Twins were recruited from the NIPHTP (25). Twins in the NIPHTP were identified through the Norwegian National Medical Birth Registry, which was established on January 1, 1967, and receives mandatory notification of all live births. Questionnaire studies were previously conducted in 1992 (twins born 1967–1974) and in 1998 (twins born 1967–1979). Altogether, 12,698 twins received the second questionnaire, and 8,045 (3,334 pairs and 1,377 single responders) responded after one reminder (cooperation rate, 63%).
