The gender binary assumes that one’s category membership is biologically determined (1), apparent at birth (2), stable over time (3), a powerful predictor of psychological variables (4) and salient and meaningful to the self (5). There is an idea that brains are gendered (i.e. male and female brains). In neuroscience requires two assumptions:

1. Sex is a dimorphic system (i.e. system that can only take two forms).
2. Effect of sex on other systems (e.g. brain) is characterized by dimorphic outcome (e.g. male vs. female brain).

For a system to show dimorphism, each of its elements should be dimorphic. Furthermore, all the elements within an individual should be internally consistent (i.e. either all in the form typical of males or typical of females). The brain sex differences are context-dependent. However, it is unlikely that brains are internally consistent and dimorphic. Mosaicism (i.e. one typical female-part in the brain and one typical male-part in the brain) is most common in the human brain.

The idea of gender binary in behavioural neuroendocrinology involves two assumptions:

1. The reproductive glands are dimorphic (i.e. male and female hormones).
2. The levels of these hormones are genetically determined and fixed.

These assumptions do not hold as both men and women have testosterone and oestrogen. Furthermore, the average level of these hormones does not differ between men and women. The differences in levels of hormones vary across the lifespan. Thus, the gender binary cannot be completed based on androgens and oestrogens.

The reproductive glands are not fixed and innate as their levels vary widely within individuals. Hormones are influenced by social context. For example, testosterone decreases with supportive environments and increases with competitiveness. Furthermore, gendered expectations and lived experiences can shape hormones.

Gender binary in psychological research involves the assumption that there are only two discrete categories of people (i.e. males and females), implicitly stating that there is no overlap between the two categories.

People possess both feminine and masculine psychological characteristics. Internal consistency in personality traits (e.g. all masculine traits) is extremely rare. Stereotypes of men and women exist but individuals who consistently match these stereotypes are very rare. T

Transgender and non-binary people have largely been ignored in psychological research. Psychological research on transgender and non-binary people lead to three major challenges to the gender binary:

1. It shows that gender-assigned categories are imperfect for predicting how a person will self-label their gender identity.
2. It shows that the assumption that gender/sex only comprises the dichotomous categories of male and female is not correct.
3. It shows that self-labelling of gender (i.e. being gender) is different from enacting gender roles and stereotypes (i.e. doing gender).

Gender identity is not invariably linked to sex category at birth. Gender could be seen as a bundle of separable constructs.

Being gender could be a precondition for doing gender in ways that are not tied to the birth-assigned gender category. This means that realizing you are a boy or a girl leads you to pay attention to the relevant stereotypes for the self-labelled categories. One’s self-assigned gender identity can focus a person’s attention on how gender is expected to be done within that category.

Gender emerges as a psychologically salient and meaningful dimension in childhood and is not the inevitable result of an innate mechanism. There are several practices that establish gender as a salient and binary category:

1. Heightening the perceptual discriminability of gender
   This is heightened due to social and cultural norms (e.g. different clothing for males and females) and this makes it more likely that children categorize each other into binary gender categories.
2. Linguistic labelling of gender
   Language affects a child’s conceptualization and makes categorization easier. Categorization in language makes inferences about category members easier. Linguistic labelling of gender leads to children using the gender binary.
3. Explicit and implicit use of gender for sorting
   Exposing children to environments that are characterized by gender sorting contributes to children’s attention to sorting people by gender. Explicit sorting makes use of linguistic labels whereas implicit sorting makes use of context.

The developmental intergroup theory states that children’s endogenous qualities (e.g. cognitive capacities) interact in dynamic ways with their environmental context to induce children to attend to, categorize others and develop stereotypes and prejudice concerning gender.

There are several costs of using the gender binary:

1. Reliance on the gender binary is an obstacle to scientific progress (e.g. only studying nurturing behaviour in women).
2. Gender binary denies and denigrates the existence of individuals who fall outside the binary.
3. Gender binary leads to unequal treatment (1), shape self-conceptions (2), undermine individuals’ performance in stereotyped domains (3) and shape legal and social policies (4).

Gender stereotypes may lead to biases in mental health diagnoses and treatment in clinical practice. It may also shape the diagnosis and treatment for issues specifically related to gender. Gender stereotypes may obscure the understanding of etiological factors as well as their choices of diagnosis and treatment.

Sex cannot be studied without consideration of gender. It is multidimensional and each component is dynamic and responsive. Gender should only be used in research to make sure that the sample is representative. It may be useful to measure gender in a continuous way. However, asking people about gender in research implies that this is important.

The term gender/sex allows researchers to develop integrated research questions and paradigms that simultaneously explore biological and sociocultural influences that relate to gender and sex.