In fashion there are two Hadid sisters. They are both successful models, but the surefire way to tell them apart is by their smile. When they are together, Gigi almost always does and Bella Hadid almost always doesn’t. Sometimes they swap, but on the whole, just as the sun rises and sets each day, as Ant stands on the left and Dec on the right, they always follow suit. Even at the Victoria’s Secret show, where maniacal grinning is almost law, Bella didn’t smile.

Facebook Twitter Pinterest Edie Campbell at the Molly Goddard show during London fashion week September 2017. Photograph: John Phillips/BFC/Getty Images for The British Fashion Council

This is helpful if you don’t know who they are – and useful if you’re deciding whether to “go Bella” or “go Gigi” during the season (ie your Christmas party) – but confusing to the rest of us. Particularly within the fashion bubble. Because, in fashion, smiling isn’t really a thing. On the catwalk, models usually don’t because they are precisely that – models, or vehicles employed to display the designer’s work, people without personality. It is a tradition that stems back to the 19th-century cartes de visite, the selfies of their day, a time when nonchalance (ie not smiling) was seen as aspirational, and not smiling while wearing the right clothes was a way into society. In short, smiling is also a class thing.

Facebook Twitter Pinterest Victoria Beckham in London 2014. Photograph: Will Oliver/EPA

So when did it become a fashionable act? The Hadids might have the status to weaponise smiling, but it’s also happening on the catwalk. On the Issey Miyake, Stella McCartney and Molly Goddard spring catwalks, models grinned. Even Victoria Beckham now smiles. As with the Supers, it’s models such as Gigi and Kendall Jenner – multi-hyphenates, with a strong social media presence, who have reclaimed their personality within the industry – who tend to smile more. The standout shot from Edward Enninful’s first Vogue featured Adwoa Aboah smiling as if her life depended on it. It also helps as the wacky gets wackier. As Vanessa Brown, a senior lecturer in design and visual culture at Nottingham Trent University, recently told Newsweek: “There must be an element of personal dignity at stake for a model forced to traverse the room in something that potentially makes them feel ridiculous” – so if you have to carry a child’s lunchbox down the catwalk, smiling will take the edge off.

Facebook Twitter Pinterest Meghan Markle at Kensington Palace in London on 27 November 2017. Photograph: Facundo Arrizabalaga/EPA

In many ways, it’s no surprise that this has happened now. Authenticity has never been more valued. Even at the royal photocall, smiling was deployed to great effect. Great smile, Meghan and Harry, said everyone afterwards. Great crow’s feet, said absolutely no one, except for those in the know, who could tell from the indiscernible little wrinkles – one of many anatomical tics according to the Duchenne smile theory that differentiates an authentic grin from a fake one – that they were genuinely happy to be there. All we need now is for Markle to be on the cover of British Vogue – the right calls are probably being patched through now – and smiling will be back in fashion. Or Vogue.