Femaleness is X and maleness is Y − Rakyat Biologi

The human male and female resemble each other more than they differ. Sexual dimorphism (marked differences in the appearance of the body) is undoubtedly very advantageous for certain species such as guppies, peacocks and praying mantises, but this is not as much the case for most primates (macaques, chimpanzees, humans). The major differences between the bodies of human males and females reside in the sexual and reproductive organs, and only to a lesser degree in body traits related to non-reproductive specialization (hunting for males, child rearing for females, for example). In psychological terms, maleness and femaleness are continuously distributed traits. Each man and each woman is psychologically more or less androgynous (depository of male and female traits). However, in the concrete (biological) sense of whether one disposes of a male reproductive system or a female one, humans are very markedly bimodally distributed: most humans are wholly male or wholly female. Men are characterized by the presence of an X and a Y chromosome in every cell of their body, while women are characterized by the presence of two X chromosomes. These chromosomes are called "gonosomes" because they determine the development of the gonads (testicles for men, ovaries for women) which in turn determine the development of further sex-specific traits. The sex chromosomes represent the 23rd pair in humans, all the other pairs being "autosomes" (meaning having to do with determining every biological trait of the self, except for the person's gender).

There are two major phases during which the body is sexualized. The first is prenatal. During the second trimester of pregnancy, the male or female gonad (testicle or ovary) begins to secrete its sex-specific steroid hormones. These hormones are secreted into the fetus's blood stream and travel throughout the body, affecting overall development, including of the brain. This first steroid bath is termed the structuring phase of sexual development. As I will demonstrate throughout this book, this phase is the one during which the basic anatomic sex differences are implemented, penises and vaginas, sexual identity, sexual orientation, and many of the subtler sex differences as well.

The second major maturational event is puberty. The gonads become particularly active, secrete high concentrations of steroids, and complete the processes of sexualization of the body and of the mind. The body traits which result from this phase include body hair in the sex-specific areas, development of breasts, etc. This phase is called the activational phase of sexual development.

These two levels of hormonal determination of behavior I have just described are standard teaching in any introductory text. However, I believe their is a third, higher level of mechanism, more subtle than the previous two, which is rarely mentioned. I call it the "physiological" level. Indeed, as I show in detail in chapter 9, hormonal variations accompanying the menstrual cycle, pregnancy, and menopause have effects on brain neurotransmitters (molecules used by neurons to communicate with each other) and even on neuronal anatomy -including in the human species. Though in humans changing concentrations of circulating hormones in adulthood can be thought to affect brain function only in women, this has been found not to be the case. Testosterone level fluctuates mildly though significantly on a seasonal basis in normal men. Contrary to what is commonly believed, it is not in spring that testosterone blood concentrations are at their highest, but in fall. It is in winter that they are at their lowest. This cycling of testosterone characterizes not only human males but infrahuman primate males as well and the difference between the two extremes reaches 300%. Several studies have tracked prevalences of male-typical behaviors over the months of the year. Monkey mounting peaks in fall and generally correlates highly with testosterone levels. In humans, incidence of rape peaks in August, as does violent crime. Doreen Kimura found that cognitive performances of men track these subtle variations in hormones. This suggested to her that even in normal men there are subtle effects of testosterone fluctuations on brain function during adulthood.

Male and female traits can be directly caused by genes located on the gonosomes without the later occurring intervention of sex hormones, the male-typical color-blindness being an example. However, most gender-specific traits, termed «sex limited», result from an interaction between the two. Male baldness is is caused by an autosomal dominant gene which produces the undesireable phenotype only with the concurrant influence of circulating testosterone -which is itself modulated by action of the gonosomes.