twin pairs (male = 311, female = 378), and 740 fraternal, or dizygotic (DZ) twin pairs (male = 230, female = 235, opposite sex = 275), for whom data were available for at least one twin at one or more waves. Twins were 12–18 years of age (mean = 14.51, S.D. = 2.10) at time 1 and 17–29 (mean = 19.63, S.D. = 2.46) at time 2. The mean age difference across waves is 5.3 years (S.D. 0.99). Twin zygosity was determined using a 9-item assessment of physical characteristics (Nichols and Bilbro, Jr. 1966) completed by the interviewer and by genotyping 11 highly informative short tandem repeat polymorphisms. Twin pairs were categorized as MZ if they had similar physical characteristics and were concordant on all markers and DZ if they presented with dissimilar physical characteristics and were discordant on any of the markers.
