Do you want a citation for that P/E ratios reflect market expectations about future value (and thus earnings), or do you want a citation for the claim that the market is good at predicting what earnings will be in the future?
The claim that the market is good at predicting future earnings.
It probably is, but economics does not yet have the empirical grounding to give me high confidence in its theories (the way I would be for fields like physics or chemistry; I still think economic theory has a strongly positive correlation with reality).
This thread is for asking any questions that might seem obvious, tangential, silly or what-have-you. Don't be shy, everyone has holes in their knowledge, though the fewer and the smaller we can make them, the better.
Please be respectful of other people's admitting ignorance and don't mock them for it, as they're doing a noble thing.
To any future monthly posters of SQ threads, please remember to add the "stupid_questions" tag.