Sex developed its 'sexual intercourse' meaning in the early part of the century (now its more common meaning), and a few decades later gender gained a meaning referring to the behavioral, cultural, or psychological traits typically associated with one sex, as in 'gender roles.' Later in the century, gender also came to have application in two closely related compound terms: gender identity refers to a person's internal sense of being male, female, some combination of male and female, or neither male nor female gender expression refers to the physical and behavioral manifestations of one's gender identity.
In the 20th century sex and gender each acquired new uses. In the 15th century gender expanded from its use as a term for a grammatical subclass to join sex in referring to either of the two primary biological forms of a species, a meaning sex has had since the 14th century phrases like 'the male sex' and 'the female gender' are both grounded in uses established for more than five centuries. The words sex and gender have a long and intertwined history.
