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What do you mean by the long term trend? Since 1959, a collateralized commodity index would have outperformed ALL asset classes, including the S&P500, with less risk.
http://papers.nber.org/papers/w10595
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Interesting study, I'd like to read it but don't want to pay for it. I'm assuming they acounted for stock dividends in their comparison. Two comments however. First, volatility isn't risk. Second, they studied commodities futures, not commodities values, and Keynes speculated that commodities future buyers are getting an enhanced return from hedgers.
I'm also curious as to why they started the study in 1959, instead over a longer period, say like from 1919. Then the same results would be even more compelling.