The American Society for Reproductive Medicine (ASRM) has published a clear statement of the scientific consensus on biological sex, drawing on the National Institutes of Health definition and decades of reproductive medicine research. The statement directly challenges legislative efforts in the United States that seek to reduce sex to a single physical characteristic observed at birth.
What science means by biological sex
The National Institutes of Health defines biological sex as a multidimensional biological construct based on anatomy, physiology, genetics, and hormones. This is not a fringe or contested position. It reflects what reproductive medicine specialists, geneticists, and endocrinologists have understood for many years. Sex is assigned at birth by a medical professional based on visible physical characteristics, but the underlying biology that shapes sex is far more varied than any single trait can capture.
Sexual differentiation takes place during foetal development, driven by a combination of genetic and hormonal factors. That process does not produce two neatly separated categories. It produces a spectrum of outcomes. Chromosomal variation alone illustrates this clearly. While XX and XY chromosomes are frequently associated with female and male sex respectively, variations such as XXY, XYY, and others occur in an estimated one in every 1,500 to 2,000 live births. Conditions such as Klinefelter syndrome, in which a person assigned male at birth carries an extra X chromosome, are relatively common and are not considered diseases or disorders requiring correction. They are natural variation.
Primary and secondary sex characteristics
The characteristics most commonly associated with sex, including genitalia, reproductive organs, body hair, and breast development, are shaped by a combination of genetics and hormones. These traits can vary considerably between individuals, even among those who share the same chromosomal profile. The assumption that any one of these characteristics can serve as a definitive, universal marker of sex is not supported by the available evidence.
Why legislation does not resolve the science
Several US states have in recent years passed laws that define legal sex by reference to reproductive anatomy or genetics at birth. A 2023 Kansas statute defines females as individuals whose reproductive systems are developed to produce ovaries, and males as those developed to fertilise ova. A 2023 Tennessee law defines sex as immutable and determined by anatomy and genetics at birth. The ASRM notes plainly that proposals of this kind are unsupported by science and oversimplify the intricate nature of human biology. Defining something by law does not change what it is in biology. The two are separate questions that require separate forms of reasoning.
What this means in a UK context
The debate about the legal and biological definition of sex is not confined to the United States. In the United Kingdom, the Supreme Court ruled in April 2025 that the word "sex" in the Equality Act 2010 refers to biological sex. That ruling determined how a specific piece of legislation should be interpreted. It did not settle the scientific question of what biological sex is or how it should be measured, and it did not override the medical and scientific literature on the complexity of sex as a biological construct. These distinctions matter when policy decisions about healthcare, sport, education, and legal recognition are being made, because collapsing a complex biological reality into a simplified legal category can have consequences for the people whose biology does not fit the assumed norm.
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Dr Helen Webberley, Gender Specialist and Medical Educator.
helenwebberley.com