MarketWatch Premium: Stock market valuations have been higher for 20 years — is the change permanent?

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Those of you who agree with Mark Twain that the future at least rhymes with the past still must decide which period on which to focus.

This is an especially urgent question in the stock market, since almost all valuation ratios have exhibited a strong upward trend over the past century.

The bears who want to wrap themselves in the flag of…