I measured the stock move on the first trading session after every quarterly earnings release by current S&P 500 companies since April 2020: 12,353 reports from 493 companies. The average move was 4.6% and the median 3.3%, about three times a normal day for the same stock. The part I didn't expect is how much bigger the moves have become.

Reports from Reports Average move Median move Moved 10% or more

Apr to Dec 2020 1,361 4.0% 3.0% 6.3%

2021 1,852 3.4% 2.6% 4.4%

2022 1,870 4.6% 3.3% 8.8%

2023 1,883 4.4% 3.3% 8.7%

2024 1,920 5.1% 3.9% 12.6%

2025 1,916 5.0% 3.6% 12.3%

Jan 1 to Oct 23, 2026 1,551 5.3% 3.9% 13.3%

In 2021 about one report in 23 moved the stock 10% or more. This year it's more than one in eight.

Five findings

1. It isn't index turnover. Newer, more volatile companies joining the S&P 500 could explain the trend, so I reran it on the 358 companies that reported in every year. Their average move went from 3.3% in 2021 to 5.2% in 2026, and the 10% share from 4.1% to 13.2%.…