Even among the most hands-off passive investors, one portfolio question tends to generate plenty of debate: How much should you allocate to international stocks?

There is no universal answer, but one useful reference point is the composition of a market-cap-weighted global equity exchange-traded fund (ETF) such as the Vanguard Total World Stock ETF (ticker: VT).

VT takes a straightforward approach. It attempts to own essentially the entire investable global stock market and weights companies according to their market capitalization. The result is a portfolio of more than 10,000 stocks, with U.S. companies representing approximately 62% of assets as of the end of July.

For investors who view market-cap weighting as an efficient way of…