[Piers]

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Armed with this information, but with no information as to what Y is about, are you justified in believing that Y is probably true?

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Well you have a position for claiming Y is more likely then 30% to be true. You seem to be considering extrapolating and assuming a linier relationship between IQ and believers just cause its pretty, where it could easily be a curve with an initial linear part with an asymptote at say 40%. So the claim that Y is probably true is much more debatable than Y is more than 30% true.

You could just use your judgement and assert that Y is probably true. Which is subjective but nether the less a reasonable position.

Of course once we know what Y is everything changes, the specifics of exactly what Y is will carry more weight to most people than the above argument.