You don’t have to believe that men come from Mars and women from Venus to appreciate that there’s a difference between the way that men and women think and act. It’s obvious right from the very start, as any parent knows. Boys and girls are simply different. It’s not simply gender stereotyping to say that boys are more interested in construction toys, cars and rough play as toddlers. Little girls do prefer to play caring, sharing games with dolls and teddies. On the whole they don’t like the rough and tumble that so engages little boys. These differences become, if anything, more marked with age.

Women are much better at judging social situations and are more sensitive to facial expressions than men. They worry about how other people are feeling, putting themselves in their shoes (empath­izing) and are keen to chat and share intimacy. Men are better at working out how things work (systemizing), be it washing machines, computer programs or armies; they prefer games and gadgets. They may start collecting things – football stats, CDs, beer mats – a hobby which is often a complete mystery to women. At the same time the appeal of watching soaps on TV and the concept of shopping for pleasure is mystifying men. Most men will run a mile before they do the ‘talking about problems’ thing.

You can look at gender in several different ways. Being XX or XY makes you female or male, of course, but there’s actually more to it than that. For instance, you could be genetically female, genitally female (i.e. you have a normal vagina), but if the hormones produced by your sex organs (gonads) have been more male than usual, these would influence your brain type and thus your sex-typical behavior.

Or you could be a man, chromosomally and genitally male, who develops female-typical behaviour (and appearance) because of more female hormones in the womb. The mechanism that turns the embryo’s genetic sex into the adult male or female depends on an immensely complex interplay of genetic, psychological, social and, naturally, hormonal factors. The chief hormonal factor is the influ­ence of testosterone in the womb.

There are supposed to be five characteristics of the male brain type: aggression, competition, self-assertion, self-confidence and self-reliance. All these are highly correlated with adult testosterone level, whether in men or women. The characteristics of the female brain type are better skills at language, sensory awareness, memory, social awareness and relationships. Male and female brain types are organized in a different way too. For instance, visual spatial perception is found in the right hemispheres in men but in the right and left hemisphere in women. Women use both hemispheres for vocabulary, men use only their left. This is dramatically demonstrated by the after-effects of stroke.

Women who have sustained damage to their left hemisphere are less likely to have language problems ( disphasia ) than men. Even if they do have some degree of disphasia, they tend to recover language more quickly than men, perhaps because the other hemisphere is taking over some of the functions of the damaged part. In people who have brain damage following accidents, left hemisphere damage is more likely to cause language problems in men, whereas in women, language test scores tend not to be affected by whether the damage is to the left or right.

This is not only about gender. Some men are empathizers — about one in five – and about one in ten women are systemizers. Nor is this about homosexuality – you don’t have to be a gay man to know the lyrics to Judy Garland songs and be able to match upholstery fabrics. This is about brain type. Logically, there should be an extreme female brain type, driven by an excess of oestrogen in the womb. Baron Cohen speculates that such a woman might be an endlessly patient psychotherapist, or alternatively a woman who is completely at sea with anything mechanical and just stands there looking helpless while men fall about wanting to help her.

There are three periods during development when there is a surge of testosterone. Now let’s examine what occurs between the eighth and twenty-fourth week of pregnancy and how it influences gender.

The default gender in the womb is female, which is perhaps hardly surprising given that the womb is an environment awash with female hormones. A genetically male fetus will therefore de­velop the female form of sexual organs until ‘maleness’ is switched on by the SRY gene on the Y chromosome, and the fetal testis starts to develop and then produce testosterone. Incidentally a func­tioning ovary is not needed to appear to be female, whereas a functioning testis is essential for a man.

A clue about the effect of hormones in the womb comes from the Diethylstilboestrol (DES) tragedy. DES is a synthetic oestrogen. From the end of the Second World War on, it was prescribed to pregnant women mainly in the US, in the mistaken belief that it would prevent miscarriage. At the time it was thought that mis­carriages were caused by a drop in oestrogen, although such a drop is now known to be a result of miscarriage, not the cause of it. In fact, DES was a potent carcinogen and twenty years on was dis­covered to cause a rare and sometimes fatal type of vaginal cancer in some young women who had been exposed to it in the womb, as well as infertility in others. It was withdrawn from use in 1971. The cancers affected females but there was also a curious effect on males exposed to DES: they were more likely to show female-typical behaviour such as enacting social themes in their play or caring for dolls.

There are also natural variations in hormone levels which provide clues to the effects in the womb. Boys with a condition called Idio­pathic Hypogonadotrophic Hypogonadism (IHH), in which the testes are very small, are poor at systematizing. Babies born with Con­genital Adrenal Hyperplasia (CAH), a disease that sends their adren­als into hyperdrive, over-produce a type of testosterone. Affected girls have stronger systematizing abilities. Curiously, affected boys are not simply better than other boys at systematizing because of those extra androgens – the rule seems to be that very high or very low androgen levels are bad news and that in the middle is best.

Professor Simon Baron Cohen, a psychologist and Director of the Autism Research Center at Cambridge University, has a special inter­est in male/female brain types, particularly in relation to autism, and wanted to find out whether levels of testosterone in the womb affected behavior. Luckily, the University hospital is a regional center for amniocentesis (a procedure in which fluid is taken from the amniotic sac for analysis) and once the necessary tests had been done, it was their policy to freeze the remainder of the sample. Baron Cohen and his team contacted women whose samples were still in the deep freeze, who now had toddlers and were happy to bring them in for an assessment of their behavior. The researchers found that the higher the levels of prenatal testosterone in the amniotic fluid, the less eye contact the toddlers made, and the smaller their vocabulary. These toddlers were seen again when they were four years old. By this time, those children that had had the highest level of pre­natal testoerone had lower social skills and more restricted interests than those who had had lower levels of testosterone in their bath of amniotic fluid.

Fetal testosterone clearly affects the brain in some way and there­fore influences behavior for the rest of the baby’s life. In a nutshell, the more you have in the womb, the more of a systemizer you are; the less you have, the more of an empathizer you are. While of course baby boys produce more testosterone, baby girls also pro­duce it, some almost to the levels of the least producing boys. Both sexes are subjected to the hormonal environment in the mother, which is yet another source of testosterone (and indeed oestrogen). In addition to the mother’s natural levels of testosterone (which vary from woman to woman), more may come from other sources.

For instance, vigorous exercise during pregnancy raises testoster­one levels, although sustained exercise, such as a long run, lowers testosterone. If a woman is under a great deal of stress during preg­nancy, as a result, for example, of bereavement or war, testosterone levels are increased.

**Norman Geschwind, a legendary US neurologist and teacher, believed that fetal testosterone has an effect on the rate of growth of the two hemispheres of the brain. The right hemisphere is involved in spatial ability, the left in communication. If the right side of the brain develops faster, systemic and spatial skills will be favored; if it’s the left side, it will improve the ability to empath­ize and communicate. One of the effects of a right-side surge, caused by testosterone, is that men are less attuned to words spoken in their left ear than those in their right. This is called the right ear advantage because auditory connections are greatest on the contra-lateral side of the brain. **( See Note on ‘Correction’ )

You might want to rearrange your pillow-talking positions … Part of the evidence for differential hemisphere growth comes from the Wada test, in which the hemispheres of the brain can be put to sleep individually with an anaesthetic. Women fluency with words is reduced no matter which side is ‘dark’. Men verbals only decrease if the left hemi­sphere is asleep.

Another interesting twist are the findings of Doreen Kimura, a neuropsychologist at the University of Western Ontario, who identified a relationship between gender-related behavior and body asymmetry. Men and women with bigger right testicles or breasts tend to exhibit more masculine traits, while those with left-sided dominance show more feminine characteristics. Norman Geschwind also hypothesized that fetal androgens promote growth not just of the right hemisphere, but also of the right side of the body. Indeed, some, but not all, studies find that in men the right foot is larger than the left, and the right testicle is larger than the left one, whereas in women the left foot and left ovary are larger than the right ones.

You don’t have to strip off to notice these asymmetries. Your fingers display them, as does your face. More angular faces are produced by higher levels of fetal testosterone, which promotes the lateral growth of cheekbones and chin (the chiseled masculine jaw which was mentioned earlier). A rounder face, with more prominent lips and higher eyebrows, speaks of lesser levels of testosterone. Facial features are a nightmare to measure in a consistent repro­ducible way but John Manning, an evolutionary biologist who worked mainly at the University of Manchester, has shown that pre­natal testosterone stimulates growth of the fourth finger (look at your left hand, palm up, with your thumb as No. i finger, your fourth is your ring finger). Oestrogen promotes the growth of the second finger (the index finger). A low 20:40 ratio (fourth finger longer than the second) is a marker for high womb testosterone. A high 20:40 ratio (second finger longer than the fourth), may be a marker for a womb environment low in testosterone.

This is fascinating stuff and finger lengths have been used to predict all sorts of things, for instance, greater proclivity towards homosexuality and higher music aptitude (in those with high 2D:4D ratios) as well as differences in fertility, although this seems to stretch the science to its limit (if not well beyond it). There is a very substantial overlap between the sexes with respect to digit ratio, far more so than there is with height, and the ratio varies with geography and race, so one suspects there is more than just fetal testosterone at work here. Nevertheless, digit ratio is clear in, say, girls with congenital adrenal hyperplasia (CAH) – that’s where the girl is exposed to high levels of androgen in the womb and also in those with Asperger’s syndrome (AS), a type of autism. It is to autism, and Simon Baron Cohen, that we now return.

**Correction** ( Credit to Jacob from www.jakestuhh.blogspot.com for alerting us ; Thanks Jacob ! )

“Norman Geschwind, a legendary US neurologist and teacher, believed that fetal testosterone has an effect on the rate of growth of the two hemispheres of the brain. The right hemisphere is involved in spatial ability, the left in communication. One of the effects of a right-side surge, caused by testosterone, is that men are less attuned to words spoken in their left ear than those in their right. This is called the right ear advantage because auditory connections are greatest on the contralateral side of the brain”

This theory is attributed to left handedness and left handed males. Prenatal testosterone promotes dominance in the right side of the brain. The left side of the body is controlled by the right brain. Thus higher prenatal testosterone levels creates a highly developed right brain, thus a left handed male. A left handed male favors the left side of his body, and thus are more attuned to their
left ear than their right.

Also when you speak of communication being favored on the left side, for normal people that is true, but when you are talking about right hemispherically dominated males, the cognitive processes for communication and empathy are more balanced in between both hemispheres.

Geschwind’s theory has nothing to do with the left or the right side of the body being made larger than the other. It only dealt with the right hemisphere being more developed than the left side. Higher prenatal testosterone levels does not promote the growth of the right side of the body. Rather it promotes the usability and sensitivity of the left side.

In a sense, males exposed to normal levels of prenatal testosterone are right handed and systematized.

Males exposed to higher levels are left handed, and subject to cerebral expansions that is not limited to the traditional notions of typical male behavior and processes.