Gains are wonky

Simple statistics, nothing else, makes it hard to beat the market.

It’s been a solid year for stocks. The S&P 500 index of big American companies is up 21%, as is the Nasdaq 100 index of mostly big tech companies. The MSCI World exchange-traded fund (ETF), which tracks stocks worldwide, has also surged 18%—a hearty return.

Yet most retail and professional investors will be sitting on less eye-popping profits. After all,…