Genetic and environmental architecture of psychotic and obsessive symptoms are not completely elucidated.
This study estimated for these symptoms (i) the genetic and environmental components, (ii) the withinindividual
association, and (iii) the extent to which this association originates from common genetic and
environmental factors. Young adult twins (N=701) from the population-based Italian Twin Register were
assessed for psychotic and obsessive–compulsive symptoms by using the Symptom Check List (SCL-90).
Multivariate Cholesky models were fitted by the Mx statistical program. No previous study used this design to
examine the same dimensions. The best-fitting model included additive genetic and nonshared
environmental components, each accounting for about half of total variance in the symptoms. Genetic
influences on the different symptoms overlapped considerably (rg=0.81 to 0.99). Phenotypic correlations of
psychotic symptoms and of psychotic with obsessive symptoms were high (r=0.61 to 0.76), with 53% to 69%
explained by shared genetic effects. This study shows substantial genetic influence on psychotic and obsessive
symptoms, and indicates that their co-occurrence may be due to genetic factors to a greater extent than to
environmental effects. These results encourage the search for genetic and environmental factors underlying
the covariance between different psychotic traits as well as between psychotic and obsessive traits.
