From Warren Buffett’s 2017 letter to Berkshire Hathaway shareholders,

I want to quickly acknowledge that in any upcoming day, week or even year, stocks will be riskier—far riskier—than short-term U.S. bonds. As an investor’s investment horizon lengthens, however, a diversified portfolio of U.S. equities becomes progressively less risky than bonds, assuming that the stocks are purchased at a sensible multiple of earnings relative to then-prevailing interest rates.

It is a terrible mistake for investors with long-term horizons—among them, pension funds, college endowments and savings-minded individuals—to measure their investment “risk” by their portfolio’s ratio of bonds to stocks. Often, high-grade bonds in an investment portfolio increase its risk.