An insider buying shares of their own company on the open market, with their own cash, is about the cleanest signal in public filings. I measured what followed 47,458 of those purchases: every Form 4 open-market buy on 3,859 US stocks from January 2020 to August 2025, grouped into one event per stock and disclosure day. The answer depends almost entirely on what you compare against, and on one test most versions of this study skip.

After Typical purchase Median listed stock S&P 500 Excess vs median stock Excess vs S&P 500

1 month -0.15% 0.00% 2.05% +0.12 pts -1.54 pts

3 months 0.95% 0.02% 5.29% +0.80 pts -3.56 pts

6 months 0.95% -0.41% 9.89% +1.07 pts -7.10 pts

1 year 2.36% 0.00% 17.84% +2.56 pts -12.95 pts

Same purchases, two answers: 13 points behind the index after a year, or 2.6 points ahead of the typical listed stock. Neither is really about insiders.…