Why a twin register?
The Netherlands Twin Register (NTR) was founded on February 1st 1987 at the Vrije Universiteit in Amsterdam for the purpose of conducting scientific research. A large number of families with young twins are registered with the NTR. These twins are followed in their development from birth. Another important research project of the NTR focuses on the health and lifestyles of adolescents and adults. More than 25,000 twins and multiples over 18 years and over 62,000 twins and multiples between 0 and 18 years are registered with the NTR. All in all over 175,000 subjects (multiples, parents, siblings, spouses etc.) are registered.
The aim of the NTR is to examine the contribution of hereditary predisposition to personality, growth, development, disease and risk factors for disease. Multiples are not different from singles, but with the help of twins we can determine to what extent differences between individuals are to be contributed to hereditary and environmental factors is unique. This makes twin research unique.
For example: one person has high blood pressure and the other low blood pressure. Are these differences the result of environmental factors, such as the use of salt, or are they hereditary? Usually both factors are important and the aim of the research is to determine how large the contribution of genetic factors is.
How do we determine whether hereditary predisposition plays a role in behaviour and disease? To do this we need twins; both monozygotic and dizygotic twins. Monozygotic twins are genetic copies, they both have exactly the same genetic material. In contrast, dizygotic twins are genetically similar to ordinary brothers and sisters; they share on average 50% of their genetic material.
Now suppose genetic factors have an influence on smoking. In that case monozygotic twins will look more alike in their smoking habits than dizygotic twins. After all, monozygotic twins have exactly the same hereditary predisposition.
We are also interested in the phenomenon "twins" as such. For instance, what determines the fact that in some families more twins are born than in other families? Does being part of a twin have specific influences on the development and experience of a child?