Zygosity for same-sex pairs was determined by two methods. During face-to-face assessment sessions throughout childhood, interviewers rated each twin pair on a nine-item assessment of physical characteristics (Nichols & Bilbro, 1966). The mean number of judgments for the 293 pairs was 18.9 for this longitudinally followed sample. Ratings were completed independently by two individuals at each occasion. These ratings were used to make a judgment of zygosity prior to the advent of routine DNA collection. A second zygosity determination was based on a more recent genotyping of 95% of the sample at a minimum of 11 highly informative short tandem repeat polymorphisms using standard polymerase chain reaction methods and ABI 377 genotyping technology. DNA was extracted from epithelial cells collected by noninvasive cheek swabbing (Meulenbelt, Droog, Trommelen, Boomsma, & Slagboom, 1995). The average heterozygosity of the markers exceeded .75 and gives a posterior probability of MZ misdiagnosis of less than .0001. Marker discordance for members of a twin pair indicated DZ origin, while marker concordance across all genotyped markers indicated their MZ origin. For all pairs in the current analyses, tester ratings and genotype data were in agreement.
