Why nature and nurture both matter

The ages-old debate around the relative importance of nature and nurture on human behaviour isn’t over, but the new science of epigenetics is helping us understand the debate better than ever before

Audrey Hepburn was one of the defining faces of the 20th century. Stylish, luminous, with a delicacy of feature that made her seem almost otherworldly, she became an icon through her role as Holly Golightly in Breakfast at Tiffany's, a performance of such lightness and charm that it's still difficult to believe it was inhabited by someone who had known real hardship.

But Hepburn was a survivor of one of the most brutal episodes of the second world war. She was a teenager during the Dutch Hunger Winter of 1944-45, when the Nazi occupation of the western Netherlands imposed a food blockade so severe that rations fell to as low as 400-500 calories a day. People ate tulip bulbs to survive. The experience left permanent marks on Hepburn's health that stayed with her for life.

It also, as science has since discovered, left marks of a different and more remarkable kind.

The Dutch Hunger Winter created something that researchers would spend decades studying: a precisely defined population of people who had experienced a single, well-documented period of severe deprivation, at known and varying stages of development in the womb. What they found there changed our understanding of what it means to be human.

The emerging science of epigenetics is dismantling one of our most persistent intellectual habits – the either/or framework of nature versus nurture. Are we the product of our genes, or our environment? The answer, it turns out, is that this has always been the wrong question. And what epigenetics is replacing it with is considerably more interesting – and unsettling.

What DNA actually does

In her landmark book The Epigenetics Revolution, molecular biologist Nessa Carey offers a deceptively simple image to explain what DNA actually does. We tend to think of it as a template – a mould from which identical copies are stamped out. But Carey argues it is better understood as a script. The same script can produce radically different performances depending on who is directing, what the conditions are, and what has happened before rehearsal began.

Epigenetics is the study of those directing decisions: the chemical modifications that sit above and around our genetic code, switching genes on or off without altering the underlying sequence. Your DNA does not change, but which parts of it are read, and how loudly, is something that life itself can rewrite. The key processes – DNA methylation and histone modification – essentially act as volume controls on gene expression, triggered by what we eat, what we breathe, how much stress we carry and whether we are loved. They are the molecular grammar through which experience writes itself into biology.

Research spanning 25 years of follow-up to the Dutch Hunger Winter found that those exposed to famine in early gestation had higher risks of cardiovascular disease, kidney problems, poorer cognitive function at 58, and higher rates of anxiety and depression.

But one detail changed everything: children conceived during the famine but born after it – who never experienced hunger directly – were delivered at normal birth weight. Their bodies had compensated in the womb, recalibrating metabolic expectations downward. The regulatory systems of their growth genes had been altered in ways that persisted, measurably, six decades later. The famine did not alter their genes; it altered how their genes behaved.

The implications reach further still. Some evidence suggests epigenetic marks can be transmitted across generations, so even the grandchildren of famine survivors may carry biological traces of an event they never lived through. The science here remains contested, but the direction of travel is clear: experience, in the right circumstances, can leave marks that outlast the individual.

The laboratory of identical twins

If the Dutch Hunger Winter is epigenetics at its most dramatic, identical twins are its most intimate example. Two people have, by definition, identical genetic scripts, but over a lifetime, they diverge: for example, one could develop schizophrenia, while the other does not (twins have rates of disease discordance well over 50% for most conditions). For decades this was a puzzle. Research has since shown that while twins are epigenetically indistinguishable in early life, older identical twins exhibit remarkable differences in DNA methylation and histone modification – and the more time a pair had spent apart, the more epigenetically distinct they had become. Two people, the same script. Two different lives, two different performances.

Robert Sapolsky, the Stanford neurobiologist whose Behave remains one of the most comprehensive accounts of why humans act as they do, makes the point with characteristic directness: there is no gene that forces us to act in any particular way. What matters as much as which genes you have is how those genes are affected by their context – by hormones, by stress, by culture, by the accumulated weight of experience. Nature and nurture, he argues, are not competing explanations. They are the same explanation, looked at from different angles.

The body's record

This is where clinical medicine picks up where the laboratory leaves off. Bessel van der Kolk's The Body Keeps the Score – one of the most widely read books on trauma of the past decade – documents in forensic detail how traumatic experience physically reshapes the brain and body. The title succinctly sums up the phenomenon: the body does not forget what the mind tries to set aside. Stress, neglect and fear in early life alter the nervous system in ways that persist long after the original cause has gone. Van der Kolk's work sits in productive tension with the molecular science: where epigenetics explains the mechanism, he maps the human consequences.

Gabor Maté's The Myth of Normal takes the argument furthest. Maté, a Canadian physician with four decades of clinical experience, asks what the science of epigenetics and trauma means for the societies we have built. His central argument is both simple and radical: much of what we call illness – chronic disease, addiction, mental ill-health – is not random biological misfortune. It is the body's reasonable response to unreasonable conditions. Maté calls on us to stop seeing disease as individual pathology. People with illness are a "living alarm," signalling that what passes as normal in this culture is neither healthy nor natural.

Maté's conception of trauma is deliberately broad. It is not only war, abuse or bereavement, but the quieter wounds too – the child whose emotional needs go unmet, the adult who suppresses authentic feeling to remain acceptable, the person carrying, without knowing it, the unresolved stress of the generation before them.

Laboratory research at McGill University by Michael Meaney had already shown in rats that the quality of early maternal care produced measurable, lasting epigenetic changes in how offspring responded to stress as adults – and that those changes could be reversed by a different environment. The molecular mechanism and the clinical observation point in the same direction.

Nature and nurture together

What should we do with this understanding?

At minimum, it should reframe arguments we have been having without understanding the science. The debate about personal responsibility looks different when you understand that the capacity for self-regulation, resilience and connection is itself partly a product of what happened to us before we were old enough to choose anything. The child who grows up in chaos does not simply lack willpower. They may, quite literally, have a differently calibrated stress response – written into their biology before they could speak.

This is not determinism. The epigenome is not a verdict, as Carey is clear. It is dynamic, responsive to new conditions, capable of change. But it does mean that the question "why don't they just try harder?" is, in many cases, the wrong question entirely.

The nature versus nurture debate was always a false binary. Nature without nurture is a script without a production. What epigenetics tells us – and what the lives of those Dutch Hunger Winter children, those slowly diverging twins, those patients in Maté's and van der Kolk's consulting rooms all confirm – is that the two are inseparable.

We are written by our genes and rewritten by our lives. And the conditions of those lives are not a private matter. They are, at the most fundamental biological level, everyone's business.