It cost Chief Justice John Roberts about 5,700 words (21 pages) to uphold the Affordable Care Act's health-insurance subsidies on Thursday. He spent three times as many in 2012, when he authored the majority ruling in National Federation of Independent Business v. Sebelius, upholding the law's individual mandate.

The brevity and clarity of Thursday's ruling is notable. In an era of longer -- and apparently grumpier -- judicial writing, Chief Justice Robert's ruling barely exceeded the median word count, which has climbed from about 2,500 words in the 1960s to more than 4,500 words in more recent years, according research by political scientists James F. Spriggs II of Washington University and Ryan C. Black of Michigan State.

But Supreme Court rulings have also become more accessible, when their authors want them to be, said Michael A. Livermore, a law professor at the University of Virginia who this year published a quantitative analysis of court rulings with computer scientists Daniel Rockmore and Keith Carlson at Dartmouth College.

"The reality is that almost every legal issue is susceptible to what makes an opinion long -- extensive discussion of legislative history, a huge review of relevant case law, every iteration of every possible counterargument," Mr. Livermore said. "It's more of a stylistic question of who the author is trying to communicate to and how the author wants to trade off the depth and breadth of the analysis against the clarity and accessibility of the opinion."

Mr. Livermore said there is no intuitive correlation between the public interest in an opinion and its length. Brown v. Board of Education, the 1954 decision that held segregated public schools unconstitutional, totaled fewer than 4,000 words.