on both masculine and feminine traits) or undifferentiated (either males or females who report themselves as low on both masculine and feminine traits). Twenge (1997) noted that men are generally more masculine than women and women generally more feminine than men, but the association between biological sex and masculinity/femininity is waning. Feminist theory and gender studies Biologist and feminist academic Anne Fausto-Sterling rejects the discourse of biological versus social determinism and advocates a deeper analysis of how interactions between the biological being and the social environment influence individuals' capacities. The philosopher and feminist Simone de Beauvoir applied existentialism to women's experience of life: "One is not born a woman, one becomes one." In context, this is a philosophical statement. However, it may be analyzed in terms of biology—a girl must pass puberty to become a woman—and sociology, as a great deal of mature relating in social contexts is learned rather than instinctive. Within feminist theory, terminology for gender issues developed over the 1970s. In the 1974 edition of Masculine/Feminine or Human, the author uses "innate gender" and "learned sex roles", but in the 1978 edition, the use of sex and gender is reversed. By 1980, most feminist writings had agreed on using gender only for socioculturally adapted traits. In gender studies the term gender refers to proposed social and cultural constructions of masculinities and femininities. In this context, gender explicitly excludes reference to biological differences, to focus on cultural differences. This emerged from a number of different areas: in sociology during the 1950s; from the theories of the psychoanalyst Jacques Lacan; and in the work of French psychoanalysts like Julia Kristeva, Luce Irigaray, and American feminists such as Judith Butler. Those who followed Butler came to regard gender roles as a practice, sometimes referred to as "performative". Charles E. Hurst states that some people think sex will, "...automatically determine one's gender demeanor and role (social) as well as one's sexual orientation (sexual attractions and behavior). Gender sociologists believe that people have cultural origins and habits for dealing with gender. For example, Michael Schwalbe believes that humans must be taught how to act appropriately in their designated gender to fill the role properly, and that the way people behave as masculine or feminine interacts with social expectations. Schwalbe comments that humans "are the results of many people embracing and acting on similar ideas". People do this through everything from clothing and