Leiden Psychology Blog

Twins: similar and unique?

Twins: similar and unique?

'Who are the Ultimate Twins?' With this slogan a Dutch TV programme set out to find the twins who were most similar in looks, personality, and behavior. One pair had defeated 229 pairs, leaving viewers wondering about how they can be so similar yet unique.

Cases of eerily similar twins are easy to find. Take for example the story of "the Jim twins": two identical twin men who were separated shortly after birth. Even though they had been raised in two different families, they still showed some striking similarities. Both bit their fingernails, both drove the same type of car and smoked the same brand of cigarettes, and both had had the same job. Some differences also emerged: one twin was better at expressing himself through writing and the other through talking, and they favored different hairstyles.

But why are identical twins so similar?

Our personality, looks, and behavior are all determined by an interplay of genes and environmental factors. Genes are for a large part responsible for hair and eye color, height, and build. In the case of identical twins, both have exactly the same genetic make-up. Environment on the other hand consists of everything that happens around us, including but not limited to the language we speak, the way our parents raise us, and the schools we go to. These factors will largely be the same for children from the same family and are referred to as shared environment. Identical twins share not only the exact same genetic background, but also the same family environment. It is therefore not surprising that identical twins can be so similar that people have a hard time telling them apart.
However, even identical twins are not completely identical in their behavior. Think, for example, about taste in music, friends, and leisure activities. These factors are more tailored to individual taste and are therefore referred to as unique environment. So even though identical twins share the same genes and family environment, they can still be distinguished as two unique persons because of their personal preferences.

Why are researchers so interested in studying twins?

Identical twins provide a great source of information about how much genes, environment, or a combination of the two contribute to a certain behavior. For example, “the Jim twins” might have had the same inherited level of intelligence, but due to their different upbringings their IQ scores could have been very different. If one twin had grown up in a low income family, his IQ could have been more influenced by his environment (e.g. lack of books, parents not promoting education) then genetic predisposition. Conversely, if the other twin had grown up in a high income family which stimulated his education in every possible way, genetics would play a larger role in explaining why that child reached a certain IQ. Because identical twins have the same genetic background, we know that differences in behavior can only be attributed to environmental factors. In people who do not share either genetic or family background we cannot tell what causes those differences.
The discovery of the importance of this interplay between genetic background and environmental influences has shed light on the development of numerous psychological disorders. We now know that autism isn’t just caused by parenting, and that schizophrenia cannot be completely attributed to genes. However, many questions remain to be answered about why some children develop well while others struggle to find their way in our society.

Twin studies in Leiden

In an attempt to answer these questions a large longitudinal twin study is currently being conducted at Leiden University, as part of the larger Consortium on Individual Development. The departments of Developmental Psychology and Child and Family Studies will collaborate in following four age groups of both identical and fraternal twins over a 6-year period. In this study, not only genes and environmental factors will be studied; measures of brain activity, personality traits, activity patterns, and interactions between parents and child will also be taken into account. This will provide a more comprehensive picture of what factors contribute to the well-being of children and might help us to understand even more about the similarity and uniqueness of twins.

Want to find out more about this study? Visit www.samen-uniek.com for more information.

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