Setting up and utilizing Norwegian twin panels

Abstract

In the late 1970s, a Norwegian twin panel was set up. It included all like-sexed twin pairs, born in the period
1915 to 1960, where both members were alive and had a known address in Norway at the time. The work
was initiated through a grant from the National Institutes of Health in the United States. The aim was to estimate
maternal effects to understand the causes of variation in traits and diseases that originate in pregnancy.
However, the twin panel was also utilized for estimating genetic and environmental effects on a series of
phenotypes, for instance lipoproteins, receptors, coagulation factors, cognitive abilities, educational attainment
and left-handedness. A short zygosity questionnaire was sent together with the first invitation letter to
the twins. Later questionnaires on general health, lifestyles and pregnancy outcomes were sent out. One
important finding was that the fetal genotype had much stronger influence on variation in fetal growth than
what had been assumed previously. In the early 1990s, the New Norwegian Twin Panel was created, based
on information on plural births in the Medical Birth Registry of Norway (MBRN). The panel included both
like-sexed and opposite-sexed pairs born in the years 1967 to 1974. After linking the MBRN to present
addresses, using a national identification number, a questionnaire on zygosity, with a few items on health
and social background, was sent out. This panel was later expanded to include twins born 1975-1979 and to
collect more data on health, well-being and lifestyle factors. The data have been utilized for a series of subprojects,
including psychiatric interviews and the collection of DNA samples. Linkage to Norwegian health
registries has provided important research opportunities for a variety of phenotypic outcomes.