Exactly, and this makes this purportedly logic problem a pyschologcial or sociological problem that is only tangentially related to the nominally pure logic of

Exactly, and this makes this purportedly logic problem a pyschologcial or sociological problem that is only tangentially related to the nominally pure logic of the question. Since we can with equal legitimacy infer either an exclusive or non-exclusive meaning to the "if", how we answer the question depends entirely on that choice. Amongst undergraduates it is not implausible that when you tell them "this is a logic problem" they will tend to assume--based on the norms of undergraduate thought--that the researcher means "iff" when they say "if". I certainly would have as an undergrad. To pretend that interpreting "if" as "iff" in an unusual situation is an error, is an error. The test subject is forced to guess what the researcher means, and for all kinds of reasons (not wanting to look stupid, etc) is unlikely to simply ask, "Do you mean if-and-only-if or not?" Almost all supposed "logic problems" are in fact psychological problems of this kind. Any problem of the form, "What is the next number in this sequence..." for example, has an infinite number of valid answers, since no finite sequence deteremines its next element. Solving such problems requires making an informed guess about the state of mind of the person who designed the sequence, and infering which of the infinite number of possible patterns they thought was the obvious one.