Kind of a silly question, but it came up in our Sequences reading group yesterday: in EY's An Intuitive Explanation of Bayes' Theorem, we found the following statement:

It's like the experiment in which you ask a second-grader: "If eighteen people get on a bus, and then seven more people get on the bus, how old is the bus driver?" Many second-graders will respond: "Twenty-five."

Anybody have any idea where this finding comes from initially? We found several people who referenced EY's post, including one second-grade teacher who claimed that he'd been largely able to replicate it (a case of guessing the teacher's password, presumably), plus a bunch of "jokes" where the reader is the driver (so the correct answer is the reader's age), but I didn't see an original source for the experimental result. Maybe my Google-fu is weak but I'm curious if anybody knows...