Nature or Nurture: Insights from the Womb

[Originally published 26th February, 2014; updated 27th December, 2024]

Leonardo da Vinci image of feotus in the womb

Born this way or made this way?

Thanks to research into prenatal development over the past couple of decades, we're starting to reframe how we approach the age-old debate about nature versus nurture. This research challenges the neat categories we find ourselves assigned to –– male, female, heterosexual, homosexual –– and forces us to reconsider what really determines the traditional boundaries we use to define ourselves.

At the heart of this shift in perception is epigenetics, which reveals how our biology is shaped by a dynamic interplay between our genes and environment, starting before we're even born.

Epigentics

Epigenetics refers to both a biological process and the study of that process. Focussing on the process itself, epigenetics refers to changes in gene expression that occur without altering the genetic code itself. Environmental factors (adversity, diet, toxins, etc.) can alter gene expression –– which genes are "switched on" or "switched off" –– without changing the DNA sequence and creating ripple effects in our development and health.

The womb is an environment.

Through epigenetics, in-utero environmental factors, such as stress hormone levels, can have long-term effects on the developing brain of a foetus (Weinstock, 2010; Charil et al., 2010; Amiel-Tison et al., 2004). For example, when prenatal stress leads to abnormally high cortisol levels at critical periods of foetal development, it can alter the expression of genes involved in neurogenesis (the process of forming new brain cells). This results in a smaller hippocampus, which is linked to depression (Charil et al., 2010; Surget et al., 2011).

Before knowing anything about epigenetics, you'd be forgiven for thinking that a physically identifiable part of the brain linked to depression, like the hippocampus, is evidence of a nature-based cause of depression. But epigentics highlights that it's more meaningful to talk about the impact of genes when you also consider environmental factors that are important in whether these genes are activated and how they are expressed (Lickliter, 2013).

Even when the DNA between individuals is identical, the way DNA functions can differ due to epigenetics. 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 in the scientific community is how identical twins can have different sexual orientations.

Sexuality

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? If genetically identical, how can one twin be homosexual, and the other heterosexual?

In male foetus development, the testes produce testosterone at around week eight (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 markers, called "epi-marks", act as switches that control how genes are switched on or off (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.

Sexual differentiation

Genes 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.

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!

But, of course, it's not really as straightforward as that. 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 externally female 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 one part of our genetic sex determination system, where the presence of the Y chromosome typically 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 Xs and the Ys), particular prenatal hormones can drastically affect the foetus' development as male or female.

Problems have arisen because the boundary between men and women is not as dichotomous and mutually exclusive as most believe it, or want it, to be. We, not pure biology, define sex boundaries. We label people 'male' and 'female' based on outward appearance and add in the detail later. The biology might be true, but how we define the boundaries based on biology is subjective.

You might think of it a bit like colour categorisation. In physics, colours are 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 a perception. 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).

Societal implications

Our historically stubborn adherence to rigid definitions of "male" and "female", despite the biological complexity revealed by epigenetics, has societal implications. This is clearly manifest in how women are defined and treated in competitive sport, a discussion that I've moved into its own blog post: Are you female enough? The continued legacy of policing women’s bodies in competitive sports.

In short, female athletes have been excluded from competing at an international level based on technicalities. Not only do you have to have XX chromosomes, you also have to fall below a threshold of testosterone –– you have to be “female enough”.

Only women face medical interventions to control naturally occurring testosterone to meet eligibility criteria. This is despite the fact that a woman with high testosterone has the same presumed advantage over other female competitors that a man with high testosterone has over other male competitors.

I also find it interesting that, when it comes to sexuality, the research focus in neuroscience, genetics, and psychology, particularly in the 19th and 20th centuries, has been more concerned with exploring and explaining male homosexuality (e.g., Simon LeVay and Dean Hamer). The historical focus on male homosexuality within scientific research, neglecting female sexual orientation, has led to a less comprehensive understanding of the biological and psychological aspects of female homosexuality.

This focus on policing women's bodies and male sexuality is in part a reflection of an unconscious human bias that positions the heterosexual male as the norm, and any variations outside this norm as "deviations". This includes women as a whole and non-heterosexual men. I discuss this in more detail in another blog post: Male Norm, Female Form.

Blurring lines

Research into prenatal development invites us to rethink how we define identity, assign categories, and make decisions about what’s ‘natural’ or ‘normal‘. It tells us that trying to determine the nature versus nurture of who we are is not always useful for really understanding ourselves or human diversity.

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.

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References

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

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?

Feigenbaum, J. (2019). Shades of gray: Sex, gender, and fairness in sport. Barbell Medicine.

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

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, R. (2008). Categorical perception of colour in the 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