Offer your thoughts or opinion on the input financial query or topic using your financial background.

Diversification reduces risk, but does this base on the assumption that expected return of each asset is always in proportion to its risk?
if I have a asset A with expected return of 100% and risk(measured by standard deviation) 1%, and asset B with expected return of 1% and risk 100%, would it be rational to put asset B into the portfolio ? In the capital asset pricing model (CAPM), investors are rational and have access to perfect information. Asset A sounds like an excellent investment,  B like a lousy one -- B is probably very far from the efficient frontier. Investors know this, so A's market capitalization will be high, B's low. According to the CAPM, you should then do the same rational thing everyone else is doing, which is to buy a lot of A and very little of B (each in proportion to its market capitalization). Of course the CAPM is just a model, and like any model it is only as good as its assumptions. However, I think this particular application of the model gives a pretty reasonable common-sense answer to the question.