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.

In the mid-20th century, a terminological distinction in modern English (known as the sex and gender distinction) between biological sex and gender began to develop in the academic areas of psychology, sexology, and feminism. Before the mid-20th century, it was uncommon to use the word gender to refer to anything but grammatical categories. In the 1970s, 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.

The social sciences have a branch devoted to gender studies. Other sciences, such as psychology, sociology, 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. Biopsychosocial approaches to gender include biological, psychological, and social/cultural aspects.