The largest pscyhological sex differences in humans are those in gender identity, the core gender identity of oneself as male or female and sexual orientation (erotic attraction to and interest in sexual partners of the same versus other gender). The vast majority of people have a core gender identity consistent with their genetic sex and a sexual orientation toward the the gender other than their own. This is not, however, true for everyone. In a small percentage of men their core gender identity is female; in a small percentage of women their core gender identity is male. (Hines, Melissa, PhD. Brain Gender. p 9-12. NY:Oxford University Press, 2004.)