retest interview (Wave 3, n=1,370, median age=19). Between 2002 and 2005, all twins from the target cohort (excluding those who had withdrawn or whose parents asked that the family not be re-contacted) were contacted for Wave 4 interviews (n=3,787, median age=22; 80% of the twins identified from birth records). Wave 5 interviews were conducted between 2005 and 2008 (n=3,428 twins; median age=24).
