When you think about the term biological sex, you may only think of male and female. However, biological sex is not binary, meaning, there are more than just male and female. Though male and female, or XY and XX are the most commonly presenting biological sex assignments, biological sex is more complicated.

It might be helpful to think about biological sex in two ways: genetic sex identification and morphological sex identification. The genetic sex of a person is their chromosomes. For example, XX and XY represent female and male chromosomal sex. Morphological sex represents reproductive organs, external genitalia, and commonly associated sexual characteristics. During pregnancy, an expecting parent may wish to find out the biological sex of their baby. Typically, sex is confirmed through a visual identification via ultrasound. This process may confirm morphological sex, but genetic testing would be required to confirm the chromosomal makeup of the fetus.

While it is most common to find XY chromosomes presenting with external male genitalia, and XX presenting with external female genitalia, this is not always the case. In some instances, the formed genitalia do not align with the chromosomal sex. Broadly, this is referred to as intersex. The prevalence of intersex individuals is somewhat controversial. Some research suggests that intersex individuals make up 1.7 percent of the population and other research suggest that is more 0.018% of the population (Sax, 2002). However, as we have learned previously, research results are dependent on how the data was collected, the inclusion criteria, and how representative the sample is, among many other factors. In later chapters we will include information on the physical and sexual development of intersex individuals.