In recent years, the debate over underage fashion models has reached a fever pitch. Ondria Hardin starred in a sultry Prada ad at 13. An Australian modeling agency recently announced that it wanted 13-year-olds because 16-year-olds were “too old.” And a good number of preadolescents are establishing their modeling bona fides, including the tween daughters of Cindy Crawford and of former Oasis guitarist Noel Gallagher. Last year, in a satirical look at girl models that wound up less high-minded than exploitative, Vogue Paris featured a 10-year-old named Thylane Blondeau in full makeup, heels, and a navel-bearing dress.

When and why did high-fashion models get so young?

The answer is complicated, but it starts with the simple problem of size. When Vogue announced recently that it would no longer use models younger than 16 in its fashion shoots, the decision was inextricably linked to its other announcement—that it would no longer use models who appear to have eating disorders. As the sample size that designers use has shrunk to a zero, the logistics of runway shows and fashion shoots has bumped up against the realities of puberty. “I probably fit a sample size once, when I was 11 or 12,” the Dutch model Doutzen Kroes has said, explaining why she left high fashion to work for brands like Victoria’s Secret. Who but an undeveloped teen could fit a skirt with a 33-inch hip?

But this answer just lets the fashion industry off the hook. Why have they put themselves in a situation where no grown woman can fit their clothes? What is it they’re after? Partly, it’s novelty. Global competition among talent scouts and the growing field of aspiring models have led to a fierce need to snag the next hot thing before anyone else does. The result is that girls get discovered sooner and flame out more quickly. In fact, given the history of the business over the last 40 years or so, it seems inevitable that we would have ended up with prepubescents on the runway.

Before the ’60s, models aspired to an adult look, with their up-dos, their severe eyebrows and their stiff poses suggesting a mature, domesticated kind of glamour. “Even if they were 21 they tried to look 30,” says Becky Conekin, a Yale historian who researches mid-century British models. Several major models of that time period were already mothers in their mid-20s by the time they broke into the business. And while the industry always had a taste for thin (one model of the era was described as “a razor-blade dressed up”), the prevailing shape was hourglass. You couldn’t be prepubescent and carry that off.

The look of models changed in the ’60s with the fashion industry’s growing recognition of the power of the youth market, and the emergence of younger and wispier It Girls such as Twiggy and Jean Shrimpton. But even then, models were often well into their teens by the time they started. Shrimpton has said she was “green as a spring salad” when her career took off at 18; these days, 18 is world-weary in the fashion world.

As Jezebel’s Jenna Sauers has pointed out, there have always been young models, including Brooke Shields, who was 15 when she boasted that nothing came between her and her Calvins, and Kate Moss, who has said she cried when removing her top for shoots at the same age. Canadian model Monika Schnarre, who was discovered at 14 in the 1980s, has recalled how hard it was to mimic sexiness at shoots: “One photographer said, ‘Listen, I know you’re a virgin, but could you just pretend?’ ”

But in the last decade, young models have become much more pervasive, their undeveloped bodies, large eyes, and soft jaws helping shape the look of fierce fragility that’s become dominant on the runway (“Fierce fragility” being the psychology of choice these days—Lisbeth Salander, Katniss Everdeen). Many in the industry trace the current model youthquake to the last 10 or 15 years—to the influence of waifish Moss, and perhaps, too, the influx of pale, angular Eastern European models.

Critics of the latest extreme youthquake often point to sample sizes—clothes that are made for runway shows and magazine shoots and that usually range from size 0 to 2. Ben Barry, whose Toronto agency has made its name representing a more diverse collection of models than typically found on catwalks, says that 20 years ago, during the era of the supermodel, runway clothes ranged from 6 to 8, sizes that some agencies now consider “plus.”

Barry says designers claim that making smaller sizes allows them to save money by cutting back on the fabric and labor involved in creating expensive couture items. But this argument falls short, he points out, when you consider the hundreds of thousands of dollars fashion houses spend on runway shows. In any case, while casting directors and magazine editors blame the designers for making such small sample sizes, designers blame the agents, Barry says. If only the girls being supplied weren’t so thin, they could make bigger clothes!

Supply and demand also feed on each other in more toxic ways. Big fashion houses want “a girl no one has used before,” British modeling agent Carole White told the Guardian earlier this year. And it can’t hurt that the younger they are, the less they ask for and the easier they are to control. “You have more power, you have more influence, to guide and direct,” says a female scout in Girl Model, the disturbing documentary on model scouting that comes out in September.

The girls not only start young; they end young. Elizabeth Wissinger, a CUNY Sociology professor, says there are so many girls being scouted and so much desire for that new face that models progress from next-thing to no-thing in no time. An agent she interviewed remarked that “a good career now is three to five years.”

Girl Model shows scouts and agents in Siberia seizing triumphantly on the most promising girls, relieved to have snagged them before anyone else. In one scene, they admire photos of a 12-year-old in panties and thigh-highs, pronouncing her “extraordinary.” In another, a scout praises a skeletal 13-year-old named Nadya, whose youthful look she predicts will be perfect for the Japanese market. “She looks young, almost like a prepubescent girl,” the scout says.

Fashion sociologist Ashley Mears has written that over time, the fashion world’s notion of ideal womanhood has been reconceptualized as something less, well, womanly. If high fashion is created by the elite for the elite, everything about it must be unattainable. And “there’s something kind of average, a little too attainable and too cheap about curves,” Mears writes.

With womanhood suspect, it makes sense to put girls on the runway. They possess a quality most grown women don’t: They haven’t yet been polluted. Marc Jacobs—whose recent use of under-16 runway models, and of a sultry and incredibly young-looking Dakota Fanning in a perfume ad, ignited controversy—once remarked that youth was sexier than anything else. “I’m by no means a pedophile,” Jacobs said (a bad way to start a sentence), “but there’s a purity to youth.” In Girl Model, an agent named Tigran puts it another way. He suggests that the pretty girls he finds in the shacks of Siberia have just two ways of escaping poverty: modeling or prostitution. Young girls have “dignity,” he says. They haven’t yet spread their legs.