Twins have been at the heart of the nature versus nurture debate since Darwin's cousin, Francis Galton, started looking at the issue over a century ago. Today, more than 1.5 million twins around the world take part in studies aiming to assess the relative roles of genes and the environment in everything from ageing to disease, and from bullying to religious belief. These studies rest on a few simple assumptions. Twins usually grow up together, so share the same environment. Identical twins develop when a fertilised egg splits in two, so their DNA is exactly the same. Non-identical twins develop when separate eggs are fertilised by separate sperm, so their DNA differs. If identical twins are more similar with regard to a particular trait than non-identical twins, the rationale goes, then that trait - hair colour, say - must be genetic. If identical twins are no more similar than non-identical twins with regard to a trait - such as the language they speak - that trait is more likely to be environmental. But cases like that of the Dutch twins threaten to throw a spanner in the works, so researchers were keen to discover what made them so physically different. The first suggestion was that the twins were not identical. ''You can find identical twins who differ genetically, but they're the exception rather than the rule,'' says Dorret Boomsma of VU University Amsterdam, a member of the team that studied these two girls. Two things can make identical twins genetically different. Sometimes, when a fertilised egg splits, mistakes are made. In extreme cases, entire chromosomes can be present in one twin but absent in the other. This turned out to be the case for identical triplets born in 1983. One lost a Y chromosome when the egg split, so the triplets developed into two boys and a girl. Even when eggs split with no genetic errors, mutations later on can lead to differences. If a mutation occurs early in development, almost all of the cells in one twin may inherit it, while none of the cells in the other twin will have it. Most mutations have no discernible effect, but occasionally they hit key genes. The characteristics of the Dutch twin with a divided spine pointed to a particular gene, because they resembled those of an unusual mouse strain with a bifurcating tail. These mice have a mutation in a gene called Axin, which helps guide body layout during development.

So the team sequenced the gene in each girl but were surprised to find no difference. That led them to wonder if something else had happened to prevent the Axin gene from working. We have long known of epigenetic marks - chemical labels added to DNA that alter the activity of genes without altering the sequence. In particular, if a stretch of DNA has lots of added methyl groups, the activity of nearby genes is suppressed. So the team took a closer look at the Axin gene in blood cells from the twins. Sure enough, the girl with the split spine had unusually high levels of methylation. So while other causes cannot yet be ruled out, the researchers think the most likely explanation is that in one twin something pushed methylation levels high enough to shut the gene down. Mystery solved? Far from it. What pushed methylation levels above a critical threshold in one twin but not in the other? ''That's the million-dollar question,'' says team member Nick Martin, of the Queensland Institute of Medical Research in Australia. What's more, many other differences between twins are also being linked to variations in methylation. It is now relatively cheap and easy to study methylation levels, so the last few years have seen a surge in research. Of particular interest are identical twins where one has a particular condition or disease and the other does not. For a wide range of disorders including cancer, rheumatoid arthritis and autism, researchers have found different methylation profiles in the affected twins.

Even more intriguingly, differences in methylation are being linked to differences in behaviour. For instance, in one pair of identical twin sisters - one a war journalist, the other a risk-averse office manager - differences were found in a gene implicated in stress and anxiety. No one is claiming that these marks alone explain the sisters' different behaviours. But they might help explain why the journalist is less anxious in dangerous situations, which could have influenced her career choice. Or it could be that the methylation differences between the women are the result of their different behaviours and environments, rather than the cause. None of the twin studies prove that methylation differences trigger diseases or alter behaviour. ''The findings are correlative,'' cautions epigeneticist Jonathan Mill, of the Institute of Psychiatry at King's College London, who has carried out such studies. Indeed, it is clear that much, if not most, epigenetic variability is driven by the world we live in. Studies show that all kinds of environmental factors, from pesticides and pollutants to diet, smoking and alcohol, can alter methylation patterns. And once methylation patterns have changed, there can be lasting effects. When smokers kick the habit, for instance, their methylation patterns rapidly return almost to normal. But some changes can persist for decades - perhaps helping to explain why ex-smokers remain at an increased risk of cancer and respiratory problems years after they quit. Many studies suggest that particular methylation changes contribute to cancers, implying that methylation changes can be both effect and cause. The environment has a key role in shaping our epigenetic profiles, which in turn influence the activity of our genes, which in turn may shape our behaviour, choices and health - our environment - and so it goes on. That might explain why the epigenomes of identical twins diverge over the years, as a 2012 study showed. ''It could be that the methylation patterns of identical twins become more dissimilar because they experience increasingly different environments,'' says Bastiaan Heijmans, of Leiden University Medical Centre in the Netherlands, who led the study. Our epigenetic profiles, it seems, mimic our individual, divergent paths, environments and experiences. They are as unique as we are.

But if so much is down to the environment, how can identical twins be different even before they are born? Researchers Jeff Craig and Richard Saffery, of the Murdoch Children's Research Institute in Melbourne, have identified unique methylation profiles in identical twins born as early as 32 weeks. This could partly be due to subtle physical differences, such as variations in the size of their umbilical cords. It might also be partly due to random events, such as a failure to copy epigenetic marks when cells divide. A small change in a single cell early in development could end up affecting many organs in the resulting adult, for example. Andrew Feinberg of Johns Hopkins University School of Medicine in Baltimore, Maryland, thinks that not only are some of the epigenetic differences between individuals a result of random events, but that this randomness is built-in - an evolved feature. Feinberg thinks it is a way for evolution to hedge its bets. Many animals have to survive in a constantly changing environment. Random epigenetic changes produce more variation in genetically similar offspring, increasing the chances that some of them will survive, he argues. If your head is starting to spin, brace yourself. It seems that the amount of random epigenetic variability can itself vary depending on the environment. In mice given certain dietary supplements, there was increased variability in their methylation patterns.

Just how important these random variations are is not yet clear. The ideal study would be to raise a batch of clones in exactly the same environment and see how they turn out. This clearly cannot be done with people, but it can be done with mice. In one such experiment, 40 radio-tagged mice spent three months living together in the same five-storey cage, decked out with flower pots, tubes and toys, while researchers recorded their every move. At first the mice behaved similarly, but over time their exploratory patterns began to differ. ''They developed different personalities,'' says team member Gerd Kempermann at the German Centre for Neurodegenerative Diseases in Dresden. The study adds to the evidence that animals can turn out differently even if their genes and environment are identical. What it also suggests is that these differences can arise through a dynamic, interactive process. So a slightly more active mouse might explore a little more than a less active one. It might bump into more of its cage mates and take an enjoyable tumble down a plastic tube, which might in turn fuel its wanderlust, making it better at climbing and more likely and able to seek out further new experiences. Kempermann found that the most adventurous mice grew the most new neurons in their hippocampus, a brain region linked to learning and memory. Tiny initial differences become amplified, feeding back to biology and behaviour, sculpting individuality. ''Epigenetics is a potential mechanism to explain our findings,'' Kempermann says. Epigenetic variations could arise randomly or as a result of physical differences in the womb, or a mixture of both. ''Twins may share the same womb, but experience it very differently,'' says Craig. These tiny initial epigenetic differences might influence gene activity and sculpt our interaction with the environment, which then feeds back into the epigenome, amplifying the message. ''The environment isn't what happens to us. We make our own environment,'' says geneticist Robert Plomin, of London's Institute of Psychiatry. Add a dash of serendipity - one twin having an accident or illness - and these experiences set them on a trajectory to individuality.

These findings suggest there is more to our uniqueness than our genes and upbringing, that even if we were just one of thousands of clones we would still all end up different in some ways. Where does this leave the nature versus nurture debate? It is clear some traits, such as hair colour, are mostly down to genes, whereas others, such as language, are due to the environment. But you could argue there's a third factor - call it chance or serendipity - in the form of random events occurring in our bodies or the environment. That may be why the Dutch twins were so different. What's more, many aspects of our bodies and behaviours seem to be the result of complex interactions between genes and the environment, mediated by epigenetics and with a large dash of chance thrown in. In these cases it seems pointless arguing about nature versus nurture. ''The debate is outdated,'' says epigeneticist Manel Esteller, of the Bellvitge Biomedical Research Institute in Barcelona, Spain. ''It doesn't make sense any more.'' New Scientist