Something leaped out at me from my colleague David Gates' provocative review of the upcoming Ken Burns World War II documentary, "The War." As he wrote, "some affiliates—which didn't seem to mind the obscenely gruesome Holocaust pictures or the scene where a machine gun blows off a soldier's head—had a problem with the four uses of cusswords, one of which is alluded to in the anagrammatic title of episode five, "FUBAR." (For you youngsters, this was a GI term standing for "F---ed Up Beyond All Recognition." Perhaps it was a snafu to include that.)

This is not news, of course. Films get in trouble with the motion picture ratings board for saying the F in FUBAR and snafu, but not for a high, gruesome body count. You can stop a pleasant dinner cold by uttering that word, but not by describing in gory detail the latest atrocity on the battlefront. For insight into the peculiarities of profanity, I turned to psychologist Steven Pinker's new book, "The Stuff of Thought."

Why is sex, which at first glance (and, if you're lucky, subsequent glances) seems like a nice thing, the source of so many taboo words, including the above? Because "sex has high stakes," Pinker writes, "including exploitation, disease, illegitimacy, incest, jealousy, spousal abuse, cuckoldry, . . . and rape." As a result, "plain speaking about sex"--and what is plainer that using variations on the f-word as noun, adjective and adverb?--"conveys the attitude that sex is a casual matter." Society as a whole does not want that conveyed, and if you think we're beyond that, Pinker counsels, notice that for all our sexual liberation most of us "still don't copulate in public, swap spouses at the end of a dinner party, [or] have sex with their siblings and children." Most people want to keep it that way. Sex-loaded terms starting with f--- threaten to erode the barriers we erect to behaviors like the above, so we treat them as taboo. Indeed, this aversion to casual sex is so embedded in the human psyche that trying to reason your way around it---surely no form of sex, casual or otherwise, is as bad as battlefield atrocities?---just doesn't work.