Sexual orientation may be determined at 6th week of foetal life

Most people, when they begin to accept that homosexuality has biological origins, tend to equate this with genetic causes. There is some evidence that this is so, but there is also evidence that hormones play a big part too.

I have just finished reading Brain Sex, by Anne Moir and David Jessel. It was published in 1992 and was a bestseller in the States, though I've only just read it. But it was worth it, and I'd like to give a synopsis of the key idea presented there about the hormonal origins of homosexuality. 

Boys and girls are different because their brains are different

The overall thesis of the book is that there are significant differences in the structure and operation of the brain between men and women. These differences manifest in the all-too-well-observed differences in behaviour between the sexes. A considerable amount of scientific research has recently accumulated, demonstrating that these brain structure differences are due to the action of hormones during the early development of the brain in a foetus.

The book points out that much of the current idea that boys and girls are socialised to be different, with men the "favoured" sex and women the "disadvantaged" one, is largely politically-correct mumbo jumbo. Just as differences in physique between males and females are due to biological reasons, so are differences in personality, interests and behaviour. They are not just brought up to fight like men or cry like women. The roots of the differences can be traced to hormonal effects on the brain and its organisation.

This in itself is a very interesting subject, and as I have presented it quite baldly in the preceding two paragraphs, there will no doubt be considerable resistance to the idea that women are differently hard-wired from men. If you wish to get into more detail about this subject, do read the book..

Before going on, it is necessary for me to cover a bit more on this before I go on to homosexuality, otherwise the next part of this essay is unintelligible.

The first counter-intuitive research finding that is worth noting is that men and women use different parts of their brains even when doing the same things, such as speaking, thinking algebraically, concentrating on an electrical problem, or helping someone in grief. In addition, male brains and female brains seem to have different specialisations, such that in certain things, women do better, and in other things, men perform more efficiently.

Examples include the fact that women are better at verbal and emotional skills, but men are better at the mathematical and the visual-spatial. Generally, females (including baby girls) are more people-oriented and males (from infants onwards) are more object oriented. Some people think that if we give baby girls trucks to play with and baby boys dolls, then we will create a more equal world. The evidence is that it's not the parents' fault. It's the damn kid who influences the choice of toys. The girls tend to choose the dolls (people-oriented interests) while the boys prefer the trucks (object-oriented interests). The parents are merely subconsciously registering the kids' preferences and catering to them.

As they grow a little older, other differences become apparent. The boys (and men) are more aggressive and competitive and they tend to create friendship circles with some sort of hierarchy and a sense of exclusiveness. The girls are more inclusive and more egalitarian among them.

After puberty, men behave differently from women when it comes to sex and love. They are less romantic than women. They are far more promiscuous, while women tend to seek more fulfilling relationships. Remember, people-oriented versus object-oriented? The world over, porn (objectified sex) is almost always a male thing. Pulp romance (imaginary lover) a female thing. And it's said: among men, sex sometimes results in intimacy; among women, intimacy sometimes results in sex.

As mentioned above, these differences in personality, behaviour and interests (the details of which are quite clearly presented in the book) can be traced to demonstrable differences in brain structure and function. But what causes the brain differences? 

Male and female brains develop differently in foetal life

Around the sixth week of a developing embryo, the chromosomes (XY in the boys, XX in girls) instruct some cells to differentiate into ovaries or testicles. The male embryo's testicles then begin producing male hormones, the chief one being testosterone. These hormones in turn affect the developing brain. The male foetus' neural network is then laid down in a typically masculine pattern.

In a female foetus, the developing ovaries produce no appreciable levels of male hormones. The brain development in the foetus is not affected, and this results in a female brain.

How did they show this? Primarily with rats. Rat embryos are born very early in life, even before the brain has begun to form up. Researchers are therefore able to test their theories. If they take a male baby rat and castrate him (thereby depriving the rat of testosterone) before the brain has begun to go down the male pattern of development, they get an adult rat that behaves very much like a female. It is much less aggressive than its male companions. It grooms and licks other rats like a good mother and is more sociable. In other words, a female brain in a male rat body.

The later the rat is castrated, the less feminine the rat turns out to be. The brain has more time to be exposed to the male hormones and become masculinised.

Researchers also castrated some rats very early, but then added replacement testosterone by injection. When they did that, the rat brain still became masculine in structure and in later behaviour. With one proviso. Once the critical period was past without replacement male hormone injections, and the brain had developed along female lines, no amount of male-hormone replacement therapy masculinised the brain again.

Likewise with female rats. They do not produce significant amounts of male hormones from their developing ovaries, and left to itself, the brain organises its structure in the female pattern. But when researchers injected testosterone into these female rats at critical periods, they could create a male brain in a female body. The rat in adulthood acted very male. It was more aggressive, it attempted to mate in the male manner, and so on.

There are a quite a few examples of people who, because of unusual chromosomes or prescriptions taken by their mothers during pregnancy, had abnormally high or low levels of male hormones during their foetal life. There are cases of girls who had absolutely no testosterone (most girls have some testosterone, albeit at a low level), and they turned out to be exaggeratedly feminine, hopeless with math, very housewifey and emotional, for example. There are girls with higher than normal exposure to testosterone while in the womb, or boys with too little or too much testosterone at around six weeks ... the psychological profiles of these children or adults matched what was expected: unusually feminine behaviour and interests, or excessively masculine tendencies and interests.

The bottom line is this: at a critical stage in a developing foetus, high or low levels of male hormones form a male- or female-patterned brain respectively. The person eventually thinks and behaves male or female accordingly.

 
Sexual behaviour remains latent, showing up much later

Some kinds of male or female behaviour do not show up in childhood. Sexual behaviour is the chief one. It takes a second surge of testosterone in boys, at puberty, to activate these parts of the brain. In girls, the surge of oestrogen and progesterone, similarly activates their brains.

What if the opposite hormone is given at puberty? Does it change the structure of the brains to that of the opposite sex? No. The male brain is largely unresponsive to female hormones. Nothing much happens. But it is very sensitive to the levels of testosterone, producing very aggressive, even violent criminal behaviour, if too high. The female brain on the other hand largely ignores the male hormones. Brain structure was fixed in the foetus. Pubertal hormones merely switch them on. 

Brain masculinisation can be subdivided into stages

Chapter Eight of Brain Sex went into further detail about homosexuality. Dr Gunther Dorner, of former East Germany, extended the above research to the development of sexuality in the brain. His theory is that if we look closely at what happens at the six-week stage of the foetus, it can be subdivided. The brain is not masculinised at one go. At slightly different times, different parts of the brain are affected, giving rise to slightly different aspects of gender. He named three stages:

1. the masculinisation of the "sex centres" where the typical physical characteristics are laid down;

2. the masculinisation of the "mating centres" (specifically the hypothalamus) which control sexual behaviour;

3. the masculinisation of the "gender-role centres", which control general behaviour such as aggression and sociability.

A refinement of this theory comes from the American, Dr Milton Diamond, who believes there are 4 stages:

1. basic sexual patterning wherein the general behaviour we associate with males or females, e.g aggression, promiscuity, or lack thereof, are organised in the brain;

2. sexual identity -- what sex people ascribe to themselves (whether they consider themselves male or female);

3. sexual object choice -- the sex of the desired partner (this is equivalent to Dorner's mating centre); and

4. control centres for the sexual anatomy, including mechanisms of orgasm.

[Though listing them as stages 1 to 4, I don't intend to mean they occur in that particular order.]

If for some reason, the male hormone levels in the male foetus is lower than usual during a specific stage of the brain's masculinisation process, then that particular aspect may not be fully masculinised, without affecting the normal development of other aspects of sexuality. 

Object of attraction is fixed during a window period

For example, taking Dr Diamond's 4-stage scheme, if something odd happens during the window period for sexual object choice, and male hormone levels in the foetus does not reach as high as it should, then the male foetus' brain may remain "feminine" in that respect, with sexual attraction in adult life directed towards other males, that is, homosexual. But in all other respects, such as identifying himself as male, and showing competitive, object-oriented or promiscuous behaviour, being good at math and visual-spatial skills, he would be no different from heterosexual males. When he grows up, he would be what is often called a "straight gay". By the way, this is far and away the most common type of gay men, in case you don't know.

Another male foetus might be affected at two different stages. One might cause effeminate behaviour, and the other might cause sexual object choice to be male. This would make an effeminate homosexual male. It takes two errors in hormone levels to make this happen. Perhaps that is why it is less common to see effeminate gay men compared to "straight gays".

You could also have a situation where the person is effeminate, but clearly heterosexual. Behaviour/mannerisms and sexual object choice are different things.

If, in another example, male hormone levels fell short during Dr Diamond's stage 2, then the male foetus' brain would pattern itself to identify as female. This person, despite having male anatomy, would see himself as female. This is a classic syndrome of transgendered people. Depending on what then happens at stage 3, this person may be attracted to males or females.

For females, the normal situation should be low levels of male hormones during the critical period of brain development. The brain then proceeds to pattern itself on the female basis. But if male hormones should be produced by the baby, or mother, or comes in through medication, then atypical situations may arise, and depending on which stage of brain formation was happening, you might get a tomboyish but heterosexual female, a tomboyish homosexual female ("butch" lesbian), or a "fem" lesbian, and so on.

The theory neatly explains the various combinations of sexuality as observed in real life. Scientists have more confidence when a theory is elegant, in that a simple explanation covers many different observations, and this is certainly one.

You can also guess how this theory can explain bisexuality, through moderate levels of testosterone at stage 3.

There is one more thing the theory can explain. While there is a lot of debate about the percentages of homosexual people in the population (and the uncertainty despite years of surveys is mostly due to different survey techniques and definitions used), one feature from almost all surveys seem to recur. Whatever number the male count turns out to be, the lesbian count turns out significantly fewer.

How does the theory explain this? The work on hormones and the embryological brain shows that the natural pattern of the brain is a female one. In computer lingo, the female brain is the "default". It takes an event -- a surge of male hormones up to certain levels in the six-week-old foetus -- to produce a male brain. Errors are therefore more likely to happen with male foetuses than with female foetuses. The surge may not happen, or it happen but slightly off-timed, or it doesn't quite surge to a high enough level on certain days .... Hence more gender variety among males than females. 

Why hormonal levels vary still not known

Now we come to the question: what may cause errors in the levels of male hormones in the six-week-old foetus? No definitive answers yet. Medication taken by the pregnant mother has been identified in a number of cases. Some prescriptions have side effects which were not known at the time. Some raise testosterone levels, others suppress it.

Stress suffered by the mother is a factor that many researchers are now focussing on. It upsets her hormonal balance, with effects on the foetus.

The foetus itself, who has to (or should not, in the case of female foetuses) produce his/her own male hormones, may have some abnormality in its developing testes or ovaries, producing too much or too little of male hormones at critical times.

And finally, these ideas lead back to genetics. Are some people (mother or child) prone to hormonal upsets? I wrote about the evidence from genetics in the article Brothers and Twins.

© Yawning Bread 

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