Diversification is the simplest and easiest way to reduce risk. In addition to those benefits, diversification can also boost returns. The following chart demonstrates that equity returns vary from year to year and that the US was not the best performer in any of the years from 2000 thru 2019 and was in the bottom half in 10 out of those 20 years.

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Source: Why Should You Diversify? by Wes Crill, Indexology Blog