Most people who identify as men have an X and a Y chromosome, while most people who identify as women have two X chromosomes. Understanding what genes are on the X versus the Y chromosome and when/how those genes are activated over the life course (beginning prenatally) is a way to examine the basis of sex differences. Similarly, for the different systems of sex determination across the animal kingdom (wikipedia).
1. Some sex determination systems in animals depend on environmental cues, such as the temperature during incubation of eggs in crocodiles.
Restricting the discussion to humans:
2) Not all humans match the pattern XY=identify as male; XX=identify as female.
3. Even for people who do match the preceding pattern, for most traits, the range for females and males overlaps. Of course, as noted in earlier posts, the fact that variation within a group is of larger than variation between the average of the groups does not mean that the groups cannot be distinguished or that it is impossible to tell if a person is male or female.
Let me affirm this last point with an example from a course I once took in multivariate statistic. We could not say with confidence whether a student was male and female on the basis of their height—there was too much overlap of the ranges—or, for the same reason, on the basis of their hip circumference. Yet a simple linear function that subtracted hip from height was very reliable in discriminating male from female students. [In this figure] height would be the x-axis, hip the y-axis; the squares the males, the triangles the females.
This said, the combination of variables can differ from culture to culture and over historical time. In any case, the separability of groups on the basis of a combination of variables does not provide grounds for using an individual´s membership in a group to formulate reliable expectations or make pre-judgements about traits for any given individual.
4. Moreover, most discussions of differences between males and females are about gender, not genital and gonadal sex. In a culture that emphasizes gender differences and justifies large inequalities in the average power that identified-as-males and identified-as-females assume, people can accentuate or be forced to conform in various traits, such as aggression, to the average for their group. In other words, the culturally shaped ratio for a given trait of variation between the average of the genders to variation within a gender to may be larger than it would be if gender were simply given by chromosomal sex, or even by genital and gonadal sex.
5. Given the complex intersection of processes involved in sex and in gender development, and the complex intersection of these two phenomena, it becomes difficult to establish a method to determine the effect of any given gene.
6. Fausto-Sterling (2000) provides a rich account of the complexities of sex, gender, sexuality for humans.
Fausto-Sterling, A. (2000). Sexing the Body: Gender, Politics and the Construction of Sexuality. New York: Basic Books.
(Introduction to this series of posts)