One cause of intersexuality seems to be the posession of an abnormal number of chromosomes only one or more than two. A second cause stems from the fact that all humans, no matter what their chromosomal makeup, have the biological capacity to develop either male or female genitalia and secondary sexual characteristics while in the womb. Developmentally, some babies are born with male or female chromosomal makeup and with both male and female genitalia, or with some of the genitalia of the opposite chromosomal sex.

Dr. [Anne Fausto-Sterling] points out that there is a smooth continuum between 100 percent biologically male and 100 percent biologically female with many possibilities in between. She calls those with both testes and ovaries "herms." Those with testes and some female genitalia but no ovaries are "merms." Those with ovaries and some male genitalia but no testes are "ferms." This gives the possibility of five rough biological groupings: male, merm, herm, ferm and female.

As a result, there are perhaps millions of XX males and XY females living in the United States today. These are cultural males with male genitalia who are genetically female, and cultural females with female genitalia who are genetically male. The film star Jamie Lee Curtis is one well-known individual who is genetically male, but phenotypically female.