The Iqbal decision now requires plaintiffs to come forward with concrete facts at the outset, and it instructs lower court judges to dismiss lawsuits that strike them as implausible.

Image In Justice Ruth Bader Ginsburgs view, the court messed up the federal rules. Credit... Kiichiro Sato/Associated Press

“Determining whether a complaint states a plausible claim for relief,” Justice Anthony M. Kennedy wrote for the five-justice majority, “requires the reviewing court to draw on its judicial experience and common sense.”

Note those words: Plausible. Common sense.

The old world was mechanical. A lawsuit that mouthed the required words was off and running. As the Supreme Court said in 1957 in Conley v. Gibson, a lawsuit should be allowed to go forward “unless it appears beyond doubt that the plaintiff can prove no set of facts in support of his claim which would entitle him to relief.” Things started to change two years ago, when the Supreme Court found a complaint in an antitrust suit implausible.

In the new world, after Iqbal, a lawsuit has to satisfy a skeptical judicial gatekeeper.

“It obviously licenses highly subjective judgments,” said Stephen B. Burbank, an authority on civil procedure at the University of Pennsylvania Law School. “This is a blank check for federal judges to get rid of cases they disfavor.”

Courts applying Iqbal have been busy. A federal judge in Connecticut dismissed a disability discrimination suit this month, saying that Iqbal required her to treat the plaintiff’s assertions as implausible. A few days later, the federal appeals court in New York dismissed a breach of contract and securities fraud suit after concluding that its account of the defendants’ asserted wrongdoing was too speculative.

The judge hearing the claims of the falsely accused Duke lacrosse players has asked for briefing on whether their lawsuit against Durham, N.C., can pass muster under Iqbal. But the judge considering a case against John C. Yoo, the former Bush administration lawyer, said it could move forward despite Iqbal because the suit contained specific allegations about Mr. Yoo’s conduct in justifying the use of harsh interrogation methods.

In the Iqbal case itself, Javaid Iqbal, a Pakistani Muslim who was working as a cable television installer on Long Island, said he was subjected to intrusive searches and vicious beatings after being arrested on identity fraud charges two months after the Sept. 11 attacks.