Why are twins so important to research?

Imagine two people who have virtually identical genes but do not necessarily develop the same diseases or live equally long lives. That is the reality for identical twins - and one of the reasons they are so interesting to researchers.

Professor Kaare Christensen, who heads the Danish Twin Registry at the University of Southern Denmark, explains what twins can teach us about health, disease and the lives we lead.

1. Why are twins so interesting to researchers?

Researchers have been fascinated by twins for more than 100 years. Partly, of course, because identical twins can look remarkably alike, while fraternal twins can be very different.

But traditionally, one of the most important contributions of twin research has been to help us understand why we differ from one another.

What makes twins special is that they give us an opportunity to investigate how much our genes contribute to a particular disease or characteristic, and how much is due to the things we experience and are exposed to throughout our lives.

2. How can twins help researchers understand the role of genes?

We compare how similar identical twins are to each other with how similar fraternal twins are.

For example, if we are studying a particular disease and find that both identical twins develop it more often than both fraternal twins do, that suggests genes play a role.

In this way, we can investigate how much genes contribute to what we are studying, and how much is influenced by the environment.

3. What can you learn from twins who lead different lives?

With identical twins, we have, in a sense, "the same person genetically", but the two individuals are exposed to different things and lead different lives.

This has helped researchers better understand some very complex relationships. For example, we know that people with more years of education tend to live longer than those with fewer years of education.

This may partly be due to a "direct effect of education": education may lead people to make healthier lifestyle choices and enjoy better working and living conditions.

But twin studies have shown that part of the explanation is also that people who are already healthy and have favourable living conditions are more likely to pursue and complete longer education programmes.

Among identical twins with different levels of education, the difference in survival was considerably smaller than we would otherwise have expected.

This suggests that education itself is not the only reason why people with more years of education tend to live longer.

4. Why is Denmark particularly important for twin research?

At the University of Southern Denmark, we are especially fortunate to have the Danish Twin Registry, which includes twins born as far back as 1870 and up to 2021 – spanning three centuries.

In total, more than 97,000 pairs of twins have contributed to research over the years.

And none of this would have been possible without the generations of Danish twins who have willingly taken part in our studies and patiently answered all our questions.

So we would also like to take this opportunity to say a very big thank you to them.

5. What has twin research taught us that we might otherwise never have discovered?

Some of the most important contributions of twin research over the years have been in the field of psychiatry.

For example, twin research helped demonstrate that schizophrenia was not caused by having a "cold mother", as was widely believed for much of the 20th century.

Instead, researchers were able to show that genetic vulnerability plays a major role in the development of schizophrenia.

Similarly, twin research has shown that obesity has a substantial genetic component. Of course, environmental factors also play a role, but research has shown that maintaining a normal weight is considerably easier for some people than for others.

These findings have had a major influence on public health thinking and on how we understand a range of diseases and conditions.

And what can twins tell us about COVID-19?

In our new study, we examined data from more than 83,000 Danish adult twins and over 32,000 of their spouses.

We found no evidence that genetic factors played a substantial role in determining who was hospitalised with COVID-19. By contrast, spouses of people who were hospitalised had a markedly higher risk of being hospitalised themselves.

This suggests that a shared environment and close contact may be more important than genetic similarity. However, our study cannot tell us exactly what explains the increased risk.