People who believe in gender-identity ideology generally describe sex as “assigned at birth” – a doctor’s best guess at how the person may later feel, to be set aside if desired by the person themselves when they are old enough to declare their own identity. Insisting on recognising an individual’s sex then becomes, in the words of some influential British gender clinicians, “reductionist” and acting to “privilege biology over lived and felt identity”. 

This belief system grew out of the claim, first made by some scientists who studied sexual behaviour in the 1960s, that everyone has a “gender identity”: an inner, gender-related awareness of who they are that is separate from their knowledge of their biological sex. Those scientists believed that this awareness usually aligns with bodily sex, but occasionally differs from it. It might be described as a preference for being read by others as more masculine or feminine, or as androgynous – neither masculine nor feminine – or as a specific mixture of the two. No one can know someone’s gender identity without being told: it is invisible and subjective.

Among those who find the idea of gender identity meaningful, some think that it is fluid. This idea originates in postmodern philosophy, in particular the work of two French philosophers, Michel Foucault and Jacques Derrida, and in later interpretations of their work by the American philosopher and gender theorist Judith Butler. 

Postmodern philosophy is very hard to write about for non-specialists. The ideas are unintuitive and often self-contradictory, in part because the language is confusing. But roughly, postmodernists regard language as playing a decisive role in shaping what we experience as being reality. They regard subjective experience as more important than objective statements, and emotional responses as more authentic than rational ones. And they think that conventional understandings of the world need to be “deconstructed” in order to reveal how traditional knowledge structures, including science and medicine, are supposedly inherently oppressive.

According to Butler, gender is “performative”, by which she means that it is not an objective fact about a person, but is created by their actions. For example, she argues that a person becomes feminine by acting in a feminine way, which means that other people see that person as feminine. A woman, then, is “someone who does womanly things”. In this sense Butler sees gender as socially constructed rather than innate. 

One piece of evidence in favour of the idea that gender identity is fluid is that some people experience themselves as having a gender identity that is not stable. They may describe themselves as  “gender-fluid” or say that their gender identity has changed over time (for example, that they used to feel very masculine and now feel more androgynous). 

Some believers in gender identity – confusingly, including some who quote Butler frequently – think it is innate. These people usually think it is fixed at a young age, perhaps even at or before birth. 

These people often posit the existence of a biological marker of gender identity (a “brain sex” that usually aligns with the body’s sex but occasionally does not). For evidence, they point to a handful of studies that have attempted to show differences between the brains of people who identify as transgender and those who do not, which they think help explain why a small minority of people say they have always experienced a deep-seated feeling of having a gender identity different from their sex. 

But studies that purport to show brain differences in trans people are generally very weak in at least one way: 

• the differences they find are small and inconsistent with each other
• they do not take account of sexual orientation (that is, they may be picking up differences between the brains of straight and gay people, for which there is a fair bit of evidence, rather than those of “cis” and trans people)
• they do not control for the use of cross-sex hormones (exactly how these affect the brain is not known, but it is very likely that they do).

Moreover, even if there were brain differences this would not necessarily mean they cause the transgender identification. They could be as a result of that identification. Or they could indicate something else that causes gender dysphoria and hence a trans identification, such as a tendency to dissociate from one’s body.

The claim that gender identity is innate sits oddly with the postmodern approach out of which it grew. The reworking of ideas of gender from fluid to fixed seems to have happened during the early 21st century, as postmodern ideology evolved online and went mainstream. 

The key to understanding it is postmodernism’s emphasis on subjective experience and opposition to fixed, objectively delineated categories. If bodies cannot be classified according to clearly defined characteristics, and if beliefs trump observations, then anyone can say “I am a man” or “I am a woman”, and no one else has authority to overrule that statement, or supply criteria against which it can be assessed. 

This reasoning has political, legal and clinical consequences. For doctors, the biggest is when patients seek life-changing medical and surgical interventions because they feel that their bodies do not represent who they feel themselves to be. This is not a situation that healthcare professionals are trained to handle. 

The whole aim of modern medicine is to use objective symptoms and research evidence to diagnose and treat diseases and disorders. But now the body is supposed to take second place to a sense of identity, and what is wanted is help in manipulating it with hormones and surgery to make it conform to an inner sense of what is “real”. This may be called “medical treatment”, but there is no evidence-based link between cause and consequence, and no expectation of the usual sorts of health benefits.