They are like collectors' dolls, big-eyed and pocket-size.

They are two halves of one whole, for that yin-yang split in all of us.

Ashley Olsen is our girlie-girl; Mary-Kate is our tomboy. Ashley is our blond overachiever; Mary-Kate is our tousled bohemian. They contain the innocence of children and the sultriness of sexpots; they are both intimidating and familiar, like a relative who becomes famous.

They are like real people, only smaller and richer; they strap on Jimmy Choos and jaunt off to Rome, carrying our fantasies of glamour with them. They are the Olsen twins, two for the price of one.

Despite their fabulous wealth and their 18th birthday on June 13, there is still about the Olsens a faint aura of kitsch. Perhaps it is because they sell their fashion lines at Wal-Mart, or because they remind us of the late '80s when we first encountered them, sharing the role of toddler Michelle Tanner on the wholesome sitcom "Full House." Back then they had a vaguely simian look, and their hair was fastened in fountains atop their heads. They had a stable of catchphrases like "You got it, dude!," which they delivered with a sort of robotic charm.

Over the years, the Olsens did not crash and burn as we expected them to -- though there's still time, as the tabloids remind us weekly with headlines like "Olsens X-Posed!" They did not enter rehab or get married in Las Vegas or star in a series of god-awful movies intended to get their careers back on track. Instead, they starred in a series of god-awful movies that bypassed theaters and went straight to video, bought and beloved by young girls everywhere. These young girls then went out and bought the Olsens' clothing and makeup and stationery and backpacks, making the twins rich. They named their brand mary-kateandashley -- one word, say it fast, the way little girls do -- and this year it and the rest of Empire Olsen are projected to do $1.2 billion in sales.

Because we first encountered them in diapers and watched them grow up on-screen, because they have always catered to the whims of children, Mary-Kate and Ashley are, in a sense, America's kids. No dollmaker could have designed a better mold for the nation's maternal instincts. They are fraternal twins but they are almost indistinguishable. Even now, their huge blue-green eyes, enhanced by sooty makeup, recall those big-eyed waif paintings of the 1960s. Their hair -- layered and styled into starlet fullness -- seems too big for them. They are both less than 5 feet 2 inches tall -- in their recent movie, "New York Minute," Mary-Kate stood on apple boxes for her kissing scene. This smallness helps reinforce the notion that two little Olsens make one big Olsen, which is, after all, the basis of their career. Who is Mary-Kate without Ashley? Who is Ashley without Mary-Kate?

And who are we without them, making unity from the division in our souls?

For the little girls who followed their television shows and their direct-to-video movies, the Olsens were the original reality television. Their lives were chronicled in as many as six movies a year, many of them short and episodic, with such titles as "You're Invited to Mary-Kate & Ashley's Sleepover Party" and "The Adventures of Mary-Kate & Ashley: The Case of the Fun House Mystery."

As they got older, the twins ceased to play themselves in their movies and instead played thinly veiled versions of themselves. Ashley played characters named "Ally" and "Alex," and Mary-Kate became "Melanie" and "Madison." The movies got longer. Usually the theme was the same: twin sisters beating a tough challenge through smarts and spunk. Often this required outfoxing parents and, as the girls got older, necessitated the involvement of cute boys. It was "safe and appropriate" entertainment, as their publicist puts it, the sort a mom could be glad her kids were watching.

When the Olsens' roles were not indistinguishable in their perkiness, they fell into stereotypes. Ashley was feminine and a perfectionist; Mary-Kate played a jock or a bohemian. In the theatrical release of "New York Minute," which came out last month amid halfhearted reviews, Ashley plays an ambitious young Republican, while Mary-Kate plays a rock-and-roll drummer with a devil-may-care attitude. In real life, the Los Angeles teenagers supposedly embody those distinctions; they are different enough to be interesting but not controversial. "Know your Olsens," says the caption beneath a photo of the girls in a recent issue of People, comparing "the funkier MK" with "the more classic Ashley."

The Olsens' brand is predicated on this sense of intimacy, this notion that what you see on-screen is what you'd get if they really did invite you to their sleepover party. The girls are the brand and the brand is the girls. Movies are just excuses for the Olsens to be themselves. Marketers say little girls see the Olsens as friends or big sisters. When the Olsens fly off to have adventures in Paris, Rome or the Bahamas, as they have in their various direct-to-video movies, girls see themselves, only with money and a sophisticated lifestyle.

There are plenty of products to foster the bond. There are Mary-Kate and Ashley dolls in caps and gowns to mark the twins' graduation from high school this year. As a Mattel spokeswoman puts it, "We allow girls to participate in Mary-Kate and Ashley's life through our product line." There are twin perfumes: mary-kateandashley one ("jasmine spice") and mary-kateandashley two ("juicy peach freesia"). There is the video game "Crush Course," in which players try to figure out who Mary-Kate and Ashley's secret admirers are. There are books about Mary-Kate and Ashley going to the beach, and to summer camp, and about their summer after graduation. "New York Minute" is set in Manhattan, where the girls will be starting college this fall at NYU. It's as if each real-life event has its well-packaged counterpart, on sale now at Wal-Mart for $4.49.

Which is arguably why the Olsens are so popular. If they are glamorous, they are also accessible. If they are rich, they are also humble. If they arrive beautiful on the red carpet, don't forget we saw them before they were potty trained. And frankly, they were kind of funny-looking.

Ah, twins. The magic and the mystique. We tend to think of them as two halves of the same whole -- either at odds or capable of mystical communion. They cooperate for the sake of high jinks, as in "The Parent Trap." They war, as did Romulus and Remus, and Jacob and Esau. They are opposites, like the characters Bette Davis played (a demure twin and a sexually manipulative one) in "A Stolen Life," and the good-and-bad-twin pairings that are the lifeblood of soap operas, and the line that cultural historian Hillel Schwartz says he once saw in the Weekly World News: "My evil twin stole my wife and my Jaguar."

There is the sense that twins have a kind of connection we singletons don't. Schwartz traces this to a theme in Western culture he calls "the legend of the vanishing twin." Schwartz wrote a book in 1996 called "The Culture of the Copy," which explored doubles of all sorts. He says that the notion of a vanishing twin got its start with the Platonic myth that people were born perfect spheres, then split in half by Zeus, forcing humans to forever search for what they lost. As Schwartz sees it, Plato's myth describes the singleton's essential loneliness.

Plato's myth is usually invoked to describe the origins of romantic love, but don't twins and soul mates represent the same thing? In a sense, aren't we all searching for our lost twin, that other half to make us whole?

There is an element of the supernatural to twins. There is the "notion that they shared experiences in the womb which we didn't have and so could've developed a pact without our knowing it," Schwartz says. Twins sometimes finish each other's sentences, playing into the idea that each must know the other's thoughts.

We are jealous of Mary-Kate and Ashley, of the way they seem always to be touching in photographs -- Mary-Kate kisses Ashley's shoulder; Ashley smushes her forehead against Mary-Kate's cheek. In one picture in a recent issue of Allure magazine, they embrace chest to chest, their hips and bellies pressed against each other, their arms clasped around each other's waist. In black and white, it's hard to tell Mary-Kate's brown hair from Ashley's blondness, but one of them -- she wears dark lipstick; it must be Mary-Kate -- is turned slightly away from us, so we can see that her gauzy sundress bares her back. A fan forces their hair to swirl upward around their faces, as if they were sinking underwater.

They're not smiling. Mary-Kate's dark lips are almost touching Ashley's cheek, and she eyes the camera with her head tipped back, as if to say, This is mine.

Last year Rolling Stone put the Olsens on the cover. The photo shows Mary-Kate leaning against Ashley, her shirt casually folded up as if by accident, revealing several inches of taut torso. The headline beside them reads "America's Favorite Fantasy."

The Olsens' sex appeal represents the joining of two male fantasies.

One has to do with youth and a virginal image, suggesting an uncorrupted innocence, and the opportunity to be corrupted. This is the basis of barely-legal porn, and of the popularity of Britney Spears back when she was an avowed virgin dancing in a Catholic schoolgirl uniform. The Olsens have about them a coyness, a closed-mouth smile and a hooded stare that suggests they don't know much, but they have an idea. Their constant touching of each other seems seductive, but unintentionally so. The fact that they grew up in front of the camera only underscores their youth. They are, as one marketing consultant puts it, like the girl next door.

"You don't really pay any attention to her," says Morris Reid, who's based in Washington. "And the next day she comes out and she's a fox."

For years, there have been Internet sites counting down to the second when the Olsens would turn 18 and be legal to do more than vote. (Never mind that in most states the age of consent is younger.) Lex Staley and Terry Jaymes, the shock jocks who host the nationally syndicated "Lex and Terry" show, say they have been waiting for the twins' 18th since the girls were barely 14. On their Web site is "The Olsen Twin Jailbait Countdown Clock!"

Staley and Jaymes are vocal about the other part of the Olsens' appeal, which we'll call the twin thing. The twin thing operates on the principle of compounded interest: two beautiful women are more than double the value of one. And women who look the same? This was the heat in those Robert Palmer videos from the 1980s, in which hordes of sexy women -- dressed precisely alike, with their hair slicked back and their eye makeup uniformly over-applied -- bopped in robotic unison. They made up an orgy of sexual sameness, their individual identities subsumed by their desire to please.

The word "twins" has a special ring in certain male circles, representing the royal flush of sexual conquests. The very phrase, Olsen twins, "either sounds very young or very pornographic," says Linda Kaplan Thaler, CEO of the Kaplan Thaler Group, an ad agency in Manhattan.

Michael Pagnotta, the Olsens' publicist, says the twins have had several offers from men's magazines to pose skimpily clad when they turn 18. They will not be accepting them. They will become presidents of their company, Dualstar Entertainment Group, and they will continue to grow up, at their own pace. As Robert Thorne, Dualstar's CEO, says, "They tend to be modest."

In magazine photos the twins have more than once posed sucking on their necklaces. In "New York Minute" there's a long scene in which they run through Times Square barely dressed, one in a towel and the other in a bathrobe. Several movie reviewers pronounced it "creepy." Pagnotta says the twins did the scene to "have some fun with how people perceive them."

And last month, when the twins hosted the season finale of "Saturday Night Live," the cast gathered onstage at the end to say goodbye to the audience and Mary-Kate shouted, "Remember, we're legal in four weeks!"

She was joking, but she was reminding us at the same time. In a sense the Olsens are the perfect metaphor for our divided world.

Pagnotta spins this nicely. He invokes the Rolling Stone headline, "America's Favorite Fantasy."

"They're a fantasy if you're a 6-year-old girl and perhaps they're a fantasy if you're a 50-year-old man," Pagnotta says. "That is, I think, the magical thing about them, is they seem to span such a broad range of demographics."

Over the past year, Mary-Kate and Ashley have reached a benchmark more significant than a birthday, crossing that invisible line that makes them fair game for celebrity magazine muckraking. The minutiae of their lives have been recorded -- that Mary-Kate is dating David Katzenberg, son of DreamWorks' Jeffrey Katzenberg; that Ashley recently broke up with Matt Kaplan, a student at Columbia University.

Stories have included a rumor that the twins purchased and are merging adjoining apartments in Manhattan for $3.5 million. One of the more recent rumors is that Mary-Kate has an eating disorder and is dangerously thin, a claim she has denied. In Touch Weekly ran close-ups of the twins purporting to reveal key differences in leg width.

Judy Swartz, the girls' design partner for their Wal-Mart fashion lines, worries that people may believe what they read.

"Do you think rumors like this could actually destroy them?" Swartz asks one day as she's being interviewed over the telephone. "My mother called me last night," asking about a story characterizing the twins as "teens gone wild." "I said, 'Mom, are you kidding me?' "

Which raises the question of what the Olsens will become once their woman-child phase has passed. CEO Thorne says that after the sisters settle into college, "they're pursuing individual acting careers."

Presumably the demands of big-screen movies will be greater than those of "You're Invited to Mary-Kate & Ashley's School Dance," and the critics will be tougher. Mary-Kate will be without her Ashley, and Ashley without her Mary-Kate. Two halves, wandering separately through the adult world, leaving us somehow lonelier.

Ashley, far left, and Mary-Kate Olsen have been a public fixture since they were toddlers on ABC's "Full House," turning that start into a twin-branded line of merchandise. But they turn 18 on June 13 amid the prospect of going their separate ways.Twin bill: Mary-Kate and Ashley are more than old enough to drink milk from a glass, as opposed to their younger days on "Full House," above right. Their mary-kateandashley line of clothes at Wal-Mart, right, is part of an empire projected to ring up $1.2 billion in sales this year.