are therefore not prevented by any formal agreement between institutions from accepting a US offer and then declining it for an overseas offer. This is a serious point in the UK as under current arrangments many sudents will not know whether they have received a government scholarship until August, several months after the US deadline.
In general US departments have to appreciate that any rules they make to regulate their own affairs are not binding on non-US departments who have not even been consulted about these rules. We can't, after all, have the APA acting like the US government.
I wonder what other philosophers think about this issue?