You walk past shop windows every day. Ever wonder what story stands behind the mannequins inside?

These props exist to show off clothes. What has their own evolution looked like?

In 2008, when Katy Perry released her debut album One of the Boys, it included a playful single called “Mannequin,” about a schoolgirl who falls in love with a mannequin. The song may also be a metaphor for unrequited love, since it borrows every trait of a mannequin — perfect, beautiful, and yet “not quite a person.”

A similar story played out in the 1987 film Mannequin, starring the British actress Kim Cattrall. The male lead falls in love with a mannequin, and this fake, perfect human comes to life for him. “I worked out a lot, because I wanted my body to have the same perfect lines as a mannequin,” Cattrall said afterward.

In the more than 100 years since the mannequin was invented, the thing has served as a screen onto which human society projects its shifting ideals of beauty. In that sense it does much the same social work as science fiction: both use hypotheticals to argue out social and ethical questions — “androgyny,” “women's liberation,” “the question of nudity,” “doll worship,” “consumerism,” “wartime food rationing.”

But look at the mannequin's history and you see that humans spent most of their energy on better materials and a more lifelike look. Thinking critically about the object itself came late.

It is said that in 1923, when the British archaeologist Howard Carter, a pioneer of Egyptology, opened the tomb of the Egyptian pharaoh Tutankhamun, he found a wooden figure with no hands or feet, proportioned like the pharaoh and dressed in the pharaoh's clothes. Dated to around 1350 BC, it may be the world's first mannequin. The wife of Nero, the tyrant of ancient Rome, kept a “stand-in” made in her exact likeness and used it to preview her outfit each day.

“Human beings are forever fascinated by their own copies,” says Michael Southgate, creative director of the mannequin design company Rootstein, “and that fascination probably begins in childhood, with rag dolls or toy soldiers.”

Past dolls and toy soldiers, the fashion dolls made in Europe may be the true forerunners of the modern mannequin. They came in many sizes, from palm-sized to life-sized, built of wax and wood. Clothing designers of the day used them to show off garments — not in shop windows, but by mailing them to customers in other countries to say, “the new clothes are here, and this is what they look like.” These fashion ambassadors were so popular that even in wartime they passed safely from country to country.

The first Industrial Revolution made it possible to manufacture mannequins at scale. Early ones had no heads, and so no facial expressions either; the teeth were fake, the eyes were glass, the feet were iron, the legs and arms were wood, the torso and head were wax. They broke easily, and they melted under strong lighting. Next to their successors they were stiff, offering only three poses — left leg forward, right leg forward, or feet together. More to the point, these mannequins, which sold for about $15, embodied Victorian taste: the “women” were full-figured, weighed as much as 300 pounds, and wore corsets.

New as they were, mannequins were not welcome. The Women's Christian Temperance Union objected to half-naked mannequins wearing nothing but a corset in store windows. Some cities then passed ordinances banning improperly dressed mannequins from shop windows, and those ordinances stayed on the books until the 1960s.

By the end of World War I, more and more women were working, some of them replacing men who had gone to the front in munitions factories. As real women's clothing grew more practical, mannequins dressed differently too: they stopped wearing hats, shed the corsets, and bared their ankles and knees.

Between 1920 and 1930, as Victorian taste collapsed, the mannequin changed again. The stiff poses gave way to a loose, slightly boyish style that echoed the Art Nouveau movement of the same years.

After the mannequin company Siegel & Stockman brought new materials to the market, these figures dropped to about 100 pounds, took heat, and stopped melting. They also gained new poses, as if singing or dancing. In the 1920s, the “Streets of Paris” exhibition at the Moulin Rouge in France showed a pair of gay mannequins.

This was also the period when the mannequin came close to its modern face, with more varied and elegant poses.

One episode is worth telling. During the Great Depression in the United States, the American sculptor Lester Gaba made a batch of mannequins he called the Gaba Girls, a tribute to the film stars Marlene Dietrich and Greta Garbo. Gaba went on to design the famous Cynthia, and he put the mannequin on a diet: Cynthia weighed only 100 pounds and looked remarkably real, her face covered in freckles, a cigarette butt pinched between her fingers. She was a sensation the moment she appeared. She ran in an opening-page ad in Life magazine, was invited to a royal wedding, and got stacks of fan mail. One columnist wrote about her as though she truly existed. Tiffany saw a chance for publicity and offered its jewelry to dress her.

Around the same time, the Australian mannequin maker Kathe Kruse gave mannequins metal skeletons, made the skin look more convincing, added poses, and varied the faces — high cheekbones, heart-shaped chins, instead of one face repeated.

In 1939, the surrealist artist Salvador Dalí was invited to design a window for the Bonwit Teller department store in New York. In his design, a bathtub lined with Persian goatskin was filled with dark water and strewn with narcissus flowers. A hand reached out of the tub holding a mirror, and the mirror pointed at a wax mannequin dressed in feathers, wearing a red wig and weeping red tears.

Complaints came quickly, and the window was banned. When Dalí heard, he rushed to the store; furious, he shoved the bathtub out into the street with his own hands. The police arrested him for it.

After Dalí's surrealist statement, mannequins entered a period of restraint that matched World War II. Their faces seemed to carry a patriotic sadness.

Between 1940 and 1950, the company Wolf & Vine put out the first plastic mannequins in history. But once they reached the windows a chemical reaction turned them green, and they had to be pulled from the floor. Clothing was changing as well — cuts grew slimmer, garments simpler — and mannequins were built smaller than before. That also saved wartime materials.

In 1992, the researchers Minna Rintala and Pertti Mustajoki compared mannequins from Italy, Japan and Malaysia made between the 1920s and the 1960s. They found that, next to the pre-World War II figures, the arms had lost 2 to 3 centimeters and the hips had narrowed to match.

Then came the mannequin's most contested years. According to the mannequin expert Dr. Marsha Bentley Hale: “In the 1940s and '50s, American companies took the nipples off mannequins, because the detail was considered too sexual.” In step with the taste of the time, the mannequins of those years looked like Marilyn Monroe: narrow waist, heavy hips, big bust.

From the 1960s into the 1970s, fashion entered the age of the supermodel. Mannequins got a new material, fiberglass, and copying celebrities became the fashion. Adel Rootstein released a Twiggy mannequin, identical to her from body to facial expression, and Twiggy became the most famous mannequin the company ever made. During the American civil rights movement, Adel Rootstein made the first mannequin with dark skin, named Donyale Luna, and gave her a wild, sensual look.

As mainstream media attacked anorexia among models — above all after Karen Carpenter, lead singer of The Carpenters, died of the illness — the mannequins of the 1970s and '80s broke free of celebrity faces. Features grew vague. Some mannequins went back to having no head, as in the earliest days; others turned into “digital faces.”

Consumers, meanwhile, seemed to sober up. Unlike the first complaints, about mannequins baring themselves for no reason on the street, someone finally noticed that mannequins were too perfect. Makers tried to build them smaller and closer to ordinary bodies, and they were still taller than ordinary people. The journalist Helen Burggraf wrote: “Mannequins are simply taller than real people, because everybody knows a little extra height is what makes a clothes hanger.”

“Clothes obviously look better on a mannequin that's thin and tall and has defined abs,” the visual director of the department store Bloomingdale's said in a 2007 interview.

Plenty of people started complaining on Twitter and Facebook. They began to question the unreal standard of beauty mannequins stood for, and those too-perfect bodies.

In 2011, Gap got a complaint from the blogger Alice Taylor: “Mannequins are always so skinny. I'm curious what Gap calls this look internally. Is it some kind of ‘famine fashion’?”

Unlike Gap, many stores began making their mannequins more varied and more real, closer to the bodies of actual shoppers. In 2010, the blogger Rebecka Silvekroon posted a photo on Facebook of a mannequin from the Swedish retailer Åhléns, praising its natural human proportions. Three years later the feminist group Women's Rights News picked the photo up and posted it on Facebook again, and this time it drew more than a million likes. That same year, the British retailer Debenhams put a size-16 plus-size mannequin in its stores.

The mannequin has now been around for more than 100 years. Its materials went from crude wood and wax paper to fiberglass and plastic. Compared with the early 20th century, today's mannequins are generally smaller. Every change in them has tracked, on time, every inch that skirts rose or fell across the history of fashion. Heavy, hourglass with full hips and bust, or tall and slender — each was human society's ideal of beauty, projected. For fashion companies, at least, mannequins must look close to real people while staying a little more beautiful than anyone can reach. Only then does a shop window drive sales.

But rather than read the mannequin's evolution as humanity's longing for beauty, call it a history of narcissism. Now, at last, some people have begun to doubt this high-minded self-love.

Perhaps this counts as a parody of the ancient Greek myth: the beautiful youth Narcissus comes to the lake, bends down to drink, sees his handsome reflection on the water, and instead of falling in love with himself on the spot, asks whether there might be something wrong with the mirror.

Cover image and in-body images from: mannequinmadness, hopesandfears, collectorsweekly, nomi, bonami.be, Visual China Group