MAJOR DEPRESSION AND PHOBIAS - THE GENETIC AND ENVIRONMENTAL SOURCES OF COMORBIDITY

In a population based sample of 2163 personally interviewed female twi ns, substantial comorbidity was observed between DSM-III-R defined maj or depression (MD) and 4 subtypes of phobia: agoraphobia, social phobi a, animal phobia and situational phobia. However, the level of comorbi dity of MD with agoraphobia was much greater than that found with the other phobic subtypes. We conducted bivariate twin analyses to decompo se the genetic and environmental sources of comorbidity between MD and the phobias. Our results suggest that a modest proportion of the gene tic vulnerability to MD also influences the risk for all phobic subtyp es, with the possible exception of situational phobias. Furthermore, t he magnitude of comorbidity resulting from this shared genetic vulnera bility is similar across the phobic subtypes. By contrast, the non-fam ilial environmental experiences which predispose to depression substan tially increase the vulnerability to agoraphobia, have a modest impact on the risk for social and situational phobias and no effect on the r isk for animal phobias. The increased comorbidity between MD and agora phobia results, nearly entirely, from individual-specific environmenta l risk factors for MD which also increase the risk for agoraphobia but not for other phobias.