The fourth wave of data collection was started by inviting the twins from the intensive sample to participate in an in-laboratory protocol at a mean age of 22 years (range 20 – 26). For those unable to participate in the full protocol (in-lab assessments conducted in Helsinki; in Oulu or Turku for some participants), we offered an abbreviated protocol. This protocol included the SSAGA interview via telephone, postal questionnaires and providing a DNA sample. Zygosity was originally determined using well-validated questionnaires in adolescence and young adulthood (Sarna et al. 1978, Goldsmith 1991). For more than 90% of the same-sex twins in the intensive sample, zygosity was confirmed from DNA and found to be correct in 94% of cases.
