Recently, I found myself at a party in Manhattan, casting sidelong glances at a woman. Her look (baby-doll dress, kinderwhore lip gloss) and behavior (exclaiming "Oh my god, totally!" and texting obsessively) screamed tweenager. You know, that hybrid archetype—the one who worships at the altar of Hannah Montana and High School Musical. Everything about the woman telegraphed jailbait—except, that is, for the crow's feet, which suggested that this wannabe tween was pushing 35.

Look around. The 35-going-on-12 woman is everywhere. Man-child Syndrome—the affliction that causes thirtysomething guys to cling to adolescence—may be rampant, but lately it's women who are taking the lead in regressing. Call it the Big Girl Epidemic: women selling versions of themselves that, when you get down to it, are pretty creepy. It's not so much a Lolita thing—the Big Girl isn't trying to be a dewy seductress—but more of a daffy, tweenage thing.

Is this the sort of girl you clawed your way into manhood to date? Think back to what your big brother's girlfriend seemed like when you were a kid: A woman. A w-o-m-a-n woman. Not someone who speaks in acronyms and carries a glitter-covered Sidekick.

"I get the feeling that a lot of women are dressing and acting that way because they think that that's what guys want," says Jean Twenge, associate professor of psychology at San Diego State University and author of Generation Me, a book about American youth culture. "It's the same thing as older women getting plastic surgery. The idea is that what men want is a woman who looks 18. Although they don't usually want a woman who acts 18." Twenge laughs, then adds, "And that's where the problem comes in."

And it is a problem—especially if you're a man who happens to find tween impersonations not only unbecoming but more than a little sad. Unfortunately, as long as our culture reinforces the Big Girl's worst inclinations, the epidemic will persist.

"I've been meditating on the question of why women in their twenties and thirties seem to be obsessed with all things teen—fashion, slang, gossip, et cetera," says Anastasia Goodstein, publisher of ypulse.com, a marketing website. "The reality is that teen culture has come to define pop culture." As the usual markers of American adulthood—marriage, career, kids—get more and more delayed, the simpleminded distractions of adolescence have extended their grip on the adult brain. Man-children may drag their old skateboards and video games with them into their thirties, but Big Girls needn't bother to cling to the pop culture of their youth. "Adult" pop culture has been conveniently colonized by a teen sensibility. Consider Gossip Girls or The Hills. Both shows are endlessly blogged about and compulsively parsed by teenage girls—but they're also cornerstones of thirtysomething women's party conversation. The audiences have converged.

Meanwhile, the female-oriented tabloids—Us Weekly, In Touch, and Star—read like Tiger Beat redux, chronicling the dysfunction of the Lindsays, Britneys, Nicoles, Heidis, and Laurens who have come to dominate the celebrity landscape at the expense of more seasoned female celebrities. And you only need look at the teen/tween style bible Teen Vogue to discover where Big Girls are taking their fashion cues from. According to the demographic stats it supplies to advertisers, nearly 2.8 million readers (almost half the total) of Vogue's little sister are adults.

Dating one of these Big Girls doesn't mean just putting up with jailbait fashion and IMs that say OMG, totally! It means potentially enduring the worst sort of navel-gazing drama. As Twenge points out, arrested development goes hand in hand with self-absorption: Narcissism "is a very adolescent personality trait. Obviously it means you focus on yourself and what's good for you." It used to be that men had the ego market cornered, but now, Twenge says, "there's virtually no difference between the sexes with regard to narcissism. Most of the change has taken place in girls and women." Basically, women have caught up to men by sinking to comparable levels of adolescent self-absorption.

Which makes the idea of actually dating a Big Girl even more unappealing.

Not that, in this postfeminist age, you have to hold out for "I am woman, hear me roar." But "I am girl, hear me giggle"? Uh, no. Totes no.

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