Altogether, 2,794 twins (44% of those eligible) were interviewed. Noncooperation was overwhelmingly the result of nonresponse to the written invitation; active refusals were rare (0.8%) (26). The study was approved by the Norwegian Data Inspectorate and the Regional Ethical Committee, and written informed consent was obtained from all participants after they received a complete description of the study. Of those interviewed, 36.5% were male, and the mean age was 28.2 years (SD=3.9).
