The members of many species of living things are divided into two or more categories called sexes. These categories refer to complementary groups that combine genetic material in order to reproduce. This process is called sexual reproduction. Typically, a species will have two sexes: male and female The female sex is defined as the one that produces the larger gamete (i.e., reproductive cell).

Fungi and some other organisms exist in more than two sexes, but still reproduce in pairs (any two differing sexes can reproduce). Some species, such as earthworms, honeybees, and geckos, are capable of both sexual and asexual reproduction. All the members of some species are hermaphrodites, that is, individuals that contain both sexes in one body.

The word sex is also used as an abbreviation to refer to sexual intercourse (the physical acts related to sexual reproduction) and other human sexual behavior, but this article will discuss the concept of sex defined above.

In mammals, birds, and many other species, sex is determined by the sex chromosomes, called X and Y in mammals, and Z and W in birds. Males typically have one of each (XY), while females typically have two X chromosomes (XX). All individuals have at least one X chromosome, the Y chromosome is generally shorter than the X chromosome with which it is paired, and is absent in some species, this pattern admitting some considerable variation. In other species, including crocodiles, and most insects, sex may be determined by various other sex-determination systems, including those controlled by environmental factors such as temperature, or controlled by age.

Intersexuals are the small minority of people who were born with bodies that are difficult to assign to the traditional "male" and "female" categories because they have both genitalia, are of indeterminate sex, or combine features of both sexes. In some cases, individuals have had, e.g., a chromosomal sex of XY but primary sexual characteristics (external genitalia, etc.) of females, and have been raised as females, only to have been discovered in adulthood to be genetically male. Despite the fact that these individuals had an underlying biological sex that was of one kind, and an apparent sex that was its opposite, in many cases the individuals as well as members of their families and communities accepted their incorrect sex determination. Such complex situations have led some scientists to argue that the two sexes are cultural constructions. Some people have sought to define their sexuality and sexual identity in non-polar terms in the belief that the simple division of all humans into "males" and "females" does not fit their individual conditions.

As part of this movement away from polar oppositions, Anne Fausto-Sterling once suggested a classification of five sexes (male, female, merm, ferm and herm). Advocates for intersexual people stated that this theory is confusing and unhelpful to the interests of intersexual people, and she has since ceased to advocate this nomenclature.

Many social scientists use "sex" to refer to the biological division into male and female, and "gender" to refer to gender roles assigned to people on the basis of their apparent sex and/or other contingent factors. (See, for instance, berdache, hijra, and xanith.) There is tremendous variation of cultural attitudes, both between and within societies, toward sex, sexuality, and gender roles. It is interesting, and perhaps significant, that some societies identify youths with atypical behavioral characteristics and, instead of giving them corrective therapy or punishing them, socialize them in such a way that their individual characteristics let them provide a needed and/or useful function for the society in a recognized and respected role. (See, for example, shaman, medicine man, tong-ki.)

See the article Pictogram for an example of a pictogram of a man and a woman, to indicate the respective toilets. It shows the man with broader shoulders (sex dimorphism) and the woman in clothing that is in the western world rarely worn by men, a dress (which functions as a gender signal). (Presumably these "male human" and "female human" pictograms are not used in countries where men wear dress-like clothing.) In most societies, it is considered improper for a person of one sex to misrepresent himself or herself as a member of the opposite sex by donning inappropriate clothing (thereby practicing transvestism or, colloquially, cross-dressing). Such behavior receives severe social and/or legal sanctions in some cultures.

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