The distinction between sex and gender differentiates sex (the anatomy of an individual's reproductive system, and secondary sex characteristics) from gender, which can refer to either social roles based on the sex of the person (gender role) or personal identification of one's own gender based on an internal awareness (gender identity). In some circumstances, an individual's assigned sex and gender do not align, and the person may be transgender,gender-nonconforming, non-binary, or intersex.

The sex and gender distinction is not universal. In ordinary speech, sex and gender are often used interchangeably. Some dictionaries and academic disciplines give them different definitions while others do not.

Among scientists, the term sex differences (as compared to gender differences) is typically applied to sexually dimorphic traits that are hypothesized to be evolved consequences of sexual selection.

Sex

Anisogamy, or the size differences of gametes (sex cells), is the defining feature of the two sexes. By definition, males have small gametes (sperm); females have large gametes (ova). In humans, typical male or female sexual differentiation includes the presence or absence of a Y chromosome, the type of gonads, the sex hormones, the internal reproductive anatomy (such as the uterus in females), and the external genitalia. People with mixed sex factors are intersex. People whose internal psychological experience differs from their assigned sex are transgender or transsexual.