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Table 1. The Components of the Finnish Twin Cohort

Birth years

Like-sexed pairs

Opposite-sex (OS) pairs


Before 1938

13,888 pairs of known
zygosity (compiled 1974)

Not identified




5017 candidate pairs,
estimated c. 85% twins

New cohort compiled
in 1996 from CPR



3047 twin pairs



21,958 pairs of which 7922
OS-pairs (compiled 1987)
born 1958-1986


FinnTwin16: twins
born 1975-79
FinnTwin12: twins
born 1983-1986 &
1987 (added later)

Follow-up in 2011-2012

A fourth wave was initiated in 2011 with a pilot study to test the questionnaire. Since the 1990 questionnaire, many twins have been asked to participate in selected substudies which have focused on specific study questions and traits, but no unselected questionnaire survey had been conducted after that. While many follow-up outcomes can be assessed by medical register linkage-studies in Finland, many health-related factors need to assessed by interview or questionnaires given to the participants. To test the acceptability of what turned out to be an extensive questionnaire, we sent 254 pairs with both members alive and 299 twins from pairs where only one twin was available (due to death, migration or lack of address of the other twin) a questionnaire in February-March 2011. The response rate was 70% for full pairs, but only 55% for ‘single’ twins. With approval from the ethical committee, we revised the invitation cover letter to the ‘single’ twins to better motivate their participation in the 35 year follow-up.


The content of the questionnaires was a balance between retention of items used in the earlier questionnaires in order to get 2,3 or 4 measures over time and a need to assess age-specific outcomes of interest, such as intentions to retire. Despite the development of better measures for some topics, we retained the original questions (such as used in 1975 and 1981) wherever possible. A total of 241 items were included in the questionnaires, covering domains such as twin pair relationships, general health and risk factor measurements (blood pressure, diabetes, cholesterol), standard symptoms (chronic bronchitis, dyspnea), musculoskeletal pain, bruxism, history of common diseases and use of various medications including hormone replacement therapy for women. We asked six questions on sleep and seven items on work and retirement. A detailed smoking history was taken again supplemented by nicotine dependence scales (FTND,  WISDM) and exposure to second-hand smoke over their lifetime. Quantity, frequency and problem measures of alcohol use and abuse were asked, including a modified MmMAST. Coffee and tea use were queried. Physical activity  and sedentariness were asked using 10 questions. Life events and traumatic experiences   in childhood and adulthood were assessed. Close social relationships and social support were asked using seven items. Depression was assessed using the CES-D scale and the short extraversion and neuroticism scales from 1975 & 1981 were repeated. Finally a history of weight, weight changes and dieting were included. The invitation letter included a tape measure, and the respondents were asked to record their waist circumference; preliminary data indicates that 98% of subjects reported their waist circumference.Of these twins, 2182 have already given a DNA sample in earlier studies of disease-related phenotypes. They will be requested to give a new consent for genotyping of study-related traits, while others will be requested to give a saliva sample for DNA. This procedure has not yet started.