Multivariate analyses can also be used to establish to what extent genetic and environmental risk factors are particular to a given personality disorder, and whether genetic effects differ over time from a developmental perspective. Studies have indicated that DSM-IV cluster (A) personality disorders using data from Axis I-Axis II Twin studies, that heritability is 21% for paranoid, 26% for schizoid, and 28% for schizotypal personality disorder. The study found no sex difference or environmental factors. Genetic liability proportion that was commonly shared with other Cluster (A) disorder were 100% schizotypal personality disorder, 29% for schizoid personality disorder, and 43% for paranoid personality disorder, which infer that schizotypal personality disorder had the strongest genetic relationship.
Kindler et al. (2008) conducted a multivariate study to establish the genetic contribution in personality disorder and included all 10 DSM-IV PDs. The study found that the best fitting model included three genetic and three environmental, in additional to disorder-specific factors. The first genetic factor indicated high loading on PDs from all three clusters including narcissist, paranoid, borderline, histrionic, obsessive-compulsive and dependent PD. This factor was much associated with genetic liability to the normal personality trait neuroticism, which is a reflection of vulnerability to PD pathology.
The second genetic aspect was very specific with considerable loadings only on antisocial and borderline PD. This indicates that there is a genetic liability to a broad phenotype for aggressive behavior. The third factor indicated a higher loading only on schizoid and avoidant PD, which can in part reveal genetic risk for schizophrenia spectrum pathology. These studies indicate that there is an element of genetic contribution in personality disorders. Twin studies have reliably validated the significance of both genetic and environmental impacts in psychiatric disorder and symptom dimensions.
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