When the U.S. Supreme Court handed down its first ruling on the Affordable Care Act in 2012, some news outlets reported incorrectly that the law had been struck down.

That caused an immediate uptick in the stock prices of large health care corporations such as Cigna and Hospital Corp. of America. But as the news got corrected and it became clear that much of the law was upheld in NFIB v. Sebelius, health care stocks slid downward, ultimately costing investors more than $6 billion over two days.

A new analysis of the stock market impact of Supreme Court decisions over the past 15 years shows that a significant number of decided cases—one in 20—“move markets” when they are handed down, demonstrating that the court's opinions have ramifications beyond the concerns of lawyers and legislators.