From Roland Barthes’ The Fashion System (1967):

Fashion understands the opposition between the feminine and the masculine quite well; reality itself requires that it do so, since reality often puts features derived from men’s clothing (pants, tie, jacket) in women’s wardrobes; in fact, between the two kinds of clothing, differential signs are extremely rare and are always situated at the level of the detail.

From “When Did Girls Start Wearing Pink?” by Jeanne Maglaty for Smithsonian.com:

But nowadays people just have to know the sex of a baby or young child at first glance, says Jo B. Paoletti, a historian at the University of Maryland and author of Pink and Blue: Telling the Girls From the Boys in America, to be published later this year. Thus we see, for example, a pink headband encircling the bald head of an infant girl.

Why have young children’s clothing styles changed so dramatically? How did we end up with two “teams”—boys in blue and girls in pink?

“It’s really a story of what happened to neutral clothing,” says Paoletti, who has explored the meaning of children’s clothing for 30 years. For centuries, she says, children wore dainty white dresses up to age 6. “What was once a matter of practicality—you dress your baby in white dresses and diapers; white cotton can be bleached—became a matter of ‘Oh my God, if I dress my baby in the wrong thing, they’ll grow up perverted,’ ” Paoletti says.

The march toward gender-specific clothes was neither linear nor rapid. Pink and blue arrived, along with other pastels, as colors for babies in the mid-19th century, yet the two colors were not promoted as gender signifiers until just before World War I—and even then, it took time for popular culture to sort things out.

For example, a June 1918 article from the trade publication Earnshaw’s Infants’ Department said, “The generally accepted rule is pink for the boys, and blue for the girls. The reason is that pink, being a more decided and stronger color, is more suitable for the boy, while blue, which is more delicate and dainty, is prettier for the girl.” Other sources said blue was flattering for blonds, pink for brunettes; or blue was for blue-eyed babies, pink for brown-eyed babies, according to Paoletti.

In 1927, Time magazine printed a chart showing sex-appropriate colors for girls and boys according to leading U.S. stores. In Boston, Filene’s told parents to dress boys in pink. So did Best & Co. in New York City, Halle’s in Cleveland and Marshall Field in Chicago.

Today’s color dictate wasn’t established until the 1940s, as a result of Americans’ preferences as interpreted by manufacturers and retailers. “It could have gone the other way,” Paoletti says.