William L. Wilby, manager of the Oppenheimer Global Fund, contends that the fact that investing overseas hasn’t offered U.S. investors much diversification is an argument for, not against, putting money into a global mutual fund.

Wilby told The Wall Street Journal: "I've come to the conclusion that the world is one big stock market, and country differences have more to do with noise in the market. That noise often stems from indexes in overseas markets' weighting certain sectors more or less heavily than their U.S. counterparts, reflecting the relative size of local industries."

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