My title is from the famous plaint by Professor Higgins in the musical My Fair Lady. The song is about the difference between the sexes. It comes very clearly from an amusingly, ludicrously biased male point of view, but it serves to remind us that the differences between men and women are a major issue in our lives. However, I might just as well have called the book Why Can’t a Man be More Like a Woman? Since men, as I shall show, are essentially biologically modified women.

So just how different are men from women, and are there important differences that are genetically determined? This is a very controversial subject, but most men and most women would think that there are significant differences, and so do I. An enormous scientific literature, often disputed, has been devoted to the subject and it is one that arouses strong passions. There has even been criticism of some of this scientific literature to add to the controversy – namely, the assertion that some investigators may have biased their research deliberately to show differences, since a result of an investigation showing no difference between males and females would be less interesting.

Are there significant biological factors that make women behave differently from men, or are all the apparent differences due to social and cultural factors? I am a developmental biologist who has studied how embryos develop from the fertilised egg. Genes control the development of the embryo by providing the codes for making proteins which largely determine how cells behave. The cells in the human embryo give rise to the structure and function of our brains and bodies. We are essentially a society of cells which come from the egg and which determine who we are. They determine whether we are male or female, and I want to understand whether important differences in the behaviour of men and women are largely controlled by their genes during development and by the action of hormones both in the womb and in later life. This is particularly relevant in relation to men and women’s intellectual, emotional and physical abilities and their social behaviour.

And why are there two sexes? This is a difficult problem related to our evolution, and most animals have two sexes. But there are some species with just one, and with these, asexual reproduction is possible, if simpler and less enjoyable. Evolution has selected differences between men and women so as to make their reproduction as successful as possible. What are the evolutionary advantages of two sexes and of the genetically specified differences between men and women? Are there differences in intelligence and mental skills, language and motor skills?

The development of the brain, which determines how we behave, involves billions of nerve cells making innumerable connections with each other, and this is controlled by genes. We need to understand the differences that develop between male and female brains. The hormone environment in the embryo, controlled by genes, clearly contributes to the development of the physical and mental differences between males and females. Genes on the sex chromosomes explain why most people are heterosexual, for example. In other animal species, research has shown that the early hormone environment clearly has long-term effects on behaviour by changing the development of the brain, but the specific changes involved in humans have only recently begun to be identified and are a controversial area.

Also controversial are the many sex differences that have been described between human male and female brains, but only a few of these seem to be relevant to sex differences in behaviour. Regions of the brain, such as the hypothalamus and amygdala, which play key roles in processing emotions and sexual behaviour, have receptors for male hormones like testosterone, but much work still needs to be done in discovering reliable links between these hormones, the development of the brain and different behaviour in men and women. I will look at the structural differences between male and female brains revealed by the powerful techniques of MRI (magnetic resonance imaging) as well as by post-mortem examinations. MRI allows one to identify which bit of the brain is active under different conditions, such as thinking about a particular problem, being under stress or engaging in sexual activity.

One well-supported scientific view is that there are inborn differences between the minds of men and women. But this view is being challenged by scientists who call this the pseudoscience of ‘neurosexism’, and are raising concerns about its implications. They emphasise instead social influences, such as stereotyping, in determining the differences in the behaviour of the two sexes. If, for example, parents tell boys that they have less chance of acquiring good verbal skills than their sisters, and tell girls that they have little prospect of developing skills in maths, this can seriously and negatively affect their acquisition of skills in these subjects.

The evidence for some genetic differences in males and females is, however, overwhelming – just consider our bodies. Yet it has been cogently argued that sex differences in behaviour do not have a genetic basis, instead being socially determined. In this book I shall question the claims made for greater ability of males or females in a number of intellectual and emotional spheres, looking in detail at maths and science, motor abilities, analysis of complex systems, empathy and language skills. Much of the recent literature claims that men and women differ in a number of emotions, linguistic ability, memory, spatial reasoning and even in their sense of humour. If the differences are significant it could mean that men and women are naturally suited to different kinds of work. For example, the small number of women employed in areas related to engineering and technology might be related to their psychological function.

Some commentators see differences to be solely a matter of biology, with no social or cultural interventions, though, as we shall see, social conditioning can shape some biological features. The Oxford English Dictionary defines feminism as ‘Advocacy of equality of the sexes and the establishment of the political, social, and economic rights of the female sex’, and some feminists promote what is called the similarities hypothesis, which contends that the psychological similarities between males and females outweigh the differences. In effect, males and females are more alike than they are different.

All this will be considered in some detail, as will differences in emotional attitudes between men and women and the nature of sexual attraction.

In discussing biological differences I have, with just a few exceptions, left out the enormous literature on other male and female mammals such as chimpanzees. There is some hesitation in accepting its direct relevance to humans and, while I think many studies are relevant, many are not. In addition, I rarely refer to behavioural and brain studies on mice – we like to think we are very distinct from them.

Helen Bradford Thompson made the earliest scientific study of sex differences in cognitive abilities in 1903 at the University of Chicago. Her pioneering work failed to show any differences in the emotional abilities of men and women, and only very small differences in intellectual capabilities, which she attributed to social conditioning. But early brain studies concluded that women were intellectually inferior simply because they had smaller and lighter brains. Since then there has been a great deal of research, but questions still remain. In an interview for the New Scientist to mark his seventieth birthday in January 2012, the theoretical physicist Stephen Hawking admitted he spent most of the day thinking about women. ‘They are’, he said, ‘a complete mystery.’

In discussing male–female differences the term ‘gender’ is not used in the same way as ‘sex’. Gender refers to socially constructed roles and characteristics used to distinguish between males and females in a given society. There is a distinction between biological sex and gender, as sex refers to the biological characteristics, namely the genetic constitution of an individual, whereas gender refers to their personal identity, beliefs and behaviour. Sexologist John Money first made the distinction between the terms almost sixty years ago. The word ‘gender’ grew to be used in relation to how individuals identify themselves. The words ‘male’ and ‘female’ are sex categories, while ‘masculine’ and ‘feminine’ are gender categories, according to the World Health Organization. Differences between men and women, as I have said, are controversial and there are many books on the subject, some of them science-based. But there are also very popular books like John Gray’s Men Are from Mars and Women Are from Venus, and Why Men Don’t Listen and Women Can’t Read Maps by Allan and Barbara Pease. These amusing books focus largely on widely held beliefs about differences in communication and the relationships of couples, and stress the possible innate differences between the minds of men and women. There are, of course, numerous scholarly books on this topic too, as we shall see.

It has been suggested by Deborah Cameron in her book The Myth of Mars and Venus...