These results should be interpreted in the context of four potentially important methodological limitations. First, the subjects in this study were all white twins born in Virginia. The degree to which these results would extrapolate to other samples is currently unknown. Second, to improve our statistical power with several of our measures, we examined subthreshold symptom levels rather than only diagnostic dichotomies. Thus, a reasonable amount of the information we used was from individuals with sub-diagnostic levels of symptoms. Third, the results of any multivariate analysis are keenly sensitive to the variables included. We lacked information on some potentially key diagnoses and personality traits. If we had been able to include information on obsessive–compulsive, bipolar illness or post-traumatic stress disorders, personality disorders such as avoidant or narcissistic or personality measures for psychopathy or impulsivity, our results might have differed meaningfully. Fourth, we used global model comparisons across the sexes and on that basis interpreted the resulting parameters as differing in males and females. However, we could have tested each individual parameter for equality across the sexes, letting the other parameters
