How Many Sexes are There?

Summary:

There are people who do not fit neatly into biologically defined "male" and "female" categories. The percentage of these people in the general population is estimated at anywhere from .02% to 2%. I don't know enough to evaluate the these numbers for accuracy.

Sex vs. Gender - One Definition

"Sex" refers to biological differences between females and males, including chromosomes, sex organs, and endogenous hormonal profiles. "Gender" refers to socially constructed and enacted roles and behaviors which occur in a historical and cultural context and vary across societies and over time.

Source: NIH

"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."

Source: American Journal of Human Biology

Authors: Melanie Blackless, Anthony Charuvastra, Amanda Derryck Castel, Anne Fausto-Sterling

Follow-up analysis has suggested smaller numbers (.018%, .37%)

Source: Leonard Sax (Psychologist, Physician), Carrie Hull (Asst. Professor of PoliSci)

Graphic showing the wide range of conditions that don't fall neatly into male or female.

Source: Scientific American

Op-ed describing wide variety of sex variations

Source: Anne Fausto-Sterling (professor of biology and gender studies)/NYT

Op-ed: "In humans, reproductive anatomy is unambiguously male or female at birth more than 99.98 per cent of the time."

Source: Colin M. Wright (evolutionary biologist) and Emma N. Hilton (developmental biologist)/WSJ 

Alternate link to op-ed (peaktrans.org)