Gender identity

A doctor takes a quick look at a newborn baby and declares, “It’s a girl!” or “It’s a boy!” Simple, right?

Maybe not. Before the 20th century, people used the terms “sex” and “gender” synonymously. But researchers began to differentiate between the two terms in the 1920s and refined their understanding over time.

As a publication from the U.S. Department of Health and Human Services (DHHS) explains, scientists today believe that gender is determined by “a person’s sense, belief, and ultimate expression of self,” rather than their sex at birth (SAMHSA, 2012, A Discussion on Gender Identity).

In other words, gender identity is distinct from biological sex, even though for most people, the two correspond. A doctor declaring "It's a girl!" or "It's a boy!" is actually describing a baby's biological sex--or more specifically, their genitalia--not their gender identity.

Most people will grow up to identify with the gender that corresponds with their biological sex. But that's not the case for everyone. People who are transgender have a gender identity that is different from their sex observed at birth.

Scientists' understanding of gender identity has evolved from a focus on strict sex-gender congruence and conforming behaviors in the 1950s, to a more nuanced concept of gender, and its distinction from biological sex, today.

In the 1950s, if a person displayed a gender identity that was different from their sex at birth, they were assumed to have a psychological problem. Beginning in the 1960s, however, researchers developed an understanding that differences between a person's "assigned sex at birth and their gender identity was of a biological, rather than psychological nature" (SAMHSA, 2012, A Discussion about Gender Identity, para. 4-6).

Where does gender identity come from if it's not determined by biological sex? Developing research has focused on the field of neuroscience, with brain scans of transgender individuals showing neurological patterns that align with the gender they identify with, rather than the sex they were assigned at birth.


Bakker, J. (2018) Brain structure and function in gender dysphoria. Endocrine Abstracts, 56(S 30.3).

Bodkin, H. (2018, May 22). Transgender brain scans promised as study shows structural differences in people with gender dysphoria. Telegraph.

Substance Abuse and Mental Health Services Administration (2012). Top health issues for LGBT populations information & resource kit. Rockville, MD: HHS Publication No. (SMA) 12-4684.

Gender Identity Gender identity icon Our core sense of who we are as a man, a woman, a mixture of both, or neither.

Gender Expression Gender expression icon How we show up in the world through choices like clothing, hair style, mannerisms or tone of voice.

Attraction attraction icon How we feel toward others sexually, romantically and/or emotionally.

Biological Sex Biological sex icon Physical attributes such as reproductive organs and genitalia, chromosomes, genes and hormone levels.

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Curious to learn more about the research on brain structure and neurological patterns, as it relates to gender identity? Read a summary of an article by scientists conducting this cutting-edge research.

Brain Structure and Function in Gender Dysphoria

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