In discussions about feminism, the sex of a person is a biological/medical characterisation and their gender is a social role.

From a scientific point of view, the sex of a person can be seen as a long tuple of parameters corresponding to representations of hormonal data, clinical sex, brain sex, chromosomal sex, bone structure and others. This needs more than seven clusters for classification (if at all possible) . The concept of 'gender' in contrast is very unscientific and corresponds to a set of socially constructed, assigned behaviors and identity patterns that may have little to do with the scientific concept of sex. If the notion of gender cannot be excluded in a backward society, then the best alternative to avoiding gender would be to assume that the "gender of a person is what the person says it is".

In biological, medical or mainstream discussions this distinction is not as commonly understood (or accepted) but distinguishing the terms in this way is important in feminist discussions. Gender is more commonly discussed than sex in feminist spaces.

Sex

Human sexes are distinguished in a number of ways, most commonly genitalia at birth (or in wealthy societies, increasingly before birth due to ultrasound imaging), but also medical analysis of hormone levels and genetic material. The sex of a human is not a binary: quite a large number of people have some biological traits that are male and some that are female, or in some cases a variant that is not assigned a side of the binary. Intersex people who cannot easily be assigned a sex at birth due to having ambiguous sex characteristics have widely been discriminated against, and used to be (and often still are) 'corrected' surgically as infants to appear to be more typical of one sex due to the preferences of medical staff and/or their own families, often with devastating medical, emotional and social consequences later in their lives.

As an example of non-binary sex determinations, the 'typical' male sex chromosomes are XY and female XX. But there are XY people who exhibit almost entirely female sex characteristics due to, for example, androgen insensitivity and there are many people who have sex chromosomes that are neither XX nor XY.

The Jaded Hippy writes:

I would agree that this is the common construction of sex, the assumption that it is a label for which secondary sexual characteristics you are born with. The problem is, Kessler is, quite clearly, not setting up that assumption to be challenged, but accepting it as fact. And let's be absolutely clear here: that is not fact.
Sex is something that is assigned to us by doctors and/or our parents when we are born, usually based on a visual examination of our genitals. If we have something that looks like what these people think of as a vagina, we're labeled "female," if we have something that looks like what these people think of as a penis, we are labeled "male."
And then there are those whose "sex characteristics" do not look like either of those things.

Gender

A person's gender is a social identity and role. Very many things play into gender presentation, including but not limited to: dress and appearance choices, choice of hobbies, choice of career, social behaviour and sexual behaviour. This social presentation is called "performance" and some parts of it vary considerably by culture.

Gender being a social part of identity does not imply that it is freely chosen or fluid for most people. Gender is also assigned to almost everyone at or before birth based on the determination of their sex and many gender role assumptions begin in infancy. Most people consider their gender a very central part of their identity and the extent to which at least this core identification, if not every aspect of gender, is a biological sex property (of brain structure, or similar) is debated. In addition, the ability to vary from assigned gender roles is strictly policed in human society: this is one of the aspects of society that feminism and other anti-gender-identity-oppression challenges.

Many people who identify as the 'other' side of the gender binary from their biological sex use the term transgender (other people are cisgender). Bi-gendered people do not fall under the umbrella term transgender. The latter term includes transwomen, transmen and cross dressers. People who do not identify exclusively or primarily with either side of the binary often use the term genderqueer.

The concept of gender should not be confused with those of gender identity and brain sex. According to recent advances gender identity of a person relates to physical characteristics of the brain which in turn is affected by  hormonal configuration.

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