All the jokey chatter around Swift's alleged non-human-ness is amusing enough, but it also inadvertently references decades of earlier American cultural history during which the female navel was seen as indescribably problematic, and a thing that should remain shrouded. In his book The Naked Ape, zoologist and ethologist Desmond Morris wrote that this immense discomfort stemmed from the fact that the female navel was essentially a "genital echo," a kind of symbolic "pseudo-vagina." The belly button served to draw the eye southward, like cleavage (or with men, like the "abdominal V").

The filmmakers of 1942's Arabian Nights were forced to remove depictions of dancers wearing belly-baring costumes. Some later movies sought to exploit censorship rules by adorning an exposed navel with a piece of jewelry, such as in Follow That Camel, featuring Anita Harris. Marilyn Monroe's final film, Something's Gotta Give, delayed from its original 1962 release, would also have been the first picture to show the actress's belly button. "I guess the censors are willing to recognize that everybody has a navel," she said before her death. When the TV show I Dream of Jeannie first aired in 1965, Barbara Eden was told to cover up her navel with high-waisted pants.

But by the early 1970s, the censors had begun to relent. Cher became the first actress to show her belly button on TV in 1975, a landmark moment somewhat undermined by her producer George Schlatter's comments in defense of the move: "As for Cher's belly button, at her weight it's not sexy and there's no violence to it at all." The film industry finally did away with its moral guidelines, known as the Hays Code in 1968, and TV broadcasters ditched their own ethics code in 1983. As depictions of nudity grew more prevalent, the navel became more normalized.

But the belly button taboo wasn't just a simple matter of propriety, or of puritanical cultural mores in the 20th century. Even navel-less photos of Eden in I Dream of Jeannie, or a sensually garbed Rita Hayworth, or a bathing-suited Monroe aren't completely demure—the images still reveal cleavage, legs, bare shoulders, or an accentuated derriere. When it comes to nudity in film and on TV, women didn't start out completely covered up, and gradually shed items of clothing on screen over the years as evolving standards allowed. Different body parts gradually became kosher at different times, seemingly without logic. But it's this high-waisted, slightly prim style that Swift has become virtually synonymous with. And she knows it: Right after posting her torso-baring Instagram photo, she ditched that bikini for another in her preferred style, captioning a photo of her wearing a halter one-piece: "'She's always wearing, like, a 1950's bathing suit.'"

In the mid-1990s, belly button piercings became de rigeur, spurred by a pre-Clueless Alicia Silverstone getting a navel ring in the 1994 music video for Aerosmith's "Cryin'." And in the years that followed, pants crept lower and lower, eventually culminating in what Slate decried in 2003 as "the crack epidemic": the horrors of the deeply impractical but trendy fashion for super-low jeans. Morris offered an one explanation for the feverish popularity of torso-baring in his book The Naked Woman: As more women began wearing pants and as cleavage became the norm, a newer erogenous zone was needed. Low-rise fashion brought whale tails and tramp stamps, but it also fueled the ascension of crop tops and hip bones jutting out from waistlines that hovered just a few inches above the crotch.

So when Swift's personal style is praised for being "classy," the underlying connotation relies on this immediate cultural memory. (That said, an entire book could be written on the navel's significance in other cultures and centuries.) In the spirit of Monroe or Hayworth decades ago, Swift can casually show off her arms, legs, and some cleavage, while remaining for the most part mom-approved. While the Federal Communications Commission isn't cracking down on every exposed belly button, that particular feature of a woman's body retains a certain sexual je ne sais quoi.