Sex and gender distinction
The distinction between sex and gender differentiates sex, the biological makeup of an individual's reproductive anatomy or secondary sex characteristics, from gender, social roles based on the sex of the person (culturally learned) or personal identification of one's own gender based on an internal awareness (gender identity). Gender is sometimes referred to as the sex of the brain. In atypical circumstances, an individual's sex and gender do not align, and the result is transgenderism.
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 is annotated as different from gender in the Oxford English Dictionary, where it says sex "tends now to refer to biological differences". The World Health Organization (WHO) similarly states that "'[s]ex' refers to the biological and physiological characteristics that define men and women" and that "'[m]ale' and 'female' are sex categories".
The American Heritage Dictionary (5th ed.), however, lists sex as both "Either of the two divisions, designated female and male, by which most organisms are classified on the basis of their reproductive organs and functions" and "One's identity as either female or male," among other definitions.
The consensus among scientists is that all behaviors are phenotypes -- complex interactions of both biology and environment -- and thus nature vs. nurture is a misleading categorization. The term sex differences is typically applied to sexually dimorphic traits that are hypothesized to be evolved consequences of sexual selection. For example, the human "sex difference" in height is a consequent of sexual selection, while the "gender difference" typically seen in head hair length (women with longer hair) is not.
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 intersexual. People whose internal psychological experience differs from their biological sex are transgender or transsexual.
From the Renaissance to the 18th century, there was a prevailing inclination among doctors towards the existence of only one biological sex. In some discourses, this view persisted into the eighteenth and nineteenth centuries. Even at its peak, the one-sex model was supported among highly educated Europeans but is not known to have been a popular view nor one entirely agreed upon by doctors who treated the general population.
In the Oxford English Dictionary, gender is defined as, "[i]n mod. (esp. feminist) use, a euphemism for the sex of a human being, often intended to emphasize the social and cultural, as opposed to the biological, distinctions between the sexes.", with the earliest example cited being from 1963. The American Heritage Dictionary (5th ed.), in addition to defining gender the same way that it defines biological sex, also states that gender may be defined by identity as "neither entirely female nor entirely male"; its Usage Note adds:
Some people maintain that the word sex should be reserved for reference to the biological aspects of being male or female or to sexual activity, and that the word gender should be used only to refer to sociocultural roles. Accordingly, one would say The effectiveness of the treatment appears to depend on the sex of the patient and In society, gender roles are clearly defined. In some situations this distinction avoids ambiguity, as in gender research, which is clear in a way that sex research is not. The distinction can be problematic, however. Linguistically, there isn't any real difference between gender bias and sex bias, and it may seem contrived to insist that sex is incorrect in this instance.
A working definition in use by the World Health Organization for its work is that "'[g]ender' refers to the socially constructed roles, behaviours, activities, and attributes that a given society considers appropriate for men and women" and that "'masculine' and 'feminine' are gender categories." However, the Food and Drug Administration uses gender instead of sex when referring to physiological differences between male and female organisms. Gender is now commonly used even to refer to the physiology of non-human animals, without any implication of social gender roles.
The Gay and Lesbian Alliance Against Defamation makes a distinction between sex and gender in their most recent Media Reference Guide. Sex is "the classification of people as male or female" at birth, based on bodily characteristics such as chromosomes, hormones, internal reproductive organs, and genitalia. Gender identity is "one's internal, personal sense of being a man or woman (or a boy or a girl).
Some feminist philosophers maintain that gender is totally undetermined by sex. See, for example, The Dialectic of Sex: The Case for Feminist Revolution, a widely influential feminist text.
The case of David Reimer who was, according to studies published by John Money, raised as a girl after a botched circumcision was described in the book As Nature Made Him: The Boy Who Was Raised as a Girl. Reimer was in fact not comfortable as a girl and later changed gender identity back to male when discovered the truth of his surgery. He eventually committed suicide.
Gender in the sense of social and behavioral distinctions, according to archaeological evidence, arose "at least by some 30,000 years ago". More evidence was found as of "26,000 years ago", at least at the archeological site Dolní Věstonice I and others, in what is now the Czech Republic. This is during the Upper Paleolithic time period.
Criticism of the "sex difference" vs. "gender difference" distinction
The current distinction between the terms sex difference vs. gender difference has been criticized as misleading and counterproductive. These terms erroneously suggest that the behavior of an individual can be partitioned into separate biological and cultural factors. (However, behavioral differences between individuals can be statistically partitioned, as studied by behavioral genetics). Instead, all behaviors are phenotypes—a complex interweaving of both nature and nurture. 
Diane Halpern, in her book Sex Differences in Cognitive Abilities, noted the problems with "sex" vs. "gender" terminology: "I cannot argue (in this book) that nature and nurture are inseparable and then... use different terms to refer to each class of variables. The ...biological manifestations of sex are confounded with psychosocial variables.... The use of different terms to label these two types of contributions to human existence seemed inappropriate in light of the biopsychosocial position I have taken." She also noted that "Pinker (2006b, para. 2) provided a clear summary of the problems with the terms "sex and gender: "part of it is a new prissiness of many people today are squeamish about sexual dimorphism as the Victorians were about sex. The word sex refers... (both) to copulation and to sexual dimorphism..." Richard Lippa writes in Gender, Nature and Nurture that "Some researchers have argued that the word "sex" should be used to refer to (biological differences), whereas the word "gender" should be used to refer to (cultural differences). However, it is not at all clear the degree to which the differences between males and females are due to biological factors versus learned and cultural factors. Furthermore indiscriminate use of the word "gender" tends to obscure the distinction between two different topics: (a) differences between males and females, and (b) individual differences in maleness and femaleness that occur within each sex." 
It has been suggested that more useful distinctions to make would be whether a behavioral difference between the sexes is first due to an evolved adaptation, then, if so, whether the adaptation is sexually dimorphic (different) or sexually monomorphic (the same in both sexes). The term sex difference could then be re-defined as between-sex differences that are manifestations of a sexually dimorphic adaptation (which is how many scientists use the term), while the term gender difference could be re-defined as due to differential socialization between the sexes of a monomorphic adaptation or byproduct. For example, greater male propensity toward physical aggression and risk taking would be termed a "sex difference;" the generally longer head hair length of females would be termed a "gender difference."
Transgender and genderqueer
Transgender does not have one definition. Instead it is a group of "identities and experiences of sex and gender variance, changing, and blending." This is when an individual's biological sex does not match up with which gender they identify themselves. Under the umbrella of transgender includes "transsexual people, cross-dressers, drag kings and queens, genderqueer people, gay men and lesbians who queer gender lines, the partners of trans people, and any number of other people who transgress binary sex." These individuals often undergo sex-reassignment surgery, take hormones, or change their style of life to feel more comfortable.
In July 2012 Gopi Shankar, a Gender activist and a student from The American College in Madurai wrote the first book on Gender-variants and coined the regional terms for genderqueer people in Tamil, Gopi said apart from male and female, there are more than 20 types of genders, such as transwoman, transmen, androgynous, pangender, trigender,, etc., and ancient India refers it as Trithiya prakirthi."
Some feminists consider sex to only be a matter of biology and something that is not about social or cultural construction. For example, Lynda Birke, a feminist biologist, states that "'biology' is not seen as something which might change." However, the sex/gender distinction, also known as the Standard Model of Sex/Gender, is criticized by feminists who believe that there is undue emphasis placed on sex being a biological aspect, something that is fixed, natural, unchanging, and consisting of a male/female dichotomy. They believe the distinction fails to recognize anything outside the strictly male/female dichotomy and that it creates a barrier between those that fit and those that are 'abnormal'. In order to prove that sex is not only limited to two categories Anne Fausto-Sterling's Sexing the Body addresses the birth of children who are intersex. In this case, the standard model (sex/gender distinction) is seen as incorrect with regard to its notion that there are only two sexes, male and female. This is because "complete maleness and complete femaleness represent the extreme ends of a spectrum of possible body types." In other words, Fausto-Sterling argues that there are multitudes of sexes in between the two extremes of male and female.
Rather than viewing sex as a biological construct, there are feminists who accept both sex and gender as a social construct. According to the Intersex Society of North America, "nature doesn't decide where the category of 'male' ends and the category of 'intersex' begins, or where the category of 'intersex' ends and the category of 'female' begins. Humans decide. Humans (today, typically doctors) decide how small a penis has to be, or how unusual a combination of parts has to be, before it counts as intersex." Fausto-Sterling believes that sex is socially constructed because nature does not decide on who is seen as a male or female physically. Rather, doctors decide what seems to be a "natural" sex for the inhabitants of society. In addition, the gender, behavior, actions, and appearance of males/females is also seen as socially constructed because codes of femininity and masculinity are chosen and deemed fit by society for societal usage.
West and Zimmerman's "doing gender"
Used primarily in sociology and gender studies, the term doing gender refers to the concept of gender as a socially constructed performance which takes place during routine human interactions, rather than as a set of essentialized qualities based on one's biological sex. The term first appeared in Candace West and Don Zimmerman’s article “Doing Gender”, published in the peer-reviewed journal, Gender and Society. Originally written in 1977 but not published until 1987, Doing Gender is the most cited article published in Gender and Society. West and Zimmerman state that to understand gender as activity, it is important to differentiate between sex, sex category, and gender. They say that sex refers to the socially agreed upon specifications that establish one as male or female; sex is most often based on an individual's genitalia, or even their chromosomal typing before birth. West and Zimmerman consider sex categories to be dichotomous, and that the person is placed in a sex category by exhibiting qualities exclusive to one category or the other. During most interactions, others situate a person's sex by identifying their sex category; however, West and Zimmerman believe that a person's sex need not align with their sex category. West and Zimmerman (1987) maintain that the sex category is "established and sustained by the socially required identificatory displays that proclaim one’s membership in one or the other category" (p. 127). Gender is the performance of attitudes and actions that are considered socially acceptable for one’s sex category. West and Zimmerman suggest that the interactional process of doing gender, combined with socially agreed upon gender expectations, holds individuals accountable for their gender performances. They also believe that while "doing gender" appropriately strengthens and promotes social structures based on the gender dichotomy, it inappropriately does not call into question these same social structures; only the individual actor is questioned. The concept of "doing gender" recognizes that gender both structures human interactions and is created through them.
Some feminists go further and argue that neither sex nor gender are strictly binary concepts. Judith Lorber, for instance, has noted that many conventional indicators of sex are not sufficient to demarcate male from female. For example, not all women lactate, while some men do. Similarly, Suzanne Kessler, in a 1990 survey of medical specialists in pediatric intersexuality, found out that when a child was born with XY chromosomes but ambiguous genitalia, its sex was often determined according to the size of its penis. Thus, even if the sex/gender distinction holds, Lorber and Kessler suggest that the dichotomies of female/male and masculine/feminine are not themselves exhaustive. Lorber writes, "My perspective goes beyond accepted feminist views that gender is a cultural overlay that modifies physiological sex differences [...] I am arguing that bodies differ in many ways physiologically, but they are completely transformed by social practices to fit into the salient categories of a society, the most pervasive of which are 'female' and 'male' and 'women' and 'men.'"
Moreover, Lorber has alleged that there exists more diversity within the individual categories of sex and gender—female/male and feminine/masculine, respectively—than between them. Hence, her fundamental claim is that both sex and gender are social constructions, rather than natural kinds.
A comparable view has been advanced by Linda Zerilli, who writes, "[Monique] Wittig is critical of the sex/gender dichotomy in much feminist theory because such a dichotomy leaves unquestioned the belief that there is a 'core of nature which resists examination, a relationship excluded from the social in the analysis—a relationship whose characteristic is ineluctability in culture, as well as in nature, and which is the heterosexual relationship.'.... Putting sex in nature, gender in society, Wittig suggests, enabled feminists to interrogate the cultural construction of femininity; but this strategy also allowed dominant discourses to acknowledge the distinction without rethinking the foundations of their diverse theoretical enterprises and their concepts of subjectivity...."[a]
- Monique Wittig, feminist theorist and author of Les Guérillères, a lesbian feminist novel
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