Some of the greatest investors in history have famously favored a substantial home-country bias for U.S. investors, meaning a portfolio overwhelmingly weighted toward American stocks.

Perhaps the most famous example is Warren Buffett . Beyond his long-running optimism ( "never bet against America" ), the Berkshire Hathaway chairman emeritus has left remarkably simple instructions for how money left for his wife should be invested after his death: 90% in a low-cost S&P 500 index fund and the remaining 10% in short-term Treasury bonds .

Vanguard founder John Bogle expressed a similar preference. Despite Vanguard becoming one of the world's largest providers of international mutual funds and exchange-traded funds ( ETFs ), Bogle argued that foreign stocks weren't necessary for a well-diversified U.S. investor. He generally favored a heavy U.S.…