Chapter 3 of the book examines the question ‘what are the real differences between men and women?’ The author compares our (humans’) sexual dimorphism (that is, the difference in body shape, size, and function between males and females) with those of other animals. By this she makes the point that our bodies are not very different at all, compared to those of some species. The chapter deals with the genetic difference between male and female bodies, namely the x and y chromosomes. The x chromosome is present in every living human, male or female. The y chromosome is present in male bodies, and it causes the gonads of a fetus to descend and become testosterone-producing testes, external genitalia to be formed, etc. This in turn causes further sex-differentiation at puberty.The book then spends time talking about hormonal differences between men and women, explaining that testosterone and estrogen are present in both male and female bodies, but in different levels and performing different functions in each. Hormonal and genetic differences interact such that in females, high levels of estrogen cause a masculinising effect, for example. The book also talks about the interactions between social situations and hormones (testosterone levels rise when you win at sports or drive a fast car, for example). After this, the book discusses brain differences between men and women, and how these differences are both biologically and socially influenced. 
        Reading The chapter, I was surprised to learn about some of the ways genes and hormones interact, and didn’t know that estrogen can cause more masculine traits in women. I was also surprised to learn how hormones affect some basic cognitive processes, such as mental rotation. What struck me the most however was the fact that we are so low on the scale of sexual dimorphism in comparison to many animals. If we were like peacocks, for instance, males would be brightly coloured and have large adornments, while females would be much smaller and darker-coloured. If we were like anglerfish, males would be a tiny fraction the size of females, and it would be biologically impossible for them to survive without a symbiotic relationship to a female.
