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Anne Fausto-Sterling’s claim that 1.7% of live births are intersex, popularized in her 2000 book Sexing the Body and a paper by Blackless et al., sounds compelling—until you peek under the hood. She argues it shows sex isn’t binary, estimating 1 in 59 babies has some “nondimorphic sexual development.” But this number isn’t what it seems. It’s a classic case of “cooking definitions”—stretching the term “intersex” so wide it loses meaning, inflating the stats to fit a narrative. Let’s break down how she did it and why it’s misleading.

Fausto-Sterling’s team cast a net over every condition deviating from a textbook male (XY, penis, testes) or female (XX, vagina, ovaries). They counted late-onset congenital adrenal hyperplasia (LOCAH)—1.5% of births—as intersex, despite these babies being born clearly male or female with matching chromosomes. LOCAH might cause later issues like excess hair, but it’s not ambiguous; most never need sex reassignment. Tossing in 88% of her 1.7% from this alone smells like padding the books to hit a target.

Then there’s Klinefelter Syndrome (XXY, 0.1%) and Turner Syndrome (X0, 0.05%). Klinefelter folks are phenotypically male—penis, testes, often fertile until puberty—and Turner folks are female—vagina, uterus, just with ovarian quirks. Neither has ambiguous genitals or mismatched sex; they’re not “intersex” by clinical standards. Fausto-Sterling also includes vaginal agenesis (0.016%), where XX females lack a vagina but have normal ovaries—hardly unclassifiable. This isn’t intersex; it’s a grab-bag of differences of sex development (DSDs).

Leonard Sax shredded this in 2002 in Journal of Sex Research. He argued “intersex” should mean chromosomal sex (XX/XY) clashing with phenotype or truly ambiguous genitals—think ovotestes or severe CAH needing surgery. By that definition, intersex drops to 0.018%—1 in 5,500 births—matching what neonatologists see (1 in 1,500–2,000 for ambiguous cases). Sax’s critique shows Fausto-Sterling’s 1.7% isn’t wrong data; it’s a definitional sleight-of-hand, lumping in conditions no doctor flags as intersex at birth.

So, when someone touts 1.7% to argue sex is a spectrum, point to the cooking: Fausto-Sterling broadened “intersex” beyond reason, counting non-ambiguous cases to juice the number. It’s not fabricated—her prevalence rates trace to real studies—but it’s misleading, designed to push a point rather than reflect reality. The true intersex rate, where sex is unclear, is closer to 0.05% or less. Next time that stat drops, you’ve got the recipe to call out the fudge.