Genetic epidemiology of mental health : twin and family studies of personality disorders, phobias, and symptoms of anxiety and depression

Abstract

In this thesis we use genetically informative samples of twins and nuclear families to estimate the relative influence of genetic and environmental risk factors for a varied set of mental disorders. <br> In the first two papers, we consider personality disorders (PDs), a class of psychopathology characterised by marked deviations from contemporary expectations of society. To date there have been no large population based twin studies of personality disorders as assessed by structured interviews. We find that most PDs are moderately heritable, but see little empirical support for a grouping of three PDs into what is referred to “cluster C”. There was some evidence of shared environmental effects in Passive Aggressive PD, but not in the cluster C disorders.<br> In paper three we conduct a multivariate twin study on five kinds of phobias, to gauge the extent to which the genetic and environmental risk factors are common across the different diagnoses. We find the best model to contain two distinct liability factors, both of which are highly heritable. The first loads principally on animal phobia, while the second loads most heavily on the complex phobias, agoraphobia and social phobia. We also find that the genetic influence on blood phobia is largely unique to this disorder. For the phobias we find no evidence for common environmental influences. In the forth paper we estimate an upper limit to the heritability of symptoms of anxiety and depression in the Nord-Trøndelag health study (HUNT). We find that these symptoms, as assessed by the ten item checklist (SCL-10), are less heritable than twin studies typically report, and we discuss possible reasons for this discrepancy.