At street level and in popular culture, Americans are freer with profanity now than ever before—or so it might seem to judge by how often people throw around the “F-bomb” or use a certain S-word of scatological meaning as a synonym for “stuff.” Or consider the millions of fans who adore the cartoon series “South Park,” with its pint-size, raucously foul-mouthed characters.

But things might look different to an expedition of anthropologists visiting from Mars. They might conclude that Americans today are as uptight about profanity...