Bigender

Bigender, or bi-gender, is a gender identity under the multigender, nonbinary, and transgender umbrella terms. Bigender people have two distinct gender identities, either at the same time, or at different times. The latter is a form of genderfluid identity, and may involve only two distinct genders, or it may involve "shades of gray between the two." The two genders of a bigender person can be the two binary genders, female and male. This is what people usually assume bigender means. However, some people who identify as bigender have a different pair of genders. For example, their two genders might be female and neutrois. Or the two genders might be both nonbinary, such as agender and aporagender. Bigender is recognized by the American Psychological Association (APA) as a subset of the transgender group.

History
A 1999 survey conducted by the San Francisco Department of Public Health observed that, among the transgender community, less than 3% of those who were assigned male at birth and less than 8% of those who were assigned female at birth identified as bigender.

In 2012, Case and Ramachandran gave a report on the results of a survey of genderfluid people who call themselves bigender who experience involuntary alternation between female and male states. Case and Ramachandran gave this condition the name "Alternating gender incongruity (AGI)." Case and Ramachandran made the hypothesis that gender alternation may reflect an unusual degree (or depth) of hemispheric switching, and the corresponding suppression of sex appropriate body maps in the parietal cortex. They said that "we hypothesize that tracking the nasal cycle, rate of binocular rivalry, and other markers of hemispheric switching will reveal a physiological basis for AGI individuals' subjective reports of gender switches... We base our hypotheses on ancient and modern associations between the left and right hemispheres and the male and female genders." These doctors think that when bigender people feel a change between their gender identities, it might have to do with a change in how they use parts of their brains. The gender change might also have to do with one of the cycles that everyone has in their body, specifically, a valve in the nose that changes sides every two days (the nasal cycle). The relationship between these gender changes and the nasal cycle is only a hypothesis, meaning that it is an interesting idea that doesn't have proof for now.

In 2014, bigender was one of the 56 genders made available on Facebook.

In 2015, an entry for "bigender" was added to Dictionary.com,  defined as "a person who has two gender identities or some combination of both."

Gender expression
Bigender people "move between feminine and masculine gender-typed behaviour depending on context. Some bigendered individuals express a distinctly ‘en femme’ persona and a distinctly ‘en homme’ persona […] others have shades of grey between the two."