Whatever biology’s influence, expressions of masculinity and femininity are culturally and historically specific. In the 19th century, both boys and girls often wore dresses and long hair until they were 7. Colors weren’t gendered consistently. At times pink was considered a strong, and therefore masculine, color, while blue was considered delicate. Children’s clothes for both sexes included lace, ruffles, flowers and kittens. That started to change in the early 20th century, writes Jo Paoletti, a professor of American studies at the University of Maryland and author of “Pink and Blue: Telling the Boys From the Girls in America.” By then, some psychologists were arguing that boys who identified too closely with their mothers would become homosexuals. At the same time, suffragists were pushing for women’s advancement. In response to these threatening social shifts, clothes changed to differentiate boys from their mothers and from girls in general. By the 1940s, dainty trimming had been purged from boys’ clothing. So had much of the color spectrum. Women, meanwhile, took to wearing pants, working outside the home and playing a wider array of sports. Domains once exclusively masculine became more neutral territory, especially for prepubescent girls, and the idea of a girl behaving “like a boy” lost its stigma. A 1998 study in the academic journal Sex Roles suggests just how ordinary it has become for girls to exist in the middle space: it found that 46 percent of senior citizens, 69 percent of baby boomers and 77 percent of Gen-X women reported having been tomboys.