2021 delivered another stellar year in US stock markets. The S&P 500 index, a commonly used benchmark to measure the US stock market, generated 26.9% excluding dividends1. The index hit 70 new all-time highs, the second highest on record since 1995 when the market hit 77 all-time highs2. This is also the third consecutive year with double digit gains. Surely all of this sounds ideal for investors. However, a look under the hood reveals some of the nuances of how the returns were generated and likely reveals a hidden risk for which investors should be aware.