What Is Endosex?

People Whose Anatomy Conforms to Binary Expectations

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Many people have questions about the difference between sex and gender. The short answer is that sex is about biology, and gender is about a person's identity.

Although there has been increasing awareness about nonbinary genders recently, there is less awareness that sex also doesn't necessarily conform to a binary. Individuals whose bodies cannot be definitively categorized as either male or female at the time of birth are described as being intersex or having differences or disorders of sexual differentiation.

Individuals born with bodies that can be clearly categorized as male or female are described as endosex.

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Meaning of Endosex

Endosex individuals are those people who are born with physical characteristics that can be clearly categorized as either male or female. More specifically, if they are XY individuals, they are born with masculine genitalia and internal structures. If they are XX individuals, they are born with female genitalia and internal structures.

Individuals whose bodies do not clearly conform to medical and social expectations of male or female are referred to as intersex or having differences of sexual differentiation. This is different than being transgender.

Endosex/intersex refers to the biology and structures of the body. Cisgender/transgender refers to a person's gender identity and internal sense of themselves.

Three Areas of Sexual Differentiation

Sexual differentiation takes place at a number of levels.

Chromosomes and Genes

The first and most basic level is at the chromosomes or genes. While we are taught that there are XX females and XY males, the truth is not so simple.

There are people who have only one X chromosome (XO, lacking a second X or a Y), and those who have two X and a Y (XXY), those with extra copies of the Y chromosome (XYY), and so on. Some people have chromosomal mosaicism, in which some of their cells have one pattern and others have another.

In addition, people may have the expected number of chromosomes but may have extra copies of genes that are important for sexual differentiation or copies that don't work.


The second level of sexual differentiation looks at what types of gonads a person has. Do they have testes, which can make testosterone and produce sperm? Do they have ovaries, which can make estrogen and progesterone and produce eggs?

Do they have structures that can't be clearly categorized as either ovaries or testes? Do they have no gonads at all or only what is referred to as "streak" gonads?


The final level of sexual differentiation is anatomic. This refers to the structures that people are born with, and there can be substantial variation even in people who have expected chromosomes and gonads.

For example, the glans can be small with a urethra at the base (typically called a clitoris), longer with a urethral opening at the tip (typically called a penis), or anything in between.

There can be a structure that appears to be a scrotum, labia, or something in the middle. People can have a clitoris and labia but no vagina. There are many types of intersex variation at the anatomic level.

Endosex Statistics

There is no accurate data about how many people are endosex and intersex. However, researchers estimate that as many as 1 in 200 people may be intersex, meaning that the rest are endosex.

This number is somewhat dependent on which people are considered to be endosex and which are not. The 1 in 200 estimate includes hypospadias and cryptorchidism among intersex traits.

More conservatively, the chromosomal abnormalities Klinefelter syndrome (XXY children) and Turner syndrome (XO) occur in around 1 in 500 to 1,000 and 1 in 2,500 live births, respectively. Those are two of the more common intersex conditions for which there is better data.

Endosex Associated Terms

When talking about endosex, there are a number of associated terms that it is useful to understand.

  • Difference or disorder of sexual differentiation: These are conditions that lead to the body developing in a way that is not clearly categorizable as male or female. They are also referred to as intersex conditions.
  • Glans: Both the clitoris and penis develop from the same embryonic structure, the genital tubercle. The glans develop into part of the clitoris or part of the penis depending on whether or not there is exposure to testosterone during prenatal development. It is one of the visible anatomical structures where differences can lead to people being categorized as intersex at birth.
  • Gonads: These are the structures that produce the majority of the sex hormones (testosterone, estrogen, progesterone) in the body. They are also where the gametes (sperm/egg) are made and matured. A person is agonadal if they don't have gonads.
  • Intersex: This is an adjective describing someone who was born with a body that could not be clearly categorized as male or female.
  • Labioscrotal swelling: This is the structure that, depending on prenatal hormone exposure, takes a developmental pathway toward becoming labia or the scrotum.
  • Sex chromosomes: Humans have 23 pairs of chromosomes, one pair of which are the sex chromosomes. Most people have either one X and one Y or two X chromosomes. However, intersex people may have different combinations of sex chromosomes.

A Word From Verywell

Why is the term endosex important? Because the alternative is to talk about intersex variations and categorize people without those variations as "normal" or the default. The truth is that biological sex is not always binary, and both intersex and endosex are expected types of developmental variation.

How do we know? Because they exist. Both intersex and endosex people can have problems with reproductive and sexual function. Both intersex and endosex people can have full, happy, and functional lives. There is nothing inherently better or worse about being either intersex or endosex.

1 Source
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  1. Lee PA, Nordenström A, Houk CP, et al. Global disorders of sex development update since 2006: Perceptions, approach and care [published correction appears in Horm Res Paediatr. 2016;85(3):180. Koopman, Peter [added]] [published correction appears in Horm Res Paediatr. 2016;86(1):70]. Horm Res Paediatr. 2016;85(3):158-180. doi:10.1159/000442975

By Elizabeth Boskey, PhD
Elizabeth Boskey, PhD, MPH, CHES, is a social worker, adjunct lecturer, and expert writer in the field of sexually transmitted diseases.