The most thorough existing research finds intersex people to constitute an estimated 1.7% of the population*, which makes being intersex about as common as having red hair (1%-2%). However, popularly misinterpretted, much referenced statistics would have you believe are numbers are much lower. Here’s why.

Some groups use an old prevalence statistic that says we make up 1 in 2000, or .05%, percent of the population, but that statistic refers to one specific intersex trait, ambiguous genitalia, which is but one of many variations which, combined (as they are in medical diagnostics and coding), constitute the 1.7% estimate by esteemed Professor of Biology and Gender Studies, Anne Fausto-Sterling, of Brown University*. A similar, slightly higher, statistic was also reported in, “How Sexually Dimorphic Are We?”, by Blackless, et al, in The American Journal of Human Biology.
“The belief that Homo sapiens is absolutely dimorphic with the respect to sex chromosome composition, gonadal structure, hormone levels, and the structure of the internal genital duct systems and external genitalia, derives from the platonic ideal that for each sex there is a single, universally correct developmental pathway and outcome. We surveyed the medical literature from 1955 to the present for studies of the frequency of deviation from the ideal male or female. We conclude that this frequency may be as high as 2% of live births. The frequency of individuals receiving “corrective” genital surgery, however, probably runs between 1 and 2 per 1,000 live births (0.1–0.2%).” Am. J. Hum. Biol. 12:151–166, 2000. © 2000 Wiley-Liss, Inc.
http://onlinelibrary.wiley.com/doi/10.1002/(SICI)1520-6300(200003/04)12:2%3C151::AID-AJHB1%3E3.0.CO;2-F/abstract These two findings are the most thorough scientific research which exists on the statistical prevalence of congenital intersex traits in humans.

The erroneous belief that we are an extremely tiny percentage of the population is often used to dismiss our need for legal rights and protections. Thus, we encourage everyone — particularly allies and/or members of the press educating others about intersex people — to please use the information and prevalence statistic provided here to accurately do so. Thank you!

* Fausto-Sterling, Anne (2000). Sexing the Body: Gender Politics and the Construction of Sexuality. New York: Basic Books. ISBN 0-465-07713-7.
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* The Intersex Society of North America (ISNA), 1993-2008, popularized the “1 in 2000″ (.05%) statistic, but clarified on its website:
“Here’s what we do know: If you ask experts at medical centers how often a child is born so noticeably atypical in terms of genitalia that a specialist in sex differentiation is called in, the number comes out to about 1 in 1500 to 1 in 2000 births. But a lot more people than that are born with subtler forms of sex anatomy variations, some of which won’t show up until later in life. Below we provide a summary of statistics drawn from an article by Brown University researcher Anne Fausto-Sterling….” (http://www.isna.org/faq/frequency).