Table 1 gives descriptive statistics for the endophenotypes and EXT. As the table indicates, MZ and DZ twins had similar mean levels and variances. Because beta and EXT are principal component scores, their values can be negative. There were small but consistent mean differences between male and female participants, with higher levels of the endophenotypes and lower levels of EXT in females. For instance, with an overall standard deviation of 0.92 on the EXT component, the difference between males and females corresponded to an effect size of d = .51 (not shown in Table 1, which is broken down by zygosity, but offered for illustrative purposes). In addition, although our analysis used the first principal component of the five disorders’ symptom counts as our EXT measure, to insure that we had meaningful variance in EXT in this population-based sample, we determined the proportion of participants with three or more symptoms of each of the individual disorders making up the EXT score. This corresponds to those who are above the symptom threshold required to qualify for a diagnosis. Results showed that, overall, 24% of the sample had three or more symptoms of at least one disorder.
