The worst rolling 10-year total return the US stock market has ever produced is a loss of about 40%, for the decade starting at the September 1929 peak. The best rolling 10-year return is a gain of about 597% — roughly 21% a year — for the decade starting in the summer of 1949. Same market, ten-year holding period, wildly different outcomes depending only on when you started.

Since 1928 the S&P 500 has finished the year higher about 73% of the time — roughly three years in four end in the green. Down years are impossible to predict in advance, but the base rate quietly favors staying invested.

The market giveth and the market taketh away. US stocks returned roughly 3,000% in total from 1979 through 1999 — then gave back 28% of their value, in total, from 2000 through 2008.

"Stocks always win in the long run" has a real exception on the books: from the spring of 1996 through March 2020 — 24 years — long-term US government bonds returned about 8.2% a year, edging out the S&P 500’s 8.0%, and did it with roughly a third less volatility.