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). The categories of sex are, therefore, descriptive of the reproductive functions that an individual is capable of performing at sometime in its life cycle.
- Alternate meaning: Human sexuality, in particular sexual intercourse
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.
Sex in fungi, plants
Fungi and some other organisms exist in more than two sexes, but still reproduce in pairs (any two differing sexes can reproduce).
Sex in animals
Some species, such as some species of earthworm, 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. 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 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 in humans
Sex in humans is a unitary concept and identity for most social purposes: we are male or female. However, when the criteria for defining male and female are looked at more closely two important facts become obvious. First, sex is definable at many different levels, some biological, some social, some psychological. Second, a significant fraction of the human population is not entirely concordant in every aspect of every level of definition.
The following table(only for threesomes) outlines some of the major levels at which we recognize a difference between human males and females. Some of the criteria are sex-dichotomous and some, such as body size, are sex dimorphic (i.e., characterize what is statistically more likely for one sex or the other). Some of the levels can be more objectively ascertained and measured than others. Some of the levels are "imputed" or defined by the surrounding society. For instance, whether male humans must wear trousers is a social issue. Some of the levels seem to be generated within each person as a subjective identity or drive.
The relationship of the biological levels of sexual differentiation to each other are fairly well understood. Many of the biological levels can be said to cause or determine the next level. For instance, for most people, the presence of a Y chromosome causes the gonads to become testes, which make hormones that cause the internal and external genitalia to become male, which cause the parents to assign and raise the child as a boy. However, the degree to which biological factors and environmental contribute to the psychosocial aspects of sexual differentiation, and even the interrelationships between the various psychosocial aspects of differentiation, are less well understood (the familiar "nature/nurture" controversy).
The second clear fact about human sex and gender is that a significant proportion of the human population is discordant at some levels of this paradigm, meaning that many people do not have every single biological, psychological, behavioral, and social characteristic in the same column.
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. People with biological discordances have taught us much about the processes of sexual differentiation, both biological and psychosocial. Some of the levels of discordance have enormous significance to the lives of the 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 of discordances. In the last several decades the public consensus of many Western societies has come to view discordances as less undesirable and more tolerable than in the much of the past and much of the rest of the world.
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. 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. (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.
See also berdache, hijra, xanith and transgender.)
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.
Note on terminology: sex versus gender
Many people, among them 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; to gender identity, a person's own feeling of belonging to a gender, and to gender perception as a description of how a person's gender is perceived.
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