Beauvoir's sentence crystallised a distinction that later thinkers would name explicitly: between sex, the biological category traced across the rest of this book, and gender, the bundle of roles, expectations and behaviours a society attaches to it. On this view, a baby is born female; she is taught, over years, to become "a woman" — to sit, speak, dress, want, and present in the ways her culture has decided femininity requires. The body is given; the femininity is built.
Through the 1960s and '70s this sex/gender distinction became one of the most influential ideas in the humanities and social sciences, taken up by writers like the sociologist Ann Oakley to argue that much of "women's nature" was actually women's training. It's a powerful lens, and the rest of this chapter is essentially a tour of its evidence — the colours, shapes and costumes that turn out to be cultural rather than natural.
One honest caveat up front, because this series doesn't pretend settled questions are settled when they aren't. Exactly where biology ends and culture begins — how much of any given trait is "sex" and how much is "gender," and even whether the two can be cleanly separated — is a live and sometimes heated debate among philosophers, scientists and feminists themselves. Beauvoir opened the question; she did not close it. What follows isn't a claim that femininity is entirely invented. It's a demonstration that an astonishing amount of it is — far more than most people assume.