How Sexually Dimorphic Are We? The Blackless et al. (2000) Review Explained

"How sexually dimorphic are we? Review and synthesis" is a 2000 review by Blackless and colleagues in the American Journal of Human Biology. It compiled published studies on chromosomal, gonadal and anatomical variation and estimated that up to about 2% of live births differ from the typical male or female pattern. Later authors have disputed that figure, depending on which variations are counted.

Citation

Who wrote it?

Six researchers share the byline. The best known is Anne Fausto-Sterling, a biologist whose later writing on sex and gender kept the review in public discussion.

What does the paper ask and find?

The paper asks how far human beings fit a strict two-category model of sex. It looks at chromosome composition, gonadal structure, hormone levels and internal and external genitalia. It is a review: the authors collected no new data, but surveyed medical literature published from 1955 onward and combined the reported frequencies.

Their abstract puts deviations from the idealized male or female body at as many as 2% of live births, and corrective genital surgery at about 0.1–0.2%. These are synthesized estimates drawn from studies with different methods and populations, not the result of a population census.

How common is intersex? Why the 2% figure is disputed

No single number answers this question, because the count depends on which conditions are called intersex. The 2000 review used a broad scope. In 2002, physician Leonard Sax replied in the Journal of Sex Research that the wide count, which he describes as about 1.7%, includes conditions such as Klinefelter syndrome, Turner syndrome and late-onset adrenal hyperplasia, which most clinicians do not classify as intersex. Under a strict definition, where chromosomal sex conflicts with body type or the body cannot be classed as male or female, he arrived at about 0.018% (Sax 2002). The gap between the figures comes mostly from definition, not from disagreement about the underlying studies.

Why is it still cited?

The review is a standard reference whenever an article, forum answer or legal commentary needs a figure for how often variations of sex characteristics occur. The New York Times, the Skeptics Stack Exchange and the Verfassungsblog have all linked to it.

How does it relate to AIS and intersex variations?

Androgen insensitivity syndrome (AIS) is one of the intersex variations. For background, start with what androgen insensitivity syndrome is and the complete and partial forms of AIS. Related pages cover what intersex means for women, medical perspectives on genital variations and related conditions.

FAQ

What is the main finding of Blackless et al. 2000?

The authors estimated that up to about 2% of live births show some deviation from the idealized male or female body, and that about 0.1–0.2% involve corrective genital surgery.

Is the 2% figure still accepted?

It is disputed. Sax (2002) argued that the broad count includes conditions most clinicians do not call intersex, and proposed a much lower figure under a narrower definition.

Where can I read the original paper?

At the publisher, through the DOI in the citation table, or in the PubMed record.

Is the paper a medical guide?

No. It is a statistical synthesis of published literature and gives no clinical recommendations.

Sources

• Blackless M, Charuvastra A, Derryck A, Fausto-Sterling A, Lauzanne K, Lee E. How sexually dimorphic are we? Review and synthesis. American Journal of Human Biology. 2000;12(2):151–166. PubMed 11534012.
• Sax L. How common is intersex? A response to Anne Fausto-Sterling. Journal of Sex Research. 2002;39(3):174–178.

Medical disclaimer

The information here is general and is not medical advice. Consult a qualified health professional.