Question : Empirical research on CAPM indicates that?

Beta is or is not an accurate predictor of a portfolios future volatility,
And diversifiable risk is, in fact, irrelevant to predictions of future volatility.


I searched and searched through my textbook, and found several spots in the text related to this, and each contradicted the other. It is noted in one spot that Robert Levy, Marshal Blume and others came to the conclusion that Beta is not an accurate predictor of future volatility. I'm lost on this question and I can't find clear reference either way.