How twins can help us understand what makes us who we are
by Professor Jeffrey Craig MRSV, Dr Evie Kendal, Associate Professor Neera Bhatia, Dr Namitha Mohandas
The Institute for Mental and Physical Health and Clinical Translation (IMPACT), Deakin University.

We are writing to you to provide RSV members with some interesting new information about how twin studies have revealed unique and important insights into what makes each one of us who we are.

We are all familiar with the nature versus nurture debate. But what if we told you there is a much-neglected third factor that makes us who we are – nature, nurture, and an element of mystery?

To explain, let’s start with twins. Research with twins helps us tease apart the factors that shape our mental and physical character. Identical twins raised together share their genes and their environments. Yet identical twins still show obvious differences. Parents of identical twin toddlers tell us there are a range of differences between them. From size to skin markings and sociability. And some dentists have even refused to believe that twins are identical because of differences in their teeth!

So, what is this mysterious third factor that makes twins and the rest of us innately unique? Researchers refer to it as ‘developmental variation’. It turns out that this type of variation affects almost every living thing. This is because developing organisms are not robots progressing along production lines. Genes only specify general rules for how we develop, not precise outcomes. For example, your genes may create the code for how to make a hand but not how to make individual fingerprints. Genes are not micro managers.

So much of how we develop is left to chance and these random processes of development manifest uniquely in each person. As the neuroscientist Kevin Mitchell said, ‘you can never bake the same cake twice’ ( Sometimes luck, or factors beyond your control, can lead to a tastier cake, such as a good batch of flour or pulling the cake out of the oven at just the right time.

There are real-life examples of how developmental variation occurs. In all humans, fertilisation usually happens in the fallopian tubes – the passage from the ovaries, where the eggs are developed, to the womb. The location within the womb at which a fertilised egg implants can affect how well it grows during pregnancy.

Identical twins happen when one fertilised egg, known as an embryo, splits in two. If this occurs before reaching the womb, the two can implant at different locations. Each will then develop together with their own placenta and umbilical cord, the source of nutrients and oxygen needed for survival and growth. And on their journey towards birth, twins can grow at different rates. Together, these factors can influence their birthweight (

Infections from the mother can also be passed to only one twin of a pair. One example of this is a set of Brazilian twins where one was born with the Zika virus infection ( This infection inhibits brain development, which greatly reduces head size. In this case, this resulted in two very different looking but genetically similar babies.

The implications of developmental variation for human health are significant. For instance, researchers have known for some time that early life factors can influence the risk of long-lasting conditions such as heart disease, anxiety, and allergies. However, placenta size and birth weight are also factors, which means that more research is needed in these areas.

You may wonder whether we can separate the cake of life’s three main ingredients: genes, environment, and developmental variation. This remains a goal of many researchers. But just as we can’t un-bake a cake to produce flour, eggs, and sugar, we can’t completely separate out the factors that make you an individual. Things are complicated because genes, environment and developmental variation interact. For example, developmental variation influences the environment we experience in the womb and biological parents interact with their children with whom they share genes. Genes can also influence us to seek out specific environments (

Data from 200,000 twin pairs showed that genetic factors are not the primary contributor to genetic disease ( The effect of environment was even lower and developmental variation was by far the largest contributor. The implications of this are profound. Without specific genes or environments to target, full disease prevention may never be possible ( But the small contribution of environment in this data does not mean that environments are never important. Extreme environments, such as abuse and trauma, can differ between individuals and populations. For example, half the world lives on less than $2.50 a day (, and undernutrition causes almost half of all child deaths ( Even in Australia, one in three children grow up in families experiencing adversity ( or in situations of domestic abuse ( Both exposures increase their chances of having problems with learning or mental health.

The extent to which we are influenced by developmental variation confirms what researchers already knew but frequently forget: data from population studies cannot be applied to individuals. It is impossible to say how much each of us is influenced by genes, environment, and developmental variation. There is no ‘average’ individual, and we are all truly unique – baked to our own perfection.

So, what does this mean for people hoping to give their children the best start to life? You are doing the right thing and you can make a difference. Watch with wonder as you see your own genetics reflected in your children and expect the unexpected.