A sex is one of two specimen categories of species that recombine their genetic material in order to reproduce, a process called genetic recombination. The somewhat similar term gender has more to do with identity than biology. Typically, a species will have two sexes: male and female. The female sex is defined as the one which produces the larger gamete (i.e., reproductive cell) and which bears the offspring. The categories of sex are, therefore, reflective of the reproductive functions that an individual is capable of performing at some point during its life cycle, and not of the mating types, which genetically can be more than two.

Sex in non-animal species

Plants are generally hermaphrodites, but this terminology is quickly complicated by variations in the degree of sexuality. As with animals, there are only two types of gametes. These are generally called male and female based on their relative sizes and motility. In flowering plants, flowers bear the gametes. In some cases, flowers may contain only one type of gamete while in others they may contain both.

In other varieties of multicellular life (e.g. the fungi division, Basidiomycota) sexual characteristics can be much more complex, and may involve many more than two sexes. For details on the sexual characteristics of fungi, see: Hypha and Plasmogamy.

Sex in the animal kingdom

Some species, such as earthworms, honeybees, and geckos, are capable of both sexual and asexual reproduction. In the insect order Hymenoptera, which includes honeybees, the queen (i.e., fully functional female) can decide to fertilize an egg or to lay it without its being fertilized. Fertilized eggs will develop into females -- workers if given standard nutrition in their larval stages and queens if lavishly fed with royal jelly. Unfertilized eggs, which have only half the number of chromosomes as fertilized eggs, develop into drones, i.e., male bees. In other species (e.g. earthworms), all individuals are hermaphrodites, that is, individuals that have male and female sex organs.

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. For mammals, 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. One interesting variation is in the platypus, a rather unusual mammal in many other ways, where sex is determined by 10 chromosomes. Males are XYXYXYXYXY and females XXXXXXXXXX. In birds, males have two of the same kind of sex chromosome (ZZ) and females have one of each type (ZW). 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. Yet other species change sex during their lifetime.

Sex among humans

In humans, sex is conventionally perceived as a dichotomous state or identity for most biological and social purposes, such that a person can only be female or male. However, when the criteria which are generally used to define femaleness or maleness are examined more closely, it becomes apparent that the assignment or determination of 'sex' occurs at multiple levels. Environmental, biological, social, psychological and other factors are all believed to have some role in this process, and the complex interaction of these factors is expressed in the diversity of biological and psychosocial 'states' or levels found amongst the human population. A significant fraction of the human population simply does not correspond exclusively to either 'female' or 'male' with regard to every level of definition expressed in the following table. This discordance is discussed in more detail below.

The table outlines the major levels at which we currently recognize a difference between human females and males. Some criteria are dichotomous and some, such as body size, exhibit sexual dimorphism (i.e. characteristics which are statistically more likely to be found in one sex than the other). Some of the levels are more amenable to scientific study or measurement than others; some are "imputed" or assigned to individuals by the society of which they are members (e.g. whether human males must wear trousers is a result of social norms); and some seem to be generated within each individual as a subjective identity or drive.

"Primary" sexual characteristics are typically present at birth and directly involved in reproduction. "Secondary" sexual characteristics typically develop later in life *usually during puberty) and are not directly involved in reproduction.

The relationship between the various levels of biological sexual differentiation is fairly well understood. Many of the biological levels are said to cause, or at least shape, the next level. For example, in most people the presence of a Y chromosome causes the gonads to become testes, which produce hormones that cause the internal and external genitalia to become male, which in turn lead parents to assign 'male' as the sex of their child (assigned sex), and raise the child as a boy (gender of rearing). However, the degree to which biological and environmental factors contribute to the psychosocial aspects of sexual differentiation, and even the interrelationships between the various psychosocial aspects of differentiation, are less well understood (see the nature versus nurture debate).

Discordance

As indicated above, the levels of this paradigm imply a certain level of 'discordance' amongst the human population as a result of diversity amongst humans.

Some discordances are purely biological, such as when the sex of the chromosomes (genetic sex) does not match the sex of the external genitalia (anatomic sex). This type of discordance is fairly well understood and is described briefly in the next section and more fully in the article on intersex.

Discordances between the biological and psychosocial levels, such as when the gender identity does not match the anatomic sex, or between the various psychosocial levels, such as when the gender role does not match the gender identity, are even more common but less well understood. These levels of definition and discordance are described below and in individual articles.

Understanding discordance is important for several reasons. We can learn much about the processes of sexual differentiation, both biological and psychosocial, from people with biological discordances. Some of the levels of discordance have enormous significance to the lives of those affected and their relationships with society. In some cases, the causes of the discordances have acquired controversial political significance. Societies vary on the values placed on some discordances. In the last several decades the public consensus of many Western societies has come to view some discordances as less undesirable and more tolerable than much of the rest of the world, although this view may itself exhibit a certain level of cultural imperialism.

Biological varieties of discordance

Human variability occurs in all the levels by which sex and gender are defined. Discordance at the biological levels is often referred to as an intersex condition. For example, some women may have an XY karyotype (chromosomal constellation). Some boys may have a rudimentary uterus, or an extra X chromosome. In a small subset of boys or girls with intersex conditions, the external genitalia may be undervirilized or overvirilized. If the degree of virilization is "in-between", the genitalia are described as "ambiguous". Many people with intersex conditions do not have ambiguous genitalia. However, for these people the relationships between biological factors (such as hormones) and environmental factors and the psychosocial levels of sexual identity such as gender identity and sexual orientation have proven to be complex, with plenty of exceptions to proposed theoretical systems. For example, there have been cases of male genetic/chromosomal sex, with female external genitalia, assigned and raised as female, but discovering or deciding upon a male gender identity by adolescence. The degree to which a person's gender identity is affected by hormones, by genetic factors distinct from hormones, by early education, by social factors, and by "existential choice" remains imperfectly understood and a subject of contention.

Psychological, behavioral, and cultural varieties of discordance

In contrast to the small percentage of people with biological discordances of sex, a fairly large proportion of human beings may be "discordant" in one or more behavioral or psychological dimensions. The vast majority of these people who are discordant in some aspect of psyche or behavior do not have any detectable biological intersex condition (although some recent studies point towards biological factors in at least some of those conditions). Human societies respond to, or accommodate, these behavioral and psychological discordances in many different ways, ranging from suppression and denial of difference to acknowledging various forms of "third sex".

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 (e.g. individuals who take on the role or customs of shaman, medicine man or 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 cross-dressing). Such behavior receives severe social and/or legal sanctions in some cultures.

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. A proponent of this movement away from polar oppositions, Anne Fausto-Sterling, once suggested we recognize five sexes: male, female, merm, ferm and herm. Although quickly rejected as a bizarre flouting of human nature and social reality, and inimical to the interests of those whom she was attempting to champion, it expresses the difficulty and imperfection of the current social responses to these variations.

Social and legal considerations

Forms of legal or social distinction or discrimination based on sex include sex segregation and sexism. Notably, some businesses, public institutions, and laws may provide privileges and services for one sex and not another, or they may require different sexes to be physically separated.

In gender theory, the term "heteronormativity" refers to the idea that human beings fall into two distinct and complementary categories, male and female; that sexual and marital relations are normal only when between two people of different genders; and that each gender has certain natural roles in life.