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I would add that I am not persuaded that international funds are a necessary component of an investor’s portfolio. Foreign funds may reduce a portfolio’s volatility, but their economic and currency risks may reduce returns by a still larger amount. The idea that a theoretically optimal portfolio must hold each geographical component at its market weight simply pushes me further than I would dream of being pushed. (I explore the pros and cons of global investing in Chapter 8.) My best judgment is that international holdings should comprise 20 percent of equities at a maximum, and that a zero weight is fully acceptable in most portfolios.
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John C. Bogle (Common Sense on Mutual Funds)