Panic syndromes in a population-based sample of male and female twins

Background. The risk for panic disorder (PD) is substantially increased in relatives of probands with PD. Prior literature provides only limited infor mation about the degree to which this increase is due to genetic factors or family environment. Methods. In personal interviews with both members of 3194 twin pairs, we as sessed the lifetime history of lifetime panic attacks and PD. Twin resembla nce was assessed by tetrachoric correlation and single and multiple thresho ld biometrical model fitting. Results. As fully syndromal PD, by DSM-III-R criteria, was too rare to anal yse usefully we examined four other dichotomous definitions of increasing s tringency: panic probe and very broad, broad and intermediate PD. For all f our definitions and for the multiple threshold analyses, the best-fit model indicated that twin resemblance was due solely to genetic factors with a m oderate heritability (33-43 %). For the broad and intermediate dichotomous definitions of PD, however, a model with twin resemblance due to familial-e nvironmental factors fit nearly as well. No gender effects were seen on the genetic risk factors for these PD-like syndromes. Conclusion. Even with large epidemiological samples of twins, studying diso rders as uncommon as PD is problematical. Despite these difficulties, our r esults suggest that: (i) narrowly and broadly defined PD are probably on th e same continuum of liability; (ii) twin resemblance for these PD-like synd romes is likely due largely to genetic factors with a moderate level of her itability although a contribution of familial-environmental factors cannot be excluded; and, (iii) the same familial risk factors impact, to a similar degree, on the liability to PD in males and females.