Gender includes the social, psychological, cultural and behavioral aspects of being a man, woman, or other gender identity. Depending on the context, this may include sex-based social structures (i.e. gender roles) and gender expression. Most cultures use a gender binary, in which gender is divided into two categories, and people are considered part of one or the other (boys/men and girls/women); those who are outside these groups may fall under the umbrella term non-binary. Some societies have specific genders besides "man" and "woman", such as the hijras of South Asia; these are often referred to as third genders (and fourth genders, etc.). Most scholars agree that gender is a central characteristic for social organization. Gender can be thought of as biopsychosocial, as it includes social, psychological, biological, cultural and behavioral aspects.
Sexologist John Money is often regarded as the first to introduce a terminological distinction in modern English (sex and gender distinction) between biological sex and gender role (which, as originally defined, includes the concepts of both gender role and what would later become known as gender identity) in 1955, although Madison Bentley had already in 1945 defined gender as the "socialized obverse of sex". Further, Simone de Beauvoir's 1949 book The Second Sex has been interpreted as the beginning of the distinction between sex and gender in feminist theory, although this interpretation is contested.
Before Money's work, it was uncommon to use the word gender to refer to anything but grammatical categories. However, Money's meaning of the word did not become widespread until the 1970s, when feminist theory embraced the concept of a distinction between biological sex and the social construct of gender. Most contemporary social scientists, behavioral scientists and biologists, many legal systems and government bodies, and intergovernmental agencies such as the WHO, make a distinction between gender and sex.
In other contexts, the term gender has been used as a synonym for sex without representing a clear conceptual difference. For instance, in non-human animal research, gender is commonly used to refer to the biological sex of the animals. This change in the meaning of gender can be traced to the 1980s. In 1993, the US Food and Drug Administration (FDA) started to use gender instead of sex to avoid confusion with sexual intercourse. Later, in 2011, the FDA reversed its position and began using sex as the biological classification and gender as "a person's self-representation as male or female, or how that person is responded to by social institutions based on the individual's gender presentation."
The social sciences have a branch devoted to gender studies. Other sciences, such as sexology and neuroscience, are also interested in the subject. The social sciences sometimes approach gender as a social construct, and gender studies particularly do, while research in the natural sciences investigates whether biological differences in females and males influence the development of gender in humans; both inform the debate about how far biological differences influence the formation of gender identity and gendered behavior. In some English literature, there is also a trichotomy between biological sex, psychological gender, and social gender role. This framework first appeared in a feminist paper on transsexualism in 1978.