Data on the male–male pairs (MM) came from a sample (birth years 1940–1974) initially ascertained, with a 72% cooperation rate, directly from registry records. The first interview (MM1) was completed largely by telephone in 1993–1996 and was followed by a second wave of interviews (MM2), conducted in 1994–1998, with a response rate of 83%. The third wave (MM3) was completed from 1998 to 2004 in 77.8% of the eligible sample. Zygosity was determined by discriminate function analyses using standard twin questions validated against DNA genotyping in 496 pairs (Kendler & Prescott, 1999). The mean (S.D.) age and years of education of the twins were 36.3 (8.2) and 14.3 (2.2) at the FF4 interview, and 37.0 (9.1) and 13.6 (2.6) at the MMMF2 interview.
