In his 1962 book “The Theory of the Avant-Garde” Renato Poggioli observed that “Like any artistic tradition, no matter how antitraditional it may be, the avant-garde also has its conventions.” The most conventional of modernist conventions has been the need to shock and offend, doing so, as the lingo has it, by “transgressing boundaries.” But once all the boundaries have been blurred, what’s left? Salman Rushdie—whose credentials at discomfiting theocrats are unimpeachable—has lamented how lame and predictable transgressive art has become: “Once the new was shocking, not because it set out to shock, but because it set out to be new. Now, all too often, the shock is the new. And shock, in our jaded culture, wears off easily.” Where does that leave the artist or curator who wants to shake things up? According to Mr. Rushdie, he “must try harder and harder, go further and further, and this escalation may now have become the worst kind of artistic self-indulgence.” Not only is it self-indulgent, it’s self-defeating. Once the problem was, as Mr. Rushdie puts it, that shock wears off. But things are so far gone that shock rarely registers in the first place. This is the natural result of decades—the better part of a century, really—of artists using up the public’s reservoir of indignation. And if transgressive art can’t shock, what does it have to offer? After all, once you’ve seen Duchamp’s “Fountain” and gotten the joke, is there anything worth revisiting in it? Whatever frisson it might once have delivered was used up in its first display. Once the shock is gone, all that’s left is a urinal. The heirs of Duchamp can’t even count on the benefit of an initial shock. Pity the poor artist who would try to get a rise out of an audience these days with the naughty shenanigans that used to work so reliably. How easy it was to pique public sensibilities 30, or even just 20, years ago, when images of nonstandard sexual activities were not quite so commonplace. “Perversion is no longer subversive,” the fashionable leftist intellectual Slavoj Žižek has written, noting that “transgressive excess loses its shock value.” And yet the same old tropes meant to signal sexual daring and deviance keep getting trotted out as though they are edgy and bold gestures. When Bob Fosse first put his dancers on stage in fishnets and bowlers, eyebrows were duly raised. How many decades will Madonna continue to wear that same costume as if it were a racy innovation? In this, our transgressive artists are rather like the frustrated scaremongers in the film “Monsters, Inc.” The job of the workers on the “scare floor” is to gather the energy of fright from the children they spook. But as innocence is lost, the kiddies get harder to scare, and soon the factory is sputtering. The monsters finally change course, making their marks laugh instead of scream. But the art world hasn’t been so nimble. Shock is their stock in trade, even in the face of alarmingly diminishing returns. Who knows how long it will take for the one-trick transgressives to realize that they don’t have the impact they imagine, and soon may have no impact at all. I look forward to the day when—like the Grinch, straining to hear the boo-hoos from Whoville only to hear singing—the artists who fancied they could shock with their trite antics discover their targets are unperturbed, unfazed and uninterested.