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  • Writer's pictureJeunese Payne

Nature or Nurture: Insights from the Womb

[Originally published 26th February, 2014; updated 25th March, 2024]

Leonardo da Vinci image of womb
Leonardo da Vinci image of feotus in the womb

Was I born or made a heterosexual female? Am I emotional because my parents were strict or because I have XX chromosomes? Am I outgoing because of my genes, or because of my birth order? Have I suffered from depression because I have a small hippocampus or because of my childhood experiences? If I had an identical twin, what would determine our similarities and differences?

The nature-nurture debate continues in many areas of Psychology. You may already have a standpoint. Most do. Many, more diplomatic types, claim it's simply a 50/50 split. I hope to show you that, actually, it isn’t one or the other. It isn’t even a case of “a bit of this, a bit of that”. In many cases, we’re not even close to really knowing.

Using the examples of depression, homosexuality, and sex –– and, of course, the psychology behind it all –– I hope to show how human experiences are not best understood in terms of a nature-nurture dichotomy, but as part of the same thing. As the title of this blog post suggests, I do so by drawing on evidence from studies on prenatal development.


Research into the long-term effects of in-utero circumstances is already successfully exploited by medical professionals in preventative medicine and in creating the optimal environment for foetal development. Research in this area is also useful in understanding how the brain is organised and developed based on the experiences of an unborn child.

For example, prenatal stress, which increases levels of cortisol (Weinstock, 2010; Charil et al., 2010; Amiel-Tison et al., 2004) can decrease neurogenesis (also known as proliferation –– the birth of neurons in the brain). This results in a smaller hippocampus –– one of the proposed causes of depression (Surget et al., 2011). Since neurogenesis is most active during prenatal development, external factors have their greatest impact before a child is born (Amiel-Tison et al., 2004).

Because the hippocampus is a physically identifiable part of the brain, the link between hippocampal neurogenesis and depression can be interpreted as evidence in favour of the nature side of the argument. But the cause –– exposure to prenatal stress –– is an environmental factor, which affects the amount of proteins that are produced. These proteins interact with our DNA and, relevant to depression, influence neuron growth in the foetus.

The study of these (and other) changes in gene activity and expression due to external factors, without altering the underlying DNA sequence, is called epigenetics. Epigentics highlights that it's not meaningful to talk about the impact of genes without also considering factors that are important in whether these genes are activated and expressed (Lickliter, 2013).

Twin studies

Epigenetics tells us that even when the DNA between individuals is identical (as for identical twins), the way DNA functions can differ. A genotype (set of genes) can produce a range of phenotypes (observable characteristics such as morphology or behaviour) in response to environmental influences. This is why, for example, identical twins have different fingerprints.

So, aside from fingerprints and freckles, how is it that identical twins can turn out so different? Don’t they have the same genes, parents, and upbringing? Prenatal research provides some insight.

Of particular interest is how identical twins can have different sexual orientations.

Generally, studies show that a higher percentage of identical male twins have the same sexual orientation than do non-identical male twins –– which suggests that genes play a role (e.g., Bailey & Pillar, 1991). But, what about the rest? How can one identical twin be homosexual, and the other heterosexual?

(By the way, I am not insensitive to the fact that I am focussing on male homosexuality here, but this is where the research is focussed, a phenomenon that is perhaps best discussed in a separate post.)

In male foetus development, the testes produce testosterone at around week 8 (Rice et al., 2012). This is responsible for “masculinsing” the body and the brain. Its impact on the hypothalamus is partially responsible for sexual orientation.

Greater exposure to testosterone in the womb is believed to translate into a greater sexual inclination towards women. Homosexuality is believed to occur when a male foetus produces less testosterone or is relatively insensitive to it.

Evidence suggests that how a foetus responds to testosterone is regulated by gene expression (epigenetics again). Temporary chemical compounds called "epi-marks" act as “switches” to regulate gene activity (Rice et al., 2012, 2013). These are sex specific and, normally, either boost sensitivity to testosterone in XY foetuses, or lower it in XX foetuses (Rice et al., 2012, 2013). These epi-marks are then erased. Occasionally, however, epi-marks passed on by the opposite-sex parent alter gene expression when they interact with the foetus’ DNA (Rice et al., 2012, 2013).

This means that, while genes play a role –– after all, homosexuality has substantial heritability (Rice et al., 2012) –– they aren't all-important when it comes to sexual orientation.

They are also not all important when it comes to sex. Note, here, that "sex" and "gender" aren't strictly the same thing. "Sex" is assigned at birth based on physiological characteristics, most notably, genitalia. "Gender" refers to how a person identifies, which might or might not align with their assigned sex. Sometimes, even the biological markers of sex don't align, as is the case with complete androgen insensitivity syndrome (CAIS), where the chromosomes (XY) don't dictate the development of the expected genitalia.

Sexual differentiation

Chromosomal sex is determined at fertilisation when a sperm and ova join. Under normal circumstances, XX produces a girl and XY produces a boy. Simple!

Here’s where the confusion can seep in: the first few weeks of prenatal development is similar for male and female foetuses. The default is to develop as female, which means if nothing changes, you can develop as female (at least externally) even if you have XY chromosomes. This is the case for individuals with XY chromosomes that have CAIS, who might only discover that their chromosomes are XY following tests when they don't start menstruation as expected.

Normally, if a Y-chromosome is present, it codes for the production of “testis determining factor” (TDF). Before the production of TDF, the body has already developed somewhat and is why, for example, men have nipples. Things can still go either way depending on the hormones secreted by the gonads.

This is not to say that we are all female at some point in the womb. This myth is regurgitated all over the Internet. In early prenatal development, we are neither male nor female. Whether XX or XY (or indeed any other, less common setup such as XXY), our bodies follow the same female blueprint, but are not yet female. The X and Y chromosomes are part of our genetic sex determination system, where the presence of the Y chromosome triggers male development, but it doesn't define the sex of a person in and of itself.

So what does this tell us about nature versus nurture? It tells us that regardless of our genetic makeup (the X’s and the Y’s), particular prenatal hormones can drastically affect the foetus' development as male or female.

"Female enough"

Problems have arisen because the boundary between men and women is not as dichotomous as most believe it, or want it, to be. We, not pure biology, dictate sex boundaries. We label people 'male' and 'female' based on outward appearance and add in the detail later.

Since 1968, at the Winter Games (Grenoble), in the interest of fair competition, sex verification was introduced to prevent men from performing in women’s events. In 1996 (Atlanta Olympics), eight athletes failed verification tests. While mandatory testing has since ceased, individual athletes can still be examined if there is any suspicion.

Since April 2011, the IAAF [now World Athletics] states that women must fall below a threshold of testosterone to compete in women’s events, even if they have XX chromosomes. This means that eligibility criteria for many female athletes now involve compulsory hormonal therapy. So, now you have to be “female enough” to compete in events. As a result, athletes have been excluded based on technicalities, even though these technicalities give them no advantage over other athletes.

In the 2012 Olympics, women could be disqualified if their testosterone levels fell in the normal male range of 7-30 nanomoles per litre (nmol/L) of blood. This is despite the fact that a woman with a testosterone level of 7 nmol/L has the same presumed advantage over her female competitors as a man who has 34 nmol/L does over his male competitors.

 [For the re-publication of this blog post, I checked to see if the rules were still the same, and, effective 2018, World Athletics (formerly IAAF) now imposes an even lower threshold (5 nmol/L) to exclude women competing in specific events.]

Interestingly, the focus is primarily on unfair advantage in women’s sports –– an emphasis that arguably stems from a combination of women being seen as a deviation from the male norm, the perception that men are more active and athletic than women, and the development of competitive sports according to male models.

Men who partake in sports in which females have an advantage, such as gymnastics, which favour flexibility and smaller body size, can focus on activities that capitalise on their strength rather than flexibility and balance, such as the rings as opposed to the balance beam [see also, Leong, 2018]. Sports images directed at women tend to be those that have an aesthetic element, which significantly reduces the number of female role-models in other, male-dominated sports (Centre for Gender Equality, 2006 [since writing this blog post in 2014, this organisation has been superseded by the Council of Gender Equality]).

Blurring lines

Research into in-utero development shows us that sex boundaries are not clear-cut. The biology is true. How we define the boundaries based on biology is subjective.

I think of it like I think of colour categorisation. At what precise point does red turn into orange? Or blue into green? Colour can be identified numerically by co-ordinates, but what we choose to call different colours that sit close together in terms of these co-ordinates depends on our culture; the boundaries between colours are mediated by the speaker’s language. For example, the boundary between some Korean colours are non-existent in most other (including English-speaking) cultures (Roberson et al., 2008).

In physics, colour is the electromagnetic waves of light that leave an object at different frequencies (blue is at a higher frequency than red, for example). In psychology, colour is the perception. Physical properties and how we perceive those properties are not causally linked.

Insights from prenatal development tell us that trying to determine the nature versus nurture of who we are is not always useful for really understanding ourselves. I haven’t discussed all the insights that research into prenatal development has to offer. There are other brain structures to consider, other chromosomal combinations, other interesting studies such as those investigating fraternal birth order, etc.

Hopefully, though, I have discussed enough to get across the idea that the line between nature and nurture isn’t just blurred, it’s arguably not even there.


Amiel-Tison, C., Cabrol, D., Denver, R., Jarreau, P-H., Papiernik, E., Piazza, P. V. (2004). Fetal adaptation to stress. Part I: acceleration of fetal maturation and earlier birth triggered by placental insufficiency in humans. Early Human Development, 78 (1), 15-27

Bailey, J. M., and Pillard, R. C. (1991). A genetic study of male sexual orientation. Archives of general psychiatry, 48 (12), 1089-1096

Boyd, A. (2018). Back to the binary: How the Olympics struggle with separation of male and female. DePaul Journal of Sports Law, 14 (1), 1-32

Charil, A. Laplante, D. P., Vaillancourt, C., & King, S. (2010). Prenatal stress and brain development. Brain Research Reviews, 65 (1), 56-79

Federation Internationale de Gymnastique (2020). Why are there four events for women and six for men?

Gilbert, S. F. (2000). Chromosomal sex determination in mammals. Developmental Biology [6th Ed].

Leong, N. (2018). Against women's sports. Washington University Law Review, 95 (5), 1251-1292

Lickliter, R. (2013). The origins of variation: evolutionary insights from developmental science. Advances in Child Development and Behavior, 44, 193-223

Medical News Today (2021). Sex and gender: What is the difference?

Rice, W. R., Friberg, U., & Gavrilets, S. (2012). Homosexuality as a consequence of epigenetically canalized sexual development. The Quarterly Review of Biology, 87 (4), 343-368

Rice, W. R., Friberg, U., & Gavrilets, S. (2013). Homosexuality via canalized sexual development: A testing protocol for a new epigenetic model. Bioessays, 35 (9), 764-770

Roberson D., Pak, H., & Hanley, J. R. (2008). Categorical perception of colour in left and right visual field is verbally mediated: evidence from Korean. Cognition 107 (2), 752-762

Surget, A. Tanti, A., Leonardo, E. D., Laugeray, A., Rainer, Q., Touma, C., Palme, R., Griebel, G., Ibarguen-Vargus, Y., Hen, R., & Belzung, C. (2011). Antidepressants recruit new neurons to improve stress response regulation. Molecular Psychiatry, 16 (12), 1177-1188

Weinstock, M. (2008). The long-term behavioural consequences of prenatal stress. Neurosicence and Biobehavioral Reviews, 32 (6), 1073-1086

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