All this talk of deadly chemicals in our atmosphere may have made you wonder what other changes they might cause. Some experts believe our sperm count and quality may have halved over 50 years – matching an increase in testicular cancer.1 Although environmental pollutants have been blamed, there is considerable controversy and conflicting data about the scale of this. In China a recent study showed low sperm counts in bisphenol (BPA) workers in plastic factories.2 Other studies have suggested that our rapidly rising obesity levels are the main cause of lower sperm count. While meta-analyses have not shown a direct effect of obesity, they do show that obesity is somehow associated with lower testosterone levels.
Could researchers be looking in the wrong place at the wrong time? When the mothers are studied, two important factors show a clear influence on lower sperm counts in their sons: obesity and smoking. These effects are actually larger in mothers than when looking directly at the sons.3 In particular maternal smoking seems to have a disruptive effect on the hormone levels in the fetus.4
Most of the evidence in humans is still indirect, but a key point is how low the doses of these toxins and hormones need to be in humans to produce an epigenetic change in the brain. I believe part of our individuality could come from these subtle influences causing changes in our genes and hormones at critical points in our brain development. Quite how subtle these changes are is clearly illustrated in animals. A small chill in the air for a caterpillar causes the female butterfly (like the squinting bush brown variety) to change from a shy demure creature and act as a predatory female. They have brighter wings and actively chase unfortunate males until they are relieved of their sperm and other valuable nutrients.5 In many reptiles and some fish a change of one degree in water temperature (or an ultra-diluted sample of bisphenol chemical) can alter the sex-determining genes and change a male fish into a female.6 Some of these temperature-dependent sex changes can be reversed by addition of hormones or hormone blockers,7 demonstrating the close relationship. A few bees fed royal jelly become super-fertile queen bees and the rest, fed on a weaker brew, become sterile short-lived workers. But other gender-determining signals in humans are even more mysterious – like the fact that the normal ratio of male to female births (currently an excess of around 5 per cent) varies over time – and in the last 50 years has been showing a male deficit in the US and Northern Europe.8
On the first day of life a baby girl reacts differently to a boy in the way she looks at faces or objects, and after only three months, clear toy preferences can be observed of trucks versus dolls. These reactions are also observed in monkeys, so they cannot be due just to exposure to the Barbie doll adverts that have programmed girls to love pink for only the last 50 years. Sex determination and development of ovaries or testicles occurs early in fetal life at around 6–12 weeks, and sets the crucial gender environment for the rest of brain development.9 The default gender program for humans is the female model (unlike Adam – the Garden of Eden version). If during these crucial weeks the gender-determining (SRY) gene on the Y chromosome produces adequate testosterone levels, changes will allow the development of a male. If the testosterone signal is below the threshold, a normal female will continue. After this crucial crossroads the brain starts to develop and organise itself with gender in mind, although many of the gender-specific pathways may not be switched on until the genes are activated by the tidal wave of hormone signals at puberty. Our brains and hormones don’t always work in ways we expect.
Chanelle and Gabrielle Pickett were good-looking identical African-American twins. When they appeared in 1993 on the American Jenny Jones TV chat show they looked attractive and very alike, with straight hair to their shoulders and light pink dresses showing off their good legs and figures. They got a mixed reception when they walked on to the show but were relaxed and articulate. The reason they had been invited – and for the audience’s odd reaction – was that ten years before, they had been cute twin boys.
At the age of around seven, Chanelle and Gabrielle both realised they were very different to other boys. They started dressing up in Mum’s clothes together, stealing make-up and secretly putting on lipstick and eyeliner. At first they did this in secret, then they got bolder, until by the age of 13 they started going out together wearing dresses. One day soon after, their mum found them at a downtown Gay and Lesbian Youth Center and she flipped.
‘We told her we knew we were different and had some problems we needed time to sort out. She was a very different person after that,’ said Chanelle. In fact she refused to have the 14-year-old kids back, and they ended up being fostered in another part of the country. Here in anonymity they dressed as girls permanently and had a great time in the local high school, where they were sociable and well accepted. That was until the headmaster told his staff ‘confidentially’ their secret and word rapidly leaked out to the pupils who had once been their friends. School became a nightmare: they were bullied and harassed. ‘It was horrible – we were suddenly treated like a freak show,’ recalled Gabrielle.
They didn’t stay in school long. They both worked hard with short-term jobs to save up for a male-to-female sex-change operation, where the testicles are removed and the penis inverted to form a vagina. With the extra confidence this gave them and with the help of regular oestrogens to stop hair growth and promote breast tissue, they were hired for better jobs. But trouble was never far behind. Although generally popular, one of them would find themselves being picked on or bullied by a colleague or boss, and they would have to move on. Although they held some part-time jobs, over time they drifted into the underground world of the transsexual sex scene, where straight male punters would pay to have sex with them. Incidentally, many straight men are turned on by transsexuals, and websites with T-girls are the fourth-most-popular Internet sex category.10
One night Chanelle was in one such (TS/TV) club called Playland in Boston, when in walked a regular, a computer programmer called William Palmer. He took Chanelle back to her flat, offered her some cocaine, and tried unsuccessfully to have a threesome with her twin. Although apparently no money was involved, they then went back to his flat. At 5 a.m. Palmer shouted from his bedroom: ‘This is not happening, keep quiet or you’ll wake the whole neighbourhood.’ The shouts turned to screams and pounding on the wall for minutes, which slowly diminished and then suddenly stopped. Palmer’s flatmates were worried and tried to open the door, which was blocked. ‘I’ve got a crazy bitch in here, but I have it under control,’ he replied, and they went back to sleep. The first thing Palmer did in the morning was to phone his lawyer.
Chanelle, who was only 23, never awoke – she had been strangled for eight minutes, her face badly beaten and had bedding stuffed in her mouth. At the trial in 1995 Palmer claimed he hit her once then went to sleep. His slick defence attorney said he hadn’t known that Chanelle was a transsexual and this must have provoked the argument. The jury acquitted him of murder and he was given a two-year sentence for the minor crime of assault and battery. Most commentators agreed that the result would have been very different if she hadn’t been transsexual, he hadn’t been white, and she hadn’t been black. Her sister Gabrielle wasn’t any more fortunate. She too was murdered in mysterious circumstances eight years later – they never found her killer.
What makes transsexuals like the Picketts go to these extremes? They changed their lives, underwent traumatic and costly operations, took dangerous hormones, and risked becoming social outcasts. In the US, there have been over 30,000 operations performed since the first in 1930. This is dwarfed by over 100,000 in an unlikely country – Iran – where the government pays for the operation as a cure for homosexuality.11 Being transsexual describes individuals who identify themselves with a gender different to their biological sex. People who feel like this, but don’t appear in sex-change statistics, are more common than you might think, and their numbers are likely rising. In the past it was thought to involve just 1 in 100,000 biological men, but current estimates are that around one in 5,000 men feel they were born in the wrong body.12 Inversely, female-to-male transsexuals account for about a third of cases. Some of these women have vivid phantom penises from early childhood and some even report phantom erections.
Men can have feelings that their penis is a foreign object that doesn’t belong to them and that they want removed. This may be a way of the brain attempting to resolve inner conflicts between the concepts of gender identity and self-image. During our fetal growth several processes of sexual development occur in parallel: the sexual organs, sexual identity, sexual attraction and sexual body image. Normally they are in tune with each other, but as we see they can often drift down different paths – particularly under the influence of hormones. Any mismatch between them can lead to physical or psychological gender identity problems that can make terms like sex and gender sometimes difficult to define, even for experts. This is exemplified by externally female athletes like the South African runner Caster Semenya, who has three times the normal testosterone levels of a female. She is reported by the Australian press to have external female genitalia and internal testes and no womb. She has been brought up as female and has been passed by the International Athletics Association as female for competitions.
Although the Picketts were identical twins, their case is relatively unusual, and most transsexual twins are discordant for gender identity – suggesting that subtle hormonal or epigenetic changes occur more to one twin than the other.13 There is also some evidence of an overlap between transsexual behaviour and autism/ASD, supporting a role of the ‘masculinised brain’ in at least some female-to-male transsexuals. The link to autism also supports a key role of differences in brain development affecting the areas involved in recognition and self – linked to gender identity, and another link between genes, hormones and the mirror neurones.14
As fetal testosterone or oestrogen cannot be safely or ethically measured directly, we have to look at non-invasive indirect methods of assessing it. Our adult bodies retain a trace like a fossil of the chemical and hormones we were once exposed to in the womb. Take a look at your right hand flat in front of you. Ignoring the thumb, you can see that the four fingers have different relative heights. Focus on the difference in length between the second digit (index) and the fourth (ring) finger. If you are female your second should on average be equal to or slightly longer than your fourth finger. If you are male your second digit is usually shorter than your fourth digit. Differences in this ratio could be a marker of your personality, aggression, criminality, sexuality and sporting prowess. These relative measures are known as 2D:4D digit ratios and reflect the relative levels of the male and female sex hormones in the womb. Broadly speaking – although there is much individual variation – the lower the 2D:4D ratio, the more of a traditional male pattern, and the greater the ratio (i.e. longer index finger), the more of a feminine pattern is represented.15
Quite why our fingers vary in length according to gender and sex hormones is unclear, as having a long ring digit as a male or a long index as a female doesn’t seem to confer any particular physical advantage that I can identify. But they do seem to be rough guides to how differently our bodies may have developed – in particular the brain and maybe the genitalia. Other Sherlock Holmes-type clues as to how androgenised or not you are include having a hairy ring finger, an early age of puberty, the ability to hear fine sounds from the inner ear (oto-acoustic emissions), and something you may not have measured recently: the distance between your anus and your urethra or penis.16 Variations around these measures are associated in rodents and humans with different sex-related traits such as fertility and sexual preferences.17 These changes occur early on in our development, shortly after the sex-determining genes are turned on, at around 12 weeks gestation. This is a time when this process is very sensitive to any changes and when natural and non-natural factors play a role epigenetically.
John Manning of Swansea University was the first to speculate on this magic finger ratio in 1998,18 although it had been noticed as an oddity since the nineteenth century.19 Nowadays it is nearly a medical speciality in its own right, with over 500 published papers on the area.20 Some of the most consistent studies showed that the smaller the ratio in male or females, the higher their level of sporting achievement.21 Other studies, not as consistently, have shown that self-reported masculine traits (deep voice, upper body strength) are also associated with testosterone levels, as well as with tendencies towards crime, facial attractiveness to women,22 and skills at computer programming – suggesting they are all under some sex-hormone control.
Although the size of the penis has in folklore and jokes often been associated with the size of the hands and feet, these chestnuts have been discredited. However, a recent study of Asian fingers has shown a reasonable correlation between having a more masculine 2D:4D ratio and a larger penis. The clever Koreans did this by identifying patients due for prostate surgery: when anaesthetised the diligent researchers stretched the flaccid organ (of the volunteers) and measured it, comparing it with the finger ratio. The stretched organ apparently gives a better indication of the erect size than the floppy one. While the press presented this as a major breakthrough – and made a handshake with Asians a potential party trick – the stats behind the study showed that as the correlation was quite weak, for any given individual its prediction was pretty useless (explaining less than 5 per cent of the variability in member size) and likely to lead to disappointment.23
A couple of genetic studies have shown that genes are at least partly responsible for variations in the magic ratio and possibly indirectly fetal testosterone. Our own twin research as part of a large collaborative study showed (as usual) a heritable component.24 A key gene associated with puberty development was convincingly associated with the ratio, but unfortunately in the opposite direction to that predicted.25 They did not find any other genes, but the study was relatively small and certainly couldn’t rule out the effect of genes on the testosterone pathway.
Other relics of our past fetal androgens have not been studied as much, but researchers in Canada painstakingly counted the hairs on the back of the middle section of the ring finger in women.26 They found that having more hairs than the average seven (and a more masculinised profile) protected women from some of the hormonal side effects of the oral contraceptive pill. For strange reasons, if you look at the same middle section of your index finger you will find many fewer hairs (assuming you don’t shave or pluck your hands). In fact, the average was less than one hair. This bizarre biological quirk suggests a straightforward relationship between circulating testosterone and how masculine a male will be (in terms of sexuality and behaviour and body size) – with finger ratios and hairy fingers a marker of this. Yet, as usual, relationships can be deceptive.
Male homosexuals are on average less keen on participating in and less gifted at some sports than heterosexual males.27 Also, aggressive male criminals have been shown on average to have larger penises, so male homosexuals might be expected in theory to have smaller genitalia than heterosexuals. However, studies actually show the opposite. Gays have been reported to be actually better endowed on average than heterosexuals.28 This (like most penis surveys) is usually based on self-report and so related to (and biased by) body image and confidence, although it could be argued that homosexuals may be more accurate, as they have considerably more experience in comparing themselves than heterosexuals. Homosexuals also have a strong male-type sex drive, although directed at other males. Lesbians in contrast (but not bisexual females) tend to have more typical female sex drives despite having longer ring fingers and being better at sports. So changes in testosterone levels probably have different and sometimes opposing effects on the developing brain, depending on the absolute levels, relative ratio to oestrogens, and also the precise timing and duration of the changes.
And let’s not forget the genetic background. While most agree that it has a role, the evidence for fetal testosterone accounting for most of the sexuality differences we are discussing is too simplistic and overstated.29 The lesson from biology is that the real explanation is nearly always much more complex.
Nigel knew he was different when he recalled at about the age of 13 ‘not feeling the same about girls as my friends did. I started to have erotic dreams about men and realised in retrospect that I never had any dreams about girls.’ No one in his family was gay, he found these feelings frustrating, and he became an angry teenager for the next three years. At a party, aged 16, he went for a walk with a girl and kissed her but remembers ‘it did nothing for me’. At age 17 in his last year at school he was teased by other kids for having an ‘inseparable friend – Simon’ and was jokingly called his boyfriend. They laughed it off, but Simon had actually been with other boys. Secretly the two got to know each other intimately and were soon engaging in masturbation and oral sex, although Simon also had a girlfriend he was cheating on. Nick realised at this point that he was definitely gay. Soon after he left school and home to be a chef, he entered the gay community and didn’t look back. He told his family soon after, including his twin brother Mark, who remembered: ‘I wasn’t that shocked to be honest. I suspected it at his last year at school – but not before.’
Mark is a nurse and like his brother hated sports – which in New Zealand was not the popular choice. They both preferred arts and music. ‘I was a late starter with girls and waited until my twenties to have sex and a serious girlfriend. I’ve never had any dreams about men, but I like and prefer the company of girls and gay men. Nigel often used to take me with him on the gay scene and I used to get offers.’ One night when Mark was 22, a particularly good-looking and persistent man chatted him up at the bar for hours and bought three bottles of champagne. Eventually he agreed to go back to his place, where they kissed and got undressed. ‘Suddenly naked in bed with this man I realised I wasn’t interested. I got up, apologised and left. I realised Nigel and I are very similar in nearly all respects, but in choice of partners I just wasn’t like my brother’. He has now been happily married for five years and has a daughter.
Nigel is 8 kg lighter than his brother, he runs several pubs and has also had a steady partner for the last five years. Their parents are relaxed about him being gay. Their older brother is straight and their mum comes from a large family with four other sisters and 11 other cousins but – as far as they know – no other gay kids. Neither twin is at all interested in sport or does any exercise. They have similar digit ratios close to 1.0 (the index and ring finger being closer in size) – higher than most men.
A similar story comes from Daniel and Thomas, retired actors born in a different era, the 1940s, who are also identical twins. They were pretty, handsome boys with very similar personalities, preferring arts to sport, and both matured late in life. Tom was always more interested in girls, while Daniel was more reticent and felt less comfortable at school dances, although the girls seemed to sense his relaxed, indifferent attitude and he was popular.
Tom was the first to have a sexual encounter, a fumble in the back seat of a Hillman Hunter with a 16-year-old would-be seductress. He recalls: ‘It was an embarrassing incident I tried hard to forget. I was very anxious and excited and came far too early and it was all over in a moment. I don’t think the girl was very happy.’ At different universities, they both decided independently that they should properly lose their virginity. Daniel duly found an obliging girl at a freshers’ week party, but it ended in disaster, with much fumbling, anxiety, embarrassment and a lack of an erection.
Tom, who was not put off despite his previous back-seat disaster, found a friendly girl, consummated, fell hopelessly in love and didn’t look back. Daniel, not keen to repeat his experience, soon met a boy and decided that it was boys that he was aroused by. Sex followed and he too fell in love. ‘After that I never really considered women sexually again’. Tom did have some nagging doubts. ‘Once aged twenty-two I went home with a good-looking gay man after a drunken actors’ party and ended up in his bed. In the 1960s it seemed like everyone was trying it. I was very drunk and to be honest not sure if penetration actually took place. But I then realised that sex with men was not for me and I definitely much preferred women.’ Both twins are popular with both gay men and women. They agree that if anything, Tom has a more feminine side to him than Daniel, but neither has since been tempted to swap places.
Human sexual behaviour falls within a wide spectrum from complete heterosexuality to complete homosexuality, with all possible combinations in between, varying in frequency and throughout life. A joke goes in the gay community: ‘What’s the difference between a straight man and a gay man? – Six pints of beer.’ The truth is that for most men it will usually take more than that. The frequency of self-declared homosexuality – defined as some adult sexual contact with a same-sex individual – is around 5 per cent in the UK for males and 2 per cent for females.30 Rates are reportedly higher in the US and in France, and rise to 16 and 17 per cent if you include any instance of sexual attraction. This appears true across all human populations and across any time studied.
What is interesting about our male twin cases and others I spoke to is the lack of bisexuality in the twins where one was gay. The straight twin in both cases did try switching once – probably consciously or subconsciously to ascertain if they might have some hidden gay tendencies. However, their single gay experience ended up reaffirming their original belief that unlike their twin they were straight. This is despite being very close to their twin in personality and other tastes, which suggests very specific differences in the brain. Most men, once they have had a gay partner or partners for a while, don’t revert to preferring women. The number of self-reported bisexual men equally turned on by men and women is actually rarer than we think – although they certainly do exist.31
Sexual preference is in part genetic, as shown by several twin-and family-derived heritability estimates ranging from 30 to 50 per cent.32 Twenty years ago Dean Hamer at the NIH claimed to have found ‘the gay gene’.33 Yet the discovery, although plausibly on the female X chromosome, disappointingly couldn’t be replicated.34 Indeed, as for nearly all complex human characteristics, there is not going to be a single defining ‘gay gene’, but hundreds of gene variants of small effects influencing susceptibility. Although many homosexuals marry (around 50 per cent in some studies), the average number of kids they conceive is much less than heterosexuals. So according to Darwin these genes should have died out long ago – unless they also have some hidden advantages.
One theory proposed initially by Simon LeVay is that the genes in females from the same gay families increase fertility and fecundity to compensate for the losses in the gay men.35 There is growing evidence that the wider families of gay men tend to be larger – particularly on the female side.36 The family’s gay genes in the women could increase the strength of their attraction and lust for men (androphilia), giving them more sexual partners. These ‘gay aunts’ may also have an increased level of fertility, by also having favourable gynaecologic and obstetric factors.
Another theory that has been proposed to explain homosexuality is the ‘immunisation of the placenta theory’. Gay men tend on average to come from larger families and often have older brothers. Studies have suggested a doubling of chances of being gay if you have at least two older brothers. The idea was that the first and second brothers’ testosterone would interact with the immunology of the mothers via the placenta, so that by the third son, antibodies to either testosterone or the male-determining gene or protein had been formed.37 A similar relationship has been shown with birth order in male-to-female transsexuals.38
I find this idea a bit far-fetched, and apart from the fact that gay men do tend to have older brothers more often than not39 there is no hard evidence for any immunological differences.40 An alternative (and speculative) explanation might be that in families with an over-representation of males, lacking X chromosomes, subtle differences might occur in epigenetic reprogramming of the germ cells so that they retain a trace of some testosterone-altering genes or signals. The same process might explain why some families for many generations can produce only sons or only daughters. There is some supporting evidence for this in studies of rodents.
In many cultures extreme environments can produce temporary changes in sexuality in most people. A good example is men and women in prisons, where some studies suggest that up to 50 per cent of males participate in homosexual activities and 20 per cent report abuse or coercion,41 although most would not call themselves homosexual – just adapting to circumstances.
In traditionally isolated and religious cultures like Afghanistan, where female sexual repression is the norm, dancing boys (aged ten to 14) are commonly employed for male parties to perform various sexual acts enjoyed by up to half of Pashtun men. This is considered the social norm and not a ‘gay’ activity, with these unfortunate children seen as available female substitutes. In an unusual boy scout custom the Sambia tribe in an isolated part of the highlands of New Guinea require the boys aged ten to 12 to every week fellate the older boys aged 14 to 16 in the belief that it is good for both groups. It ‘apparently’ helps the older boys prepare for manhood and having sex with women. Swallowing the semen also ‘helps’ the younger boys grow into men and become stronger.42 The Sambia men when older seem normal and are attracted to and marry women. They have reported adult homosexual rates of around 5 per cent – around the same as most other populations.
These unusual groups, and the identical twin pairs I interviewed who shared very similar lifestyles, suggest that the environment while growing up is not critical in permanently changing gender preference. However, the growing brain while in early development is certainly very sensitive to subtle changes, and in different individuals even with the same genes can produce different amounts of masculinity, aggression and sexual preferences, which are not always related.
As we saw from our case histories and confirmed by the larger twin studies, most twins (around 70 per cent) with identical genes do not develop the same homosexual preferences as each other. This shows that genes alone are not enough. There are rare stories of men who become homosexual after strokes or others who change sexual identity several times in a day – a state known as alternating gender incongruity or AGI. These rare examples show the potential plasticity of sexuality.43 In conversations with gay men about ongoing research into discovering gay genes – now inevitable within a few years – many expressed concern. They were worried that the excuse of genetic causation would be used by the religious right to screen babies and try to convert them by prayer or hormones, or would lead as in India or China to abortions or infanticide of females.
But our studies suggest that even identifying all the possible gene variants will not itself permit the accurate prediction of homosexuality in an individual. So far, religious gay conversion programmes (common in the US and in countries like Malaysia, where ‘sissy camps’ are compulsory) with bonding, wrestling, prayers and group therapy at $500 per day have not ascertainably succeeded.44 Once the brain has developed, it seems unlikely that any chemical or hormones acting epigenetically would be effective in childhood. So these fears seem for the moment unfounded. For males, it appears that in most cases early life experiences are not crucial to later preferences, and that once a choice for homosexuality has been made, males rarely revert to preferring females.
Women are quite different though. The fluctuating, fluid sexuality of bisexual women over a lifetime is well documented, showing that female sexual identity is not the rigid construct we imagine.45 In contrast to the treatment of gay men, societies seem more tolerant: there are few ‘butch conversion camps’ for females. Gay genes in females are perhaps just genes for flexibility or plasticity that in some circumstances or environments can lead to being gay or bisexual, and in others to being straight – but all in the same life.
Louise was seen as a bit backward by her family. She was painfully shy and more of a loner than her identical twin sister Barbara. She was inclined to be moody and sometimes depressed. ‘I lived in a bit of a dream-world I suppose. I always thought my sister was more attractive and successful than me. She was really into boys at age fifteen, so I reluctantly went out with her and other boys because of loyalty and social pressure.’
Louise’s first sexual intercourse with a boy occurred when she was 18. ‘It was OK, I suppose, but nothing special. Although I was a bit of a tomboy, I never really thought that I was gay, but definitely noticed something about me was different. Although I dated and had sex with about ten to twelve boys, it just never felt really right.’ She had her first lesbian relationship when she was 27 and ‘At last it did feel right.’ She has since had another ten female sexual partners and had her longest relationship – 12 years – with a woman. For the last four years she has lived with an older female partner who used to be married and has grown-up kids.
Her sister Barbara remembers that by the age of 11 ‘Louise had already started to dress in a more masculine way. By the age of fifteen when I was interested in boys my sister would find excuses to stay home and study. I felt she never respected men and this caused a rift between us. Once, because I know we have the same genes, I was worried whether I too was gay, but when one of Louise’s girlfriends tried to sort of seduce me, I realised I wasn’t at all interested.’
Barbara has had a total of five sexual partners, has been married with three kids for over 20 years and has always been faithful. Louise, now aged 55, does still sometimes fantasise sexually (and dream) about men. ‘It is odd that I do this. These usually involve submissive/dominant situations, like him tying me down or forcing me to have oral sex.’ Last time she slept with a man was about six years ago: ‘I blame the alcohol – but it was quite fun.’ Her ring finger is slightly longer than her index and longer than her sister’s, and compared with her sister she was always a bit better at sports.
This story is not untypical of a lesbian woman, who will often have had sex with equal numbers of male and female partners and switch back and forth over a lifetime.46 Only a small minority remain solely homosexual throughout life.47 In our recent study of 4,400 UK twins, female homosexuality was around 25–30 per cent heritable,48 which was slightly lower than for males and similar to the 19 per cent found in a large Swedish study.49 This suggests a greater relative role of chance or the environment. For lesbian or bisexual women – although research is sparse – no clear environmental components have been consistently identified. The key differentiating factor is that in females as opposed to men there is much more lifelong flexibility and plasticity. The genes for this trait could be providing a slight relative advantage by aiding greater sporting prowess, more masculine thought patterns and the option of finding both males and females potentially attractive – which improves social networking. When women perceive that they are gender-atypical, when mild it may lead to having more sexual partners and offspring, and when severe – which is rarer – it may lead to being a lesbian with no offspring. The twin studies show us that simply having identical genes is not enough to make someone gay. Other subtle environmental differences in hormones or chemicals are needed, acting on our brain neurodevelopment – again probably through epigenetics.
Dean Hamer and his team looked some time ago at differences in mothers of gay men to see which of their two X chromosomes was inactivated (by imprinting).50 This process is usually random, but they found a clear pattern, not found in straight men’s mothers. This suggested that some sets of genes on the X chromosomes were being altered epigenetically – although we don’t know which as yet. Simple yeast also have their equivalent of male and female forms turned on by epigenetic signals, suggesting that this is a common mechanism for most animals.51
In the last chapter we discussed chemical hormone disruptors like the fungicide vinclozolin. Taking this can also alter an individual’s sexual attraction to others – at least in rats. Males whose great-grandparents were originally treated with vinclozolin, which subtly affects male sex hormones (androgens), were tested for their sex appeal by Mike Skinner’s lab. Even after three generations of normal chemical-free upbringing, these males were less sexy to females, who consequently mated with them less often.52 Perhaps this is the epigenetic equivalent of an unpleasant but permanent aftershave. Other studies of rats whose mothers were fed bisphenol found this slightly reduced the males’ sexual performance and in contrast made the treated females more sexually active.53
While this is as yet unproven, it is possible that epigenetic changes related to our environment – whether from stress, cigarettes, diet or even drinking expensive mineral water from plastic bottles – could influence the sexual preferences of our kids and grandchildren. While male and female sexuality is mainly determined in the womb, it seems clear that it is very difficult to modify male sexuality or orientation after puberty, whereas females seem amenable to small changes all their lives. Let us now explore what drives individual variations in heterosexual sexual behaviour – lust and fidelity.