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For every 104 male babies, we have 100 female babies and there are statistics which are more reliable. (Those tell us that we have 997 female infants born per 1000 male infants born.) What is the reason for that? Why do we have more male babies than female babies?
Note: This is based on literature searches I've done a while ago out of general curiosity. I'm in no way an expert on human reproduction.
First, I'm not sure if you are asking about evolutionary reasons or the developmental causes for for a difference in sex ratio. Here, I will focus on the developmental causes.
There is much evidence for a male bias in human sex ratio at birth (secondary sex ratio), and the ratio have been shown to change over time, e.g. as in 20th centrury Europe after WWI and WWII (see data in Gellatly, 2009),in relation to environmental or parental factors (see e.g. Jacobsen et al, 1999), or between ethnic groups.
A bias in sex ratio can be due to (at least) three different mechanisms (Irving et al (1999)):
- deviations in X/Y-ratio of sperm
- selection of sperm within the female reproductive tract
- biases in implantation success and/or survival for embryos of different sex
I guess that success of fertilization/zygote formation could maybe be considered a separate step (between 2-3). From what I've seen, there is little evidence for explanation one, see tests in e.g. Irving et al (1999) and Graffelman et al (1999). Graffelman et al (1999) also tests for an effect of male age, but do not find support for this idea.
A problem is that it is extremely difficult to study these factors under natural conditions (practically and ethically). Interestingely, there is also evidence for an excess loss of male fetuses in recognized pregnancies (Ingmarsson, 2003, Boklage, 2005), and this would indicate that the sex ratio close to conception is even more skewed towards males than the sex ratio at birth. This is even more curious with the background of an unbiased sex ratio in sperm (explanation 1 above). However, it is very problematic to study what happens in the first part of pregnancies, since a number of weeks have usually passed before the pregnancy is even recognized. I haven't found any papers reporting the human primary sex ratio (sex ratio at conception), which could be affected by factor 1 & 2 above.
Even so, the most likely explanation seems to be a strong bias during embryogenesis, as mentioned in Boklage (2005):
The usual excess of males is present through pregnancy beyond clinical recognition, but not present at fertilization. The difference, therefore, must arise between fertilization and clinical recognition, through a preferential loss of females during embryogenesis.
The same paper also points out that fewer than 25% of natural human fertilizations go full term (probably even less), and 2/3 of these fail before the pregnancies have been clinically recognized. By back-calculating from the secondary sex ratio and the excess of failed male pregnancies, the sex ratio close to conception has been estimated to 125-135 male:100 female (Pergament et al, 2002))
The exact causes for such a bias during is not entirely known from what I've seen. However, Boklage (2005) mentions a couple of different explanations. For instance, among mammals in general, the earliest stages of embryogenesis proceeds more rapidly in male embryos:
Since some of the products of these earliest stages are signals from one part of the embryo to another, or from the embryo to the maternal physiology, signals which are required for continuation and maintenance of the pregnancy, the establishment of viable, clinically recognizable pregnancy is correspondingly more efficient in general for male embryos (Krackow, 1995; Kochhar et al., 2003).
The same issue is also mentioned in Ingvarsson (2003):
Studies in recent years indicate that sex differentiation begins at conception. The SRY gene on the Y-chromosome is already transcribed at the 2-cell stage and triggers growth acceleration in the XY embryos. This accelerated growth is believed to be important for the male embryo as it allows complete testicular differentiation before the levels of oestrogenic hormones become too high as pregnancy progresses.
Boklage (2005) also mentions genomic imprinting and chromosome X-Y segregation as possible explanations for a bias (see references in paper for more).
So, in summary; there is evidence for an male bias in secondary sex ratio (sex ratio at birth), even though there is usually a excess of failed male fetuses in recognized pregnancies, which means that the male bias must be even higher very early in pregnancies. There is some evidence that this might be due to a higher success of embryogenesis of male fetuses, but the evidence isn't strong and the issue is difficult to study.
The evidence provided by these results is indirect. That is unlikely to change. Ethical, technical and financial considerations argue against the destruction for karyotyping of statistically sufficient numbers of products of natural human fertilizations. Granted, any bias of the sort we seek to discover and understand might, after all, operate differently, or not at all, in sham fertilizations of hamster oocytes, versus oocytes from artificially induced human ovulations, versus naturally fertilized human oocytes. The best evidence we have, or are likely to have in the foreseeable future, indicates that the consistent excess of males observed in human births does not originate in a consistent bias in spermatogenesis or in fertilization. (Boklage, 2005)
Finally, it should also be mentioned that the bias in sex ratio at birth can be partially due to sex-selective abortion and infanticide, but to what extent is not entirely known (see Wikipedia: Sex-selective abortion).
You might also find this related question on the heritability of sex ratio interesting: The gender of offspring of Twins?.
There is an article in the Journal of Popular Science from 1885. However, I do know there is at least one more recent article floating around some where since I read it. At the moment, I can't find it but will update if or when I do.
The article goes on to state that during times of scarcity the number of male births outweighs the number of females whereas in times of plenty, the number of female birth outweighs the number of males. I will let you read linked article but give you the information I recall from the more recent article I can't locate at the moment.
The reason for the differences in births has to do with the lifestyles men and women lead. Men tend to do more dangerous activities at younger ages which causes unnatural deaths early on. Due to these unnatural deaths, in times of scarcity, the reproducing population converges to 1:1 (not exactly) male to females due to the lifestyle choices of men. It was also hypothesized that women are almost guaranteed to find a mate so having less females during rough times wont hurt the population since they should all find a mate.
Additionally, the phenomenon of environmental factors influencing male and female births is well documented in alligators. The first linked article also goes on to examine this occurring with other species as well.
Thanks to @Anne, who sent me a discover article, we have the name of two sets of researchers on this topic. Here is an exert from the article Famine Can Tilt the Sex Ratio of Future Generations. But Why? (Discover, Nov. 2013). Additionally, we can look up the articles by the respected researchers now. I believe the research by Robert Trivers and mathematician Dan Willard will be the most applicable.
While demographers were struggling to understand sex ratio anomalies in the context of culture, evolutionary biologists had largely embraced an idea put forth in 1973 by biologist Robert Trivers and mathematician Dan Willard. The Harvard-based pair theorized that as the physical condition of a female declines - if she's nutritionally deprived, for example - she'll tend to produce a lower ratio of male to female offspring. Evidence of the theory came from red deer and humans; in both species, adverse conditions in the mother's environment during pregnancy are correlated with a shift toward female births.
Although natural selection ideally favors a 50/50, or .500, sex ratio in a population, mammals typically produce slightly more males than females. Because sex ratio is biased toward males, the figure is expressed by dividing male births by total births. It's estimated that women give birth to 3 percent more boys, for a standard .515 sex ratio (with 48.5 percent female births). When fewer boys and more girls are born than that, it's described as a sex ratio decline.
Evolutionary biologists say male mortality, which is overall higher than that of females, explains the male bias in sex ratio: A slightly skewed sex ratio at birth that favors males ensures that there are roughly an equal number of males and females of reproductive age. (Theoretically, a .500 sex ratio at birth may be possible if the gender difference in mortality is eliminated.)
Again thanks to @Anne, who provide this link as well, we see that men do outpace female births. We can see the world data here.
The 'Biological Urge': What's the Truth?
Birth rates may be plummeting in these economic times, but it's not stopping a phenomenon that happens to women in particular -- the time that comes in every woman's life when an uncontrollable "urge" comes over her and she feels a calling from deep within to become a mother.
This phenomenon has commonly been called the "biological urge," and it's seen as part of women's biological instinct to have children. We're taught that it's something that's supposed to happen to women at some point in their lives, but what do we really know about the biology at work that creates this "urge"?
We know that biology is at play when women are pregnant. Estrogen and progesterone kick in at conception and continue through pregnancy, along with the neurohormone oxytocin, which fires at the time of delivery. Research also tells us that biology is at work once the baby is born, including how the mother's brain responds differently to different baby behaviors.
While we typically don't talk about men having the same kind of "urge," there are biological factors at work for them as well. According to Dr. Ethylin Jabs at Johns Hopkins, we do know that "the bottom line is as men age, the percentage of damaged sperm they carry in their testes tend to increase," and the greater the risk of having a baby with a birth defect.
But for both sexes, what are the hard-wired biological processes that create the desire for a child?
Here's the truth that's not talked about -- For women, there is no real evidence to support the notion that there is a biological process that creates that deep longing for a child. And the same for men there's no real evidence linking biology to the creation of parental desire.
So what's behind the "urge" if it's not biological?
Similar to the origins of what I call "Fulfillment Assumption" in The Baby Matrix , the answer first goes back to pronatalist notions that were created about parenthood generations ago, when society needed to encourage people to have lots of children. In addition to pushing the idea that parenthood was "the" path to fulfillment in life, another had to do with the idea that "normal" women experience an instinctual longing from within to have a child, and if they didn't there was something wrong with them. This belief is part of the larger pronatal "Destiny Assumption" that was created many years ago, that, like the Fulfillment Assumption, has stuck long after its usefulness.
The deep feelings of wanting to have a child have their roots in a learned desire from strong, long-standing social and cultural pronatal influences -- not biological ones. And we've been influenced so strongly for so long that it just feels "innate."
Early feminist Lena Hollingsworth gets to the heart of why it isn't: If the "urge" was actually innate or instinctual, we would all feel it, she argues -- and we don't. If it were instinctive, there would have been no need to introduce social messaging to encourage and influence reproduction. If it were instinctive, there would be no need for social and cultural pressures to have children.
When it comes to the "biological urge," it's time to shift our thinking to reflect what is real. Realizing that the "longing" is not something that will automatically descend upon us allows us to better explore its origins within us. Researcher and psychoanalyst Frederick Wyatt puts it this way: "When a woman says with feeling she craved her baby from within, she is putting biological language to what is psychological."
When we can't just chalk up the longing to biological instinct, we can better reflect on the craving from within and ask ourselves questions like, "What is at the essence of this feeling of longing? Is it truly to raise a child, or is it another yearning I think a child will fill for me in my life?"
Realizing that a yearning for parenthood is not a biological imperative allows us to look harder at why we think we want children and ferret out how much of it comes from external conditioning. Seeing the truth about the "biological urge" ultimately helps us make the best parenthood choices for ourselves, our families and our world.
Brains seem to change
The brain also appears to undergo structural changes to ensure that fathers exhibit the key skills of parenting. In 2014, Pilyoung Kim, a developmental neuroscientist at the University of Denver, put 16 new dads into an M.R.I. machine: once between the first two to four weeks of their baby’s life, and again between 12 and 16 weeks. Dr. Kim found brain changes that mirrored those previously seen in new moms: Certain areas within parts of the brain linked to attachment, nurturing, empathy and the ability to interpret and react appropriately to a baby’s behavior had more gray and white matter between 12 and 16 weeks than they did between two and four weeks.
Dr. Kim thinks this bulking of the brain reflects a ramping up of the skills associated with parenting — such as nurturing and understanding your baby’s needs — and the inevitably steep learning curve that both new moms and new dads have to surmount. In particular, because men do not experience the hormonal surges that accompany pregnancy and childbirth, “learning how to emotionally bond with their own infants may particularly be an important part of becoming a father,” Dr. Kim suggested. “The anatomical changes in the brain may support fathers’ gradual learning experience over many months.”
But while both new mothers and new fathers show activation in the brain regions linked with empathy and understanding their child’s emotional state and behavioral intentions, a 2012 study by neuroscientists at Bar-Ilan University in Israel suggested that the parts of the brain that light up the most are startlingly different for each parent. For moms, regions closer to the core of the brain — which enable them to care, nurture and detect risk — were most active. But for dads, the parts that shone most brightly were located on the outer surface of the brain, where higher, more conscious cognitive functions sit, such as thought, goal orientation, planning and problem solving.
Shir Atzil, a psychologist at the Hebrew University of Jerusalem in Israel and lead author of the study, said — along with Dr. Kim — that dads’ brains seem to have adapted in similar but different ways to ensure that they can bond with and care for their babies, despite not having given birth to them. Meaning both mothers and fathers are primed to “demonstrate similar levels of motivation and attunement to the infant,” Dr. Atzil said.
Beyond this, the differing areas of brain activation may reflect a difference in role, and different but equally strong attachments, between mothers and fathers. It is a cliché that children run to Mom for a hug when they’re hurt, while Dad is the “fun” parent. But evidence suggests that mothers and fathers get different neurochemical “rewards” after certain parenting behaviors, eliciting these differences in stereotypes.
Ruth Feldman, a social neuroscientist based in Israel, published a study of 112 mothers and fathers in 2010 which found that peaks in oxytocin (and by association, dopamine) occurred for women when they nurtured their children. In contrast, the peak for men occurred when they took part in rough-and-tumble play. Because young children’s brains seem to mimic the same oxytocin levels as their parents’ — meaning they’ll get a similar blast of feel-good oxytocin when playing with Dad and when being nurtured by Mom — they’ll be more likely to engage in that behavior over and over again specifically with that parent, which is critical to their development. Rough-and-tumble play not only cements bonds between father and child, but also plays crucial roles in a child’s social development.
There are, of course, many questions still to answer in the relatively new field of the biology of fatherhood. After 10 years of study, we now need to replicate our findings on larger and more diverse groups. But if I get the chance, I tell new fathers that evolution has primed them to parent just as it has primed women. Biology has their back.
Why Are More Boys Born Than Girls?
Worldwide, there are 107 boy babies born for every 100 girl babies. This skewed ratio is partly due to sex-selective abortion and "gendercide," the killing of female infants, in countries such as China and India where males are more desired. But even discounting those factors, the completely natural male-to-female sex ratio still hovers around 105:100, meaning that women are inherently more likely to give birth to boys. Why?
Several factors influence whether a sperm containing a Y sex chromosome or one containing an X chromosome will be first to fertilize an egg, including parental ages, their environmental exposure, stress, the stage in the mother's ovulation cycle and even whether she has had children previously all these forces combine to set the average sex ratio at fertilization at 105:100. But what good is this built-in bias?
Many demographers have speculated that the gender imbalance at birth may be evolution's way of evening things out overall. Male infants more often suffer from health complications than female infants. The disadvantage runs to adulthood, too, as adult men kill each other more often, take more risks and have more health problems, on average, than women, all of which cause them to die younger. This doesn't balance the sex scales exactly, but it does come close: Among the total human population, the ratio of men to women is 101:100. [Why Do We Have Sex? ]
Why isn't the ratio perfectly even? Well, it is in the United States, all of Europe, Australia and many other developed countries (in fact, these countries have slightly more adult women than men). The small bias toward males that remains in the sex ratio of the total world population probably results from social factors hinted at earlier: abortion of female fetuses and gendercide in Southeast Asia and much of the Middle East, where, in general, there is a strong cultural preference for males.
Equally intriguing as our species' slight gender imbalance at birth is the issue of why there ought to be a balance, or near-balance, in the first place. Men produce an ungodly amount of sperm , while women have a finite egg count. As far as evolution is concerned, why couldn't humanity make do with fewer men and more women?
The widely-accepted answer to this question was first put forth by Sir Ronald Fisher, a renowned evolutionary biologist who worked in the first half of the 20th century. Fisher's Principle holds that differences in the sex ratio will tend to diminish over time because of the reproductive advantage automatically held by members of the minority sex. [What If There Were More Than Two Sexes? ] Suppose, for example, that male births were far less common than female births. If this were the case, then newborn males would naturally have better mating prospects than newborn females, and could expect to have more offspring. Parents who are genetically disposed to produce males will thus tend to have more grandchildren, and so their male-producing genes will spread, and male births will become more common. Gradually the population will approach a gender balance.
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The equatorial enigma: Why are more girls than boys born in the Tropics &ndash and what does it mean?
Aristotle once suggested that the sex of a child was determined by the ardour of the man at the time of insemination, whereas other ancient Greek philosophers thought that it had something to do with the left and right sides of the body.
Two millennia later, an 18th-century French surgeon writing under the pseudonym of Procope Couteau took up the idea and advised men wishing to have baby boys to cut off their left testicle – a procedure no more painful than extracting a tooth, he said.
In more recent times, prospective parents wishing for either a boy or a girl have been offered all manner of remedies and food supplements to affect the sex of a baby. But none of these folk recipes – even those involving crystals under the bed – has been able to alter the fundamental biology that determines the 50:50 sex ratio.
A study published yesterday, however, has revealed a new twist to an ancient story. Scientists have found that the probability of giving birth to a baby girl rather than a baby boy increases significantly the nearer the mother lives to the equator. Conversely, the higher the latitude – and the further away from the equator – the greater the chances of a woman having a baby boy.
Kristen Navara of the University of Georgia in Athens studied the sex ratio of newborn boys to girls in 202 countries, from northern Europe to equatorial Africa, and found a clear link between latitude and a skewed sex ratio. The nearer to the equator the greater the probability of baby girls, according to the study published in the journal Biology Letters.
The natural sex ratio at birth is, in fact, slightly biased towards males in humans, with about 106 boys being born to every 100 girls. This sex ratio of 51.5 per cent in favour of boys is believed to be nature's way of balancing the slightly increased risk of premature death in young males, and so bringing the overall sex ratio in the child-rearing age groups nearer to the natural balance of 50:50.
Dr Navara, however, found that this average sex ratio at birth masks an underlying geographical trend. Using data on global birth rates compiled by the Central Intelligence Agency, Dr Navara found that countries in tropical latitudes produced significantly fewer boys – 51.1 per cent males – compared to countries in temperate and subarctic regions, where the sex ratio is 51.3 per cent in favour of boys.
The difference may seem small, but it is nevertheless statistically significant, Dr Navara said. It was even larger between some of the countries in the study. In tropical Central African Republic, for instance, the sex ratio was 49 per cent boys, whereas in more temperate China it was 53 per cent in favour of baby boys, she said.
"We found that this difference was independent of other cultural variables, including socio-economic status. It was an over-arching pattern and this effect remained despite enormous cultural variations between the countries we looked at," she said.
The sex ratio is an important biological factor in evolution and any shift away from the 50:50 norm provokes fierce debate among evolutionists. But the determination of sex itself is not controversial.
Sex in mammals is determined by the type of sperm that fertilises the egg. A sperm carrying the X chromosome of the man will become a female embryo whereas a sperm carrying the Y chromosome will produce a female embryo at conception. In theory, men produce equal numbers of X and Y sperm which means that the sex ratio at birth should be 50:50.
Evolutionary biologists have shown using mathematical models that any movement away from the 50:50 ratio should become unstable, which is why there should be equal numbers of baby boys and baby girls being born in the population. However, there are possible exceptions to this rule.
One exception is if male embryos and newborn boys are more likely to die prematurely. As a result of this increased risk for males, nature has compensated by skewing the birth rate in favour of boys, or so it was believed.
Another could come about if food is at risk of being in short supply. In hard times, it should in theory be more advantageous to give birth to males rather than females because females need more energy than males because of the effort of producing eggs and being pregnant.
A study in Italy has, for instance, found that couples are more likely to conceive a boy in autumn, while those who want a girl should conceive in spring. It was thought that nature favours conception of boys from September to November and girls from March to May. One explanation may be the evolutionary necessity of keeping the overall sex ratio close to the 50:50 norm. Another could be due to seasonal variations in the availability of food.
This underlying biological trend may now be showing itself up more clearly in the latest study on latitude.
Dr Navara said that the difference in the birth sex ratio between higher and lower latitudes may reflect an ancient evolutionary mechanism reflecting the fact that food resources in more northerly regions are more varied than in the tropics.
"This study really reminds us of our evolutionary roots. Despite enormous cultural and socio-economic variability, we continue to adjust reproductive patterns in response to environmental cues, just as we were originally programmed to do," she said.
This biological trend works independently of cultural factors that may work in favour of one sex over another. In some societies in Asia and Africa, for instance, baby boys are favoured over girls and the rise in selective abortions and infanticide has skewed the overall sex ratio in favour of males.
Dr Navara said she took this into account in her study by taking out those countries where selective abortions based on the sex of the foetus are known to occur. "I eliminated some Asian and African countries to get rid of any sex-specific abortion," she said. The trend in favour of women giving birth to girls the nearer they are to the equator was still significant, she said.
But the research does not suggest that simply having a romantic holiday in a tropical country could increase a woman's chances of ending up with a baby girl. The data used in the study only applied to women who were born in the country under consideration.
Evening the Scales
While there are more men than women from a global perspective, when you look at different demographics and geographic locations, the balance is often flipped. As mentioned earlier, male infants are more likely to die from complications during birth and in the first few weeks of life, but the imbalance starts even earlier than that. Research has also shown a higher mortality rate for male embryos in the first week of pregnancy and in the final trimester (source).
As you move up the age scale, particularly past the age of 60, there is a clear advantage for women, who tend to have longer life expectancies than men. In most parts of the world, women live longer than men, mainly owing to a higher risk of cardiovascular diseases in men. The global average life expectancy for women is 71.1 years for men around the world, life expectancy averages 67 years (source).
In some countries, like Belarus, Russia and Lithuania, the life expectancy gap is even greater &ndash female life expectancy is more than 11 years longer than men (source).
Male mortality is on the decline in European established democracies and the gender gap in life expectancies is slowly becoming smaller. Current trends in Iceland and other Scandinavian countries indicate that, by the year 2050, the difference between life expectancies will whittle down and men will live as long as women. Unfortunately, this gap continues to grow in Russia where women live on the average 13 years longer. (Source)
In terms of geographical location, some countries in the Middle East have more than 250 men : 100 women (e.g. United Arab Emirates and Qatar). At the other extreme, the Caribbean island of Martinique has a ratio of 85 men : 100 women (Source). Each country and demographic has a unique set of factors that affect the balance of men and women, so if anything, it is remarkable that the balance of men and women is as close as it is!
When a Person Is Neither XX nor XY: A Q&A with Geneticist Eric Vilain
Eric Vilain discusses the biology and politics of mixed-sex individuals, arguing that terms such as "hermaphrodite" and "intersex" are vague and hurtful.
About one in 4,500 babies show ambiguous genitalia at birth, such as a clitoris that looks like a penis, or vice versa. For the Insights story, "Going Beyond X and Y," appearing in the June 2007 issue of Scientific American, Sally Lehrman talked with noted geneticist Eric Vilain of the University of California, Los Angeles, about the biology of sex determination, gender identity and the psychology and politics behind both. Here is an expanded interview.
When did you first discover your interest in intersex individuals and the biology of sex development?
I started in Paris as a medical student, and my first assignment was to a unit of pediatric endocrinology in a Paris hospital, and it was the center of reference for all of France for babies born with ambiguous genitals. And I was actually literally shocked by the way decisions were made on these patients. I felt it didn't rely on solid scientific evidence. I mean, I'm a scientist, I'm a big believer of you can't just do things without being supported by evidence. In this case it was more like people would say it was just common sense&mdashif the clitoris sticks out this much, you have to fix it. Or if the penis is really too small, it has to be bigger. Otherwise what life is this child going to have? And you know, I was never convinced by common sense. I kept asking, "How do you know?" There was no good answer to that.
There were a lot of patients and it was always the same discussions. And it was mainly about clitoral reduction.
So there were sexual politics there, too?
Yes. I was reading at the time this book by Michel Foucault. He has book that's called Herculine Barbin. He basically tells the story of this girl who clearly has a large clitoris. She goes and gets sexually aroused as she sleeps in the bed of other girls, as it was normal for girls to do. She goes to this religious institution for girls until eventually someone finds out, and then it's a big scandal. She becomes a pariah, and she ends up committing suicide. I was reading that, I was pretty young, I was like 18.
Defining normality has always been an obsession of mine. How do you define what's abnormal versus normal? I guess it's the philosophical roots of the French educational system.
But why choose to study intersex questions for the rest of your career?
My scientific inclination was excited by this because not only was it understanding a rare condition that makes people different, all of these social aspects, but also it has scientific implications in the basic biology of developing male or female. Always in biology, you want to look at the exception to understand the general. So understanding intersex individuals makes us understand how typical males and typical females do develop.
So what has your research overall been able to say about sex development?
We've identified new molecular mechanisms of sex determination. In particular we've discovered genes, such as WNT4, that's female-specific and not present in males, and that's sort of shifted the paradigm of making a male as just activation of a bunch of male genes. In fact it's probably more complicated. What we've shown is that making a male, yes, is activating some male genes, but it's also inhibiting some antimale genes. It's a much more complex network, a delicate dance between pro-male and antimale molecules. And these antimale molecules may be pro-female, though that's harder to prove.
It sounds as if you are describing a shift from the prevailing view that female development is a default molecular pathway to active pro-male and antimale pathways. Are there also pro-female and antifemale pathways?
Modern sex determination started at the end of the 1940s&mdash1947&mdashwhen the French physiologist Alfred Jost said it's the testis that is determining sex. Having a testis determines maleness, not having a testis determines femaleness. The ovary is not sex-determining. It will not influence the development of the external genitalia. Now in 1959 when the karyotype of Klinefelter [a male who is XXY] and Turner [a female who has one X] syndromes was discovered, it became clear that in humans it was the presence or the absence of the Y chromosome that's sex determining. Because all Klinefelters that have a Y are male, whereas Turners, who have no Y, are females. So it's not a dosage or the number of X's, it's really the presence or absence of the Y.
So if you combine those two paradigms, you end up having a molecular basis that's likely to be a factor, a gene, that's a testis-determining factor, and that's the sex-determining gene. So the field based on that is really oriented towards finding testis -determining factors. What we discovered, though, was not just pro-testis determining factors. There are a number of factors that are there, like WNT4, like DAX1, whose function is to counterbalance the male pathway.
Why are genes such as WNT4 and others necessary for sex development?
I don't know why it's necessary, but if they're doing this then probably they're here to do some fine-tuning at the molecular level. But these antimale genes may be responsible for the development of the ovary. And WNT4 is likely to be such a factor. It's an ovarian marker now, we know. But if you have an excess of WNT4, too much WNT4 in an XY, you're going to feminize the XY individual.
Is the conceptual framework for sex determination changing, then, because of these discoveries?
I think the frame has slightly changed in the sense that even though it's still considered that the ovary is the default pathway, it's not seen as the passive pathway. It's still "default" in the sense that if you don't have the Y chromosome, if you don't have SRY, the ovary will develop. [ SRY, or sex-determining region Y, encodes the so-called testis determining factor.] That's probably the new thing in the past 10 years, that there are genes that are essential to make a functioning ovary. That really has changed, and WNT4 is one of the reasons for it.
What do you feel are your group's most important contributions to the sex biology field so far?
The two things that we contributed was, one, to find the genes that are antimale, and reframing the view of the female pathway from passive to active. And the second thing is in the brain. We're the first ones to show that there were genes involved in brain sexual differentiation, making the brain either male or female, that were active completely independently from hormones. Those were probably our two main contributions.
Do you think this difference in gene expression in the brain explains anything about gender identity?
About identity, it says nothing [yet]. It might say something. So those genes are differentially expressed between males and females early during development. They're certainly good candidates to look at to be influencing gender identity, but they're just good candidates.
At a recent international meeting to discuss management of people with genital and gonadal abnormalities, you successfully pushed for a change in nomenclature. Instead of using terms such as "hermaphrodite" or even "intersex," you recommended that the field use specific diagnoses under the term, "disorders of sex development." Why did you and other geneticists feel a nomenclature change was necessary?
For the past 15 to 16 years now, there really has been an explosion in the genetic knowledge of sex determination. And the question being, how can we translate this genetic knowledge into clinical practice? So we said maybe we should have a fresh approach to this.
The initial agenda was to have a nomenclature that was robust but flexible enough to incorporate new genetic knowledge. Then we realized there were other problems that were in fact not really genetic, but that genetics could actually answer them. Ultimately individuals who are intersex will each have their diagnosis with a genetic name. It's not going to be some big, all-encompassing category, like "male hermaphrodites." And that's much more scientific, it's much more individualized, if you will. It's much more medical.
How did the conference participants respond to the proposal?
The majority of health care professionals were very happy with it. There were some, there was a conservative side that said, "Why change something that was working?" There was significant minority dissent that was saying, "Why do we care?" Because it was working, for us it's an intellectual frame that has worked. So it required a little bit of education, saying, you know, it's important not only because it's more precise and it's more scientific, but also the patients would benefit from it by removing the word "hermaphrodite" and so forth. About the change to disorders of sex development, there was no issue at all in the group.
Why is the medical emphasis of this new term problematic for some?
The one piece in the nomenclature that remains highly controversial is the replacement of "intersex" with "disorders of sex development." And I'll say a few things about that. One is that intersex was big. Sometimes we wouldn't know who to include and who not to include.
"Intersex" was vague and "disorders of sex development" at least is a very medical definition, so we know exactly what we're talking about. For instance, if there are chromosomal abnormalities, if you have a patient who is missing one X chromosome&mdashTurner syndrome&mdashor having an extra X&mdashKlinefelter's syndrome&mdashboth those, now we do include them in "disorders of sexual development." They're not ambiguous. They do belong in this large category of people with "medical problems," quote-unquote, of the reproductive system. So intersex was vague, DSD is not vague.
What were some of the social issues you were trying to address?
There was another issue with the old nomenclature, which was the actual word, "hermaphrodite." "Hermaphrodite" was perceived by adult intersex individuals as demeaning. It also had some sexual connotation that would attract a flurry of people who have all sorts of fetishes, and so the intersex community really wanted to get rid of the term.
Cheryl Chase, executive director of the Intersex Society of North America (ISNA), said she has been promoting a nomenclature change for some time. Why?
People like Cheryl would say intersex issues are not issues of gender identity, they are just issues of quality of life&mdashwhether early genital surgery was performed appropriately or not, and that's really what has impaired our quality of life. She and others at ISNA do support the change because of an interesting side effect&mdashbecause it becomes a very medicalized definition, the medical science should apply. It should apply strongly. That means it's not as if now we're talking about something that's not a disorder, that is just a normal variant, a condition. If it's just a condition that's a normal condition, then there is no need for medical attention.
So basically my point of view is really, let's separate the political from the medical, the science. There's a whole psychology to this, you know, the surgeons often are under the impression that there is this tiny, vocal minority of activists who just want to destroy their work.
Intersex individuals are really distinct from, for instance, the gay and lesbian community that does not have any a priori medical issue, there is no difference in the development of any of the organs, or they don't need to see a doctor when they're a newborn. I think it's quite different. Sure, some intersex are gay or lesbians, but not all are.
Why was it necessary for intersex individuals to take an activist stance at one time?
Because otherwise nothing would have changed in the practice. Otherwise this consensus conference would just not have happened. It was really in response to activism. They put the problem on the table and it required, it really forced the medical community to address an issue that was rare enough not to be addressed.
Some have called the new term a political setback, because it pathologizes what could be seen as normal human variation.
First of all, we can call normal variants everything we can call cancer a normal variant. Of course, it kills you in the end, but it is a normal variant. We can play with words like that, but for practical purposes these "normal variants" have a lot of health risks that require lots of visits to the doctor for a bunch of issues that intersex patients have: fertility issues, cancer issues (the testis inside the body can increase the risk of cancer), sexual health issues. So if you're to start going to the doctor a lot for your condition, you can call it a normal variant, but that's not really useful. You're calling it a normal variant for political purposes. I'm calling it a disorder because I want all the rules and the wisdom of modern medical practices to be applied to the intersex field. I don't want intersex to be an exception: To say, "Um, you know, it's not really a disease," so therefore [physicians] can do whatever they want. That's what has been driving this field, people saying, well, you know, we can experiment, it's a normal variant.
There has been considerable controversy over whether surgeons should immediately make a decision about an infant's sex and quickly correct ambiguous genitalia. The consensus statement seems to promote a more cautious approach to surgery, while still assigning gender rapidly. What is your view?
I'm saying intervene [with surgery] only if you've proven that intervention is actually of benefit to the patient. Not of benefit to the parent. Because you know that surgery is used a lot to help the parent psychologically. It's a quick fix, if you will. The child looks different, it's very distressing for everyone, and one way to make it go away is just to make the kid look like everyone else. And that's really psychological help for the parents. But that should not be a parameter for surgery. We're talking about psychological distress to the parents, and that should be treated appropriately by a psychologist or psychiatrist, but not by surgery of the child.
Do you think this consensus statement will change the common practice of performing sex-assignment surgery early on?
(laughing) Well, yes. See, the consensus statement is a house of cards. You build it once, and there's no one that really inhabits it it can be destroyed. They're not guidelines. I think it will change, but it will require some additional work. One of the things I think should happen next is to have a few leading clinics actually apply all the consensus recommendations and then do studies showing whether they actually impact the health and the well-being of the patient. It's not easy to do, because some of the recommendations require money. Like saying, "We need a psychologist"&mdashthat's easier said than done. There's no funding for having a psychologist in all these clinics. So I think it will influence some things. For instance, the nomenclature will change. I get a lot of phone calls and e-mails from authors of major textbooks, they're going to change. Also from editors of journals who publish articles about intersex, so that's going to change. But will that change the general outcome of patients? I don't know. I hope so. I think it's a step in the right direction.
Many physicians and geneticists look at intersex simply as a medical condition that should be addressed. You seem to take patients' social and political concerns very seriously, too. Why?
I've always been interested in the fact that medicine is very normative, and reductionist&mdashit reduces people to their pathologies .'' Medicine should be in the business of making people as a whole better, rather than just curing the disease. And anyway, I'm not the only one saying that. Actually, I always use cancer as an example. A lot of cancer doctors are very well aware of this. They're offering options that sometimes do not include treatment just because they're aware of the fact that the treatment would ruin the quality of life so much that it's just not worth it.
How do you handle working in a field that is so volatile socially and politically? Everything that you do, people jump on and make claims about sexuality or gender.
I interpret everything conservatively. You have to not make the mistake of overinterpreting anything. That's my way of trying to navigate that. You also have to be aware of the social sensibilities. You can't just have an autistic approach to it and say, I'm just going to ignore it completely. If you're aware of the social sensibilities, and if you don't overinterpret your data, you're in good shape.
How do you stay aware and informed?
Being part of ISNA is one way [as a member of its medical advisory board]. It forces me to listen to what the patients have to say, which is really not part of the medical culture, at least in this field. The way to assess the well-being of a patient is to really listen to what the patient has to say.
'Gay genes': science is on the right track, we're born this way. Let’s deal with it.
I n a recent Guardian article , Simon Copland argued that it is very unlikely people are born gay (or presumably any other sexual orientation). Scientific evidence says otherwise. It points strongly to a biological origin for our sexualities. Finding evidence for a biological basis should not scare us or undermine gay, lesbian and bisexual (LGB) rights (the studies I refer to do not include transgendered individuals, so I’ll confine my comments to lesbian, gay and bisexual people). I would argue that understanding our fundamental biological nature should make us more vigorous in promoting LGB rights.
Let’s get some facts and perspective on the issue. Evidence from independent research groups who studied twins shows that genetic factors explain about 25-30% of the differences between people in sexual orientation (heterosexual, gay, lesbian, and bisexual). Twin studies are a first look into the genetics of a trait and tell us that there are such things as “genes for sexual orientation” (I hate the phrase “gay gene”). Three gene finding studies showed that gay brothers share genetic markers on the X chromosome the most recent study also found shared markers on chromosome 8. This latest research overcomes the problems of three prior studies which did not find the same results.
Gene finding efforts have issues, as Copland argues, but these are technical and not catastrophic errors in the science. For example, complex psychological traits have many causal genes (not simply “a gay gene”). But each of these genes has a small effect on the trait so do not reach traditional levels of statistical significance. In other words, lots of genes which do influence sexual orientation may fall under the radar. But scientific techniques will eventually catch up. In fact there are more pressing problems that I would like to see addressed, such as the inadequate research on female sexuality. Perhaps this is due to the stereotype that female sexuality is “too complex” or that lesbians are rarer than gay men.
Genes are far from the whole story. Sex hormones in prenatal life play a role. For example, girls born with congenital adrenal hyperplasia (CAH), which results in naturally increased levels of male sex hormones, show relatively high rates of same-sex attractions as adults. Further evidence comes from genetic males who, through accidents, or being born without penises, were subjected to sex change and raised as girls. As adults these men are typically attracted to women. The fact that you cannot make a genetic male sexually attracted to another male by raising him as a girl makes any social theory of sexuality very weak. Genes could themselves nudge one towards a particular sexual orientation or genes may simply interact with other environmental factors (such as sex hormones in the womb environment) to influence later sexual orientation.
The brains of gay and heterosexual people also appear to be organised differently. For example patterns of brain organisation appear similar between gay men and heterosexual women and between lesbian women and heterosexual men. Gay men appear, on average, more “female typical” in brain pattern responses and lesbian women are somewhat more “male typical”. Differences in brain organisation mean differences in psychology and study after study show differences in cognition between heterosexual and gay people. Thus gay differences are not just about who you fancy. They are reflected in our psychology and the ways we relate to others. The influence of biology runs throughout our sexual and gendered lives and those differences, that diversity, is surely to be celebrated.
Some writers tend to wave off the scientific evidence by urging us to look to the history of sexuality or claim that homosexuality is a social construction (cue Michel Foucault and the like). But these accounts are mere descriptions at best and not scientific theories. Social constructionist accounts generate no hypotheses about sexual orientation and are not subject to systematic testing. So why should we take their claims seriously? Social constructionism and postmodernist theory question the very validity of empirical science in the first place. That makes it no better than climate science denial.
Some will argue that our common sense experiences are full of people who are “fluid” in their sexual orientations or change their sexualities. This won’t do either because our experience fools us all the time. Change is widely used to argue against biological explanations. Critics will say that if behaviour changes, or is “fluid”, then surely it can’t have a biological basis? This is false because it is our biology that allows us to learn, respond to socialisation, and helps generate our culture. So showing evidence of change is not an argument against biology. There is indeed some fluidity in sexuality over time, predominantly among women. But there is no “bell shaped curve” to sexual orientation. People may change the identity labels they use and who they have sex with but sexual attractions seem stable over time.
Remember, sexual orientation is a pattern of desire, not of behaviour or sexual acts per se. It is not a simple act of will or a performance. We fall in love with men or women because we have gay, straight, or bisexual orientations and not because of choice. So let’s stop pretending there is choice in sexual orientation. Who truly “chooses” anything of substance anyway? Surely our choices are the result of things we didn’t choose (our genes, personalities, upbringing, and culture).
People worry that scientific research will lead to “cures” for homosexuality (which is an odd worry to have if you don’t believe in the “born this way” argument). They worry more about this than the consequences of choice or environmental explanations, which are not without risk either. But clearly none of the direst predictions have materialised. Sexual minority identities have not been medicalised nor has there been any genetic testing. Genetic tests would never result in 100% accurate identification of LGB individuals because, as I said, genes are less than one-third of the story. On the social policy and legal front we’ve gone in the direction of more rights and more freedoms for LGB people (at least in the West) and not less.
So should the causes of sexuality influence how we view sexual minority identities? No. The causes of a trait should not influence how we see it. But the science shows us that sexuality has a biological basis: that is simply how the science turned out. It’s no use denying it. So let’s use it to supplement, but not replace, a discussion about LGB rights and social policy. The biology of sexuality diversity tells the world to deal with it. We are who we are, and our sexualities are part of human nature.
My worry about the claims of social construction, choice and such like is that it plays into the hands of homophobic ideology, into the hands of the “aversion therapists”, and into the hands of a growing culture which seeks to minimise gay differences. It reminds me of something Noam Chomsky alluded to : if humans were entirely unstructured creatures we would be subject to the totalitarian whims of outside forces.
Dr Qazi Rahman is an academic at the Institute of Psychiatry, King’s College London. He studies the biology of sexual orientation and the implications for mental health and is the co-author of Born Gay? The Psychobiology of Sex Orientation
The protein targeted by the antibodies, called NLGN4Y, is thought to play a role in how brain cells connect to each other, says Bogaert. “So it could affect brain structures that moderate attraction,” he says. “The mother’s immune response may alter the typical function of these brain structures.”
“This is a very important study because it provides a plausible mechanism to explain the fraternal birth order effect, perhaps the most firmly established phenomenon related to human sexual orientation,” says Marc Breedlove at Michigan State University. “Given that the protein is known to be important in synapse formation, you can see how maternal antibodies might affect the wiring of the fetal brain, and that might explain why each subsequent son is more likely to grow up gay.”
However, the team’s study only looked at a very small number of people, so strong conclusions cannot be drawn yet.
“The significance of this preliminary observation, if it can be replicated, is that it identifies specific molecules in the brain that may be important for heterosexual as well as homosexual development,” says Dean Hamer, a pioneer of researching the biological determinants of sexual orientation. The finding “could pave the way to a detailed neurobiological and genetic understanding of this fascinating aspect of human development”, he says.
Journal reference: PNAS, DOI: 10.1073/pnas.1705895114