The genes conundrum-why some twins talk and think in unison

An interview with Paula and Bridgette Powers – identical twins who witnessed their mother’s carjacking – recently went viral. The way they spoke and gestured in unison has captivated global audiences. Genetically, identical twins are clones. They result from the splitting of an early embryo, meaning they share the same genes. In contrast, fraternal twins are the result of two eggs being fertilised by two different sperm.

On average they share 50 per cent of their genes – the same as any siblings who share both their biological mother and father. So, when identical twins talk and gesture in unison (known as synchrony), is it down to genes? The answer can be complicated.

Genes aren’t the only influence on looks, language and like-minded thinking.

Let’s break down the factors that might lead some twins to speak – and apparently think – in unison. A close bond in a shared environment Almost all twins, even “identical” ones, show some differences in physical, mental and emotional traits. They also regard themselves as distinct individuals and typically don’t like being referred to as “the twins”.

Yet we know most people naturally mimic the way those close to them speak and move, even without realising it. This phenomenon is called automatic mimicry and may be part of healthy social development, helping people synchronise behaviours and share emotions. For identical twins who grow up in the same home, school and community, the effect of a shared environment and close bond may be particularly intense.

Paula and Bridgette Powers, for example, have shared an environment: not only the same parents, home and upbringing, but also the same job, running a bird rescue charity. Twins may know each other so well that they can intuitively sense what the other twin is about to say — and may feel like their brains are in sync.

What about genetics? Studying identical and fraternal twins separated at birth can help us unravel how much of our behaviours – intelligence, personality and temperament – are influenced by genes and environment. Even when identical twins grow up apart, they tend to closely resemble one another – not only physically, but in their personality, interests and behaviours. Fraternal twins, in general, are much less alike. This tells us that genes matter.

One of us (Nancy) was a researcher with the Minnesota Study of Twins Raised Apart, which lasted from 1979 to 1999 and looked at more than 100 sets of twins (and triplets) separated at birth and raised apart. Twins were separated for various reasons, such as the stigma of single motherhood, inadequate family resources and maternal death. The study comprehensively examined factors affecting a wide range of psychological, physical and medical traits. The researchers wanted to understand the impact of differences in their life histories on both identical and fraternal twins, reared apart and how they affected the current similarities and differences between them. A striking finding was identical twins raised apart are as similar in personality as identical twins raised together. For example, the Minnesota researchers found little difference in traits such as wellbeing and aggression, whether identical twins were raised together or apart. This shows genes play an important role in shaping our personality. Genes also affect the way we process speech and language.

It also found that when identical twins were reunited they formed closer relationships with each other than reunited fraternal twins did. This suggests perceptions of similarity in behaviour might draw people together and help keep them connected. We now know genes and environment each account for half the person-to-person differences in personality. However, the life events we individually experience remain the most important factor shaping how our unique traits are expressed and who we ultimately become.

(Jeffrey Craig is with Deakin University and Nancy Segal is associated with California State University, Fullerton)