When the women’s liberation movement arrived in the mid-1960s, with its anti-feminine, anti-fashion message, the unisex look became the rage—but completely reversed from the time of young Franklin Roosevelt. Now young girls were dressing in masculine—or at least unfeminine—styles, devoid of gender hints. Paoletti found that in the 1970s, the Sears, Roebuck catalog pictured no pink toddler clothing for two years.

“One of the ways [feminists] thought that girls were kind of lured into subservient roles as women is through clothing,” says Paoletti. “ ‘If we dress our girls more like boys and less like frilly little girls . . . they are going to have more options and feel freer to be active.’ ”

John Money, a sexual identity researcher at Johns Hopkins Hospital in Baltimore, argued that gender was primarily learned through social and environmental cues. “This was one of the drivers back in the ’70s of the argument that it’s ‘nurture not nature,’ ” Paoletti says.

Gender-neutral clothing remained popular until about 1985. Paoletti remembers that year distinctly because it was between the births of her children, a girl in ’82 and a boy in ’86. “All of a sudden it wasn’t just a blue overall; it was a blue overall with a teddy bear holding a football,” she says. Disposable diapers were manufactured in pink and blue.

Prenatal testing was a big reason for the change. Expectant parents learned the sex of their unborn baby and then went shopping for “girl” or “boy” merchandise. (“The more you individualize clothing, the more you can sell,” Paoletti says.) The pink fad spread from sleepers and crib sheets to big-ticket items such as strollers, car seats and riding toys. Affluent parents could conceivably decorate for baby No. 1, a girl, and start all over when the next child was a boy.

Some young mothers who grew up in the 1980s deprived of pinks, lace, long hair and Barbies, Paoletti suggests, rejected the unisex look for their own daughters. “Even if they are still feminists, they are perceiving those things in a different light than the baby boomer feminists did,” she says. “They think even if they want their girl to be a surgeon, there’s nothing wrong if she is a very feminine surgeon.”

Another important factor has been the rise of consumerism among children in recent decades. According to child development experts, children are just becoming conscious of their gender between ages 3 and 4, and they do not realize it’s permanent until age 6 or 7. At the same time, however, they are the subjects of sophisticated and pervasive advertising that tends to reinforce social conventions. “So they think, for example, that what makes someone female is having long hair and a dress,’’ says Paoletti. “They are so interested—and they are so adamant in their likes and dislikes.”

In researching and writing her book, Paoletti says, she kept thinking about the parents of children who don’t conform to gender roles: Should they dress their children to conform, or allow them to express themselves in their dress? “One thing I can say now is that I’m not real keen on the gender binary—the idea that you have very masculine and very feminine things. The loss of neutral clothing is something that people should think more about. And there is a growing demand for neutral clothing for babies and toddlers now, too.”

“There is a whole community out there of parents and kids who are struggling with ‘My son really doesn’t want to wear boy clothes, prefers to wear girl clothes.’ ” She hopes one audience for her book will be people who study gender clinically. The fashion world may have divided children into pink and blue, but in the world of real individuals, not all is black and white.

Correction: An earlier version of this story misattributed the 1918 quotation about pink and blue clothes to the Ladies’ Home Journal. It appeared in the June 1918 issue of Earnshaw’s Infants’ Department, a trade publication.