Nature and nurture
Nature and nurture go together. Everybody finds themselves at all times in an environment and they bring along a number of inherited characteristics into this environment. Researchers are able to observe a product of the interaction between nature and nurture. All the cells of the body contain the same genome. Twins contribute with useful information when the object is to study to what extent and how genes and environment affect complex properties such as development and health.
Twin studies are conducted to find out why people develop differences. This applies to illnesses, behavior, and other conditions. Twins are interesting in research because they are alike. They have shared the same intra-uterine life. They were born at the same time. They go together chronologically in time. Usually twins grow up in the same family with the same childhood conditions and – if they are identical twins – they share exactly the same genes. If monozygotic twins are more alike in a certain trait than dizygotic twins are, then it indicates that genetic factors play a role for that trait. Twins are not always alike – often it is only one of the twins who catches a disease, while the other twin is not affected. Why? Epidemiological research is trying to find the answers to these questions.
The role of genes
It is difficult to determine what is heritable. We know that family members are alike – but we do not know whether it is because they share the same environment or because they share genes. Therefore it may be considered as nature’s gift to research when individuals are born who are genetically identical – i.e. the same individual twice. This means that we can follow e.g. the aging process of monozygotic twins and compare with dizygotic twins who share approximately half of their genes and thus are alike as ordinary siblings.
The Danish Twin Registry
In Denmark we have exceptionally good possibilities for carrying out twin research projects. This is because The Danish Twin Registry comprises almost all twins born in Denmark since 1870 and up until today. The Danish Twin Registry is among the largest and most comprehensive twin registries in the world.