From Romulus and Remus – the myth of the twin brothers who founded the city of Rome – to the synchronised speech of Australia's ’Twinnies’ Paula and Brigette Powers, identical siblings have long inspired our fascination and imagination.

At school, those of us who aren’t twins were often envious of siblings so alike they could swap around, tricking their teachers or parents.

Identical twins, also known as monozygotic twins, form when a single fertilised egg splits into two embryos. Picture: Shutterstock

At Twins Research Australia, we work with 75,000 twins to understand more about both their similarities and their differences, and how that relates to human health.

Recently, my colleague Professor Sarah Wilson and I contributed to the TV Show Australia’s Most Identical, where 100 sets of twins take tests and challenges to try and win the crown of the nation's most identical pair.

So, what's the the science of identical twins? How do they form? How alike are they really? And what can they tell us about genetics, environment and human development?

How identical twins are formed

Identical twins, also known as monozygotic twins, form when a single fertilised egg, called a zygote, splits into two embryos.

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This usually happens in the first two weeks after conception and the timing of the split influences whether the twins share a placenta or an amniotic sac.

Earlier splits result in more independent development. Later ones can lead to shared structures – or, in rare cases, conjoined twins.

For many years, scientists believed this was a simple 'fission' or division process. But newer theories suggest the picture is more complex.

Some researchers propose that two cell clusters may develop independently within one embryo, rather than splitting.

But the exact mechanism is still unclear.

Baby twin girls with their Dad — At birth, identical twins have almost identical epigenetic profiles. But over time, differences emerge. Picture: Supplied

Twin synchrony and 'twintuition'

Identical adult twins, like Paula and Bridgette Powers, who have spent almost every moment of their lives together, often report a sense of 'twintuition' – an intuitive awareness of what their twin is thinking, feeling, or doing.

Sometimes, this even results in them speaking in perfect synchrony.

It’s easy to see why people refer to this as 'twin telepathy'.

But it becomes harder to explain when twins who aren’t in the same room say they can feel or even see what their twin is experiencing.

When we tested this ability in Australia’s Most Identical, we saw fascinating results – but you can decide for yourself whether this phenomenon might actually be real.

How genetically identical are identical twins?

Identical twins are often said to share 100 per cent of their DNA. While this is mostly true, modern genetic technology tells a more detailed story.

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After the embryo splits, tiny changes – called post-zygotic mutations – can occur in one twin, but not the other.

These changes are usually harmless, but they mean that no two identical twins are completely identical at the DNA level.

These genetic differences are subtle – just a few letters out of three billion. But they can influence traits or health risks, especially if they affect genes that regulate development or immune responses.

This helps explain why one twin might develop a condition like cancer or autism while the other does not.

How epigenetics contribute to the differences between identical twins

Epigenetics refers to the chemical tags that sit on our DNA and help control which genes are turned on or off.

Young twins in striped shirts sharing strawberries — Identical twins usually occur randomly, at a constant rate of about three to four per 1000 births. Picture: Getty Images

These tags don’t change the DNA itself, but they do affect how our cells read it. Epigenetic patterns are influenced by things like diet, stress, pollution and maternal health during pregnancy.

At birth, identical twins have almost identical epigenetic profiles. But over time, differences emerge.

One twin may exercise more, eat differently or experience different stressors.

These experiences change their epigenetics, which in turn affects how genes are expressed. Over the years, these changes can lead to differences in personality, health or how each twin responds to medications.

Other factors create differences between identical twins

Even before birth, twins can have very different developmental experiences. For example, where each embryo implants in the uterus affects how much blood flow and nutrition it receives, which can influence their birth weight and growth.

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This is known as developmental variation. It’s a key reason why genetically identical twins can end up so different.

Genes are like a recipe, but the environment and random chance can influence how that recipe turns out.

As the neuroscientist Kevin Mitchell said, “you can’t bake the same cake twice.”

Are twins really hereditary?

There’s a common belief that identical twins run in families – but that’s not true.

Fraternal (dizygotic) twins, who come from two separate eggs, can be hereditary, especially if the mother has a tendency to release more than one egg at a time.

Twin girls in car — From tiny genetic changes to the random events of development, identical twins are both alike and distinct. Picture: Supplied

Identical twins usually occur randomly. They appear at a constant rate around the world – about three to four per 1000 births – regardless of ethnicity or family history.

There is some evidence that assisted reproductive technologies (like IVF) may slightly increase the chances of identical twinning.

Twins can have different health conditions

Identical twins don’t always share the same health outcomes. One might develop asthma, diabetes or depression – while the other stays healthy.

These differences can arise from epigenetics, from unique life experiences or from variations in how the twins developed in the womb.

For instance, if one twin received more nutrients during pregnancy, that might affect their lifelong risk of heart disease or obesity.

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Infections, medications and trauma can also affect one twin but not the other. These differences show us how important environment and timing are in shaping health.

How twins help research

Twin studies are a powerful way to understand what shapes human biology. By comparing identical and fraternal twins, researchers can estimate how much of a trait comes from genetics and how much from the environment.

Studies of identical twins, in particular, allow researchers to infer cause and effect.

Twins Research Australia, based at the University of Melbourne, has been conducting these studies for over 40 years. Its work has revealed genetic links to breast cancer and epilepsy and shown how lifestyle affects brain ageing.

It has also helped us understand how genetics influences learning, memory, and mental health.

A father holds his twin daughters upside down — Twin studies are a valuable window into what shapes us all. Picture: Getty Images

With over 35,000 twin pairs in its database, Twins Research Australia is helping to answer some of life’s biggest questions – about health, personality and how we become who we are.

So, just how identical are identical twins?

The answer is: not as identical as you might think.

From tiny genetic changes to the random events of development, identical twins are both alike and distinct.

It’s precisely these small differences – and powerful similarities – that make twin studies such a valuable window into what shapes us all. And this is exactly what identical twins themselves tell us: they are true individuals.

Australia’s Most Identical airs on Channel 9 from Wednesday 25 June 2025.

Twins of all ages, identical and non-identical, are welcome to join in research. Learn more at twins.org.au

This article was first published on Pursuit. Read the original article.

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