When the Supreme Court ruled last month in favor of a woman sent to death row in Oklahoma based on lurid sexual evidence, it used a judicial tool that was once commonplace but has all but disappeared in recent years.

That tool is the summary reversal. Its decline is a mystery.

Summary reversals are neither full-blown rulings issued after oral arguments nor terse orders on emergency applications on what critics call the shadow docket. They are a third thing: unsigned decisions on the merits based only on what is ordinarily the first round of briefs in the case, the ones arguing over whether the justices should grant review at all.

“We use them when a lower court decision is squarely contrary to one of our precedents,” Justice Samuel A. Alito Jr. explained in a 2021 speech at Notre Dame.

A new study to be published in The Columbia Law Review found that in the first 15 terms after Chief Justice John G. Roberts Jr. joined the court in 2005, it issued an average of more than seven summary reversals each term.

That is a significant number in an era in which the court issues fewer than 70 signed decisions per term in argued cases.

In the past four terms, by contrast, there was an average of about one summary reversal, Kalvis E. Golde, a law student at Columbia and a Scotusblog columnist, found in the study.