“Among the world’s democracies,” Professors Law and Versteeg concluded, “constitutional similarity to the United States has clearly gone into free fall. Over the 1960s and 1970s, democratic constitutions as a whole became more similar to the U.S. Constitution, only to reverse course in the 1980s and 1990s.”

“The turn of the twenty-first century, however, saw the beginning of a steep plunge that continues through the most recent years for which we have data, to the point that the constitutions of the world’s democracies are, on average, less similar to the U.S. Constitution now than they were at the end of World War II.”

There are lots of possible reasons. The United States Constitution is terse and old, and it guarantees relatively few rights. The commitment of some members of the Supreme Court to interpreting the Constitution according to its original meaning in the 18th century may send the signal that it is of little current use to, say, a new African nation. And the Constitution’s waning influence may be part of a general decline in American power and prestige.

In an interview, Professor Law identified a central reason for the trend: the availability of newer, sexier and more powerful operating systems in the constitutional marketplace. “Nobody wants to copy Windows 3.1,” he said.

In a television interview during a visit to Egypt last week, Justice Ruth Bader Ginsburg of the Supreme Court seemed to agree. “I would not look to the United States Constitution if I were drafting a constitution in the year 2012,” she said. She recommended, instead, the South African Constitution, the Canadian Charter of Rights and Freedoms or the European Convention on Human Rights.