Linking gender with these colors is relatively recent, according to Jo Paoletti, a University of Maryland American studies professor. In the 19th century pastel colors were fashionable in most of Europe and the United States and were worn “to flatter the complexion, not denote gender,” she says. In the early part of the 20th century, gender distinction in clothing hues began to emerge, she says—and by 1940 pink and blue took root as the intensely gender-associated colors they continue to be today.