I mentioned this to an officemate years ago once, but I never was quite satisfied with the explanation that there is a certain amount of excess return built into the price of a company’s stock as its risk premium, but which can be diversified away through some portfolio of disparate stocks.

If, merely by holding a diversified portfolio, the aggregate risk can be reduced, then one would expect the return demanded for such a portfolio to be less than the sum of the returns of the individual stocks. Yet this is not the case. So we must assume either the portfolio is overperforming, the individual stocks are underperforming, or both. On the other hand, it would seem that the risk premium of any individual stock would be arbitraged away by people holding the most diversified portfolios containing it, thus it is strange that an individual stock could retain an undiversified risk premium. Thus it must not, at least not fully. Its return (and the price it sells for) is also determined by the availability for sale of other not-fully-correlated stocks and their characteristics, even ones that have no material effect on the company’s performance. This is a sure sign of value arising from the demand for the instrument unrelated to its underlying. It would seem to be mispriced as an individual stock. Perhaps an equilibrium will be struck, but it is still paradoxical to talk about the risk premium of individual stocks as a property of that stock alone.

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