Why “biological sex” may not mean what you think

There are a lot of terms to refer to a person’s sex: sex, birth sex, natal sex — and more contentiously, assigned sex at birth and biological sex.

The nuances and differences aren’t always important. But there are differences, and when writing about trans and intersex people — and the politics about us — it’s important to be accurate. Journalists can’t be accurate without understanding the political connotations these terms have taken on that invariably influence audience perception.

Take biological sex, which is commonly used in both medical literature and journalism. It may seem straightforward, but people — including journalists — who use it may mean different and sometimes contradictory things. In Alabama, it could mean reproductive organs; in Kentucky, it could mean what is recorded at birth. Other times, biological sex focuses on whether someone has a Y chromosome, sometimes called chromosomal sex.

As journalists, it’s our job to use clear language that says what we actually mean. 

Table of Contents

• Explaining the terms and how they relate
• Contradictory and/or medically inaccurate policy definitions
• How biological sex and assigned sex at birth may differ
• Key questions for journalists

Explaining the terms and how they relate

Assigned sex at birth (ASAB) is the sex recorded on a baby’s birth certificate based on a decision by doctors. Typically, ASAB is based on external genitalia and doesn’t involve chromosome testing or hormone measurements. Birth sex and natal sex are sometimes used as synonyms for ASAB. Assigned sex at birth is a term that has been used in medical literature since at least 1960; it refers specifically to this initial determination by doctors at a baby’s birth. 

Biological sex is a more ambiguous term, and it doesn’t have an agreed-upon scientific definition. There’s no single, measurable trait that tells you someone’s biological sex. Biologists include at least five different factors that make up sex:

• Chromosomes
• Gonads (ovaries or testes)
• Genitalia
• Hormones
• Secondary sex characteristics (like breasts or facial hair)

In common usage, biological sex can be used to refer interchangeably to any one of these factors.

For most people, throughout most of their lives, these biological factors all point to the same sex. But this isn’t always the case. Up to 2% of people are intersex, meaning their bodies display less common combinations of “male” and “female” biological traits. And both non-trans and trans people may alter some of their biological sex traits. For example, hormone replacement therapy changes hormone levels.

There can be confusion between biological sex and legal sex. Reporters usually determine biological sex by the sex marker on someone’s birth certificate or driver’s license. These can be changed legally, and they don’t necessarily reflect a person’s biology. Journalists who have current vital documents but not medical records likely cannot verify someone’s birth sex.

Policy definitions of biological sex often contradict each other and/or are medically inaccurate.

• The Trump administration issued an executive order defining biological sex by gonads (ovaries or testes). This order ignores the fact that some people are born with ambiguous gonads that can’t be clearly defined as male or female.
• A definition from the U.S. Department of Health and Human Services includes gonads, and also states, “the sex of a human, female or male, is determined genetically at conception,” which might be a reference to chromosomes.
• A Tennessee law passed in 2023 defines sex essentially as ASAB: “a person’s immutable biological sex as determined by anatomy and genetics existing at the time of birth.” 
• A Utah law regulates bathroom usage in public schools and other government facilities by biological sex. The law includes exceptions for trans people who have legally amended their birth certificate and undergone a “primary sex characteristic surgical procedure,” including procedures that aren’t externally visible, such as a hysterectomy.

Biological sex and assigned sex at birth don’t always line up.

Here are a few examples:

• An intersex woman was assigned female at birth based on her external genitalia, but doctors didn’t realize she had internal testes. Later in life, she learns that her biological sex could be considered male based on her gonads and hormone levels, even though her assigned sex at birth was female and her legal sex has always been female.
• A trans man was assigned female at birth. He takes testosterone for many years. Some sports leagues disqualify him from the women’s team due to his hormone levels, while others require that he participate on the women’s team due to his chromosomes, based on varying definitions of biological sex.
• A trans woman is assigned male at birth. As an adult, she updates the sex on her birth certificate and other identity documents to female. Her assigned sex at birth was male, but her current legal sex is female. Based on just this information, we don’t know anything about how doctors would classify her biological sex.
• A clerical error leads to a cisgender boy’s assigned sex at birth being recorded as female, meaning that his ASAB doesn’t reflect his biological sex.
• A child’s parents have opted to omit sex on vital documents, which means the child’s assigned sex at birth has no relationship to their biological sex. 

These are not theoretical examples. Intersex activists Alicia Roth Weigel, River Gallo, and Pidgeon Pagonis are just a few of those who have spoken publicly about their experiences with the medical system and public scrutiny of their identities. Transgender wrestler Mack Beggs has competed on teams for multiple genders, depending on their rules; such dilemmas are common for trans adults and youth alike.

Key questions for journalists:

Ask yourself these coverage questions during your reporting. Allow the answers to guide you.

When in doubt, be specific and transparent about what you mean.

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