Twin studies use a method where identical twins (monozygotic) sharing 100% of their genes are compared to non-identical twins (dizygotic) who share 50% of their genes.
Disorders that are entirely environmental would be found at the same rates in non-identical twins and indentical twins. Disorders with a strong genetic contribution would be more likely to be found in monozygotic twin (where one twin is affected) than in a dizygotic twin (where one twin is affected).
Antisocial personality disorder (ASPD) is an early-onset disorder characterized by aggressive behavior, frequent rule breaking and failure to consider the effect of one's behavior on other individuals. It represents one of the most valid of the personality disorder diagnoses as many defining characteristics are behavioral in nature. More common in men than in women, this condition contributes significantly to the burden of violent and non-violent crime, substance abuse and prison crowding.
Kendler and colleagues recently published a twin study of the DSM-IV criteria for ASPD in a cohort of twins from the Virginia Adult Psychiatric and Substance Use Disorders. Data from 4291 twins were used in the analysis and included a factor analysis of the ASPD phenotype followed by twin analysis.
The research group found two distinct ASPD phenotypes in the factor analysis with specific ASPD criteria loading on each factor:
Factor 1: nonconformance, irritability/aggression and reckless disregard for others
Factor 2: deceitfulness, impulsivity/failure to plan, irresponsibility and lack of remorse for behavior
The subsequent twin analysis confirmed two genetic factors in adult ASPD with a confirmation of the factor analysis criteria loading. Heritability (an estimate of genetic contribution) for individual ASPD criteria varied from .12 to .57.
Additional support for these two factors came from a study of discriminant validity--the ability of the genetic factors to describe a unique clinical presentation. The clinical features found in each of the two factors were:
Factor 1: presence of childhood conduct disorder, early age at first alcoholic drink, high number of maximum drinks consumed in 24 hours, treatment seeking for alcohol dependence and lower educational level
Factor 2: novelty seeking personality and history of major depression
The authors note their study supports a view of two types of adult antisocial phenotypes. Factor one matches the general description of the psychopath with many traits persistent throughout adult life. Factor two matches primarily a deficit of disinhibition the may "reflect disturbances in anterior brain systems mediating affective and behavioral control".
This is an important study the supports more vigorous phenotypic description of adults with antisocial personality. These subtype descriptions may play a key role in understanding cognitive, structural and functional imaging studies in adult ASPD.
In the next few posts, I will review some of the emerging imaging research related to ASPD.
Photo of olive sparrow from the author's file.
Kendler KS, Aggen SH, & Patrick CJ (2012). A Multivariate Twin Study of the DSM-IV Criteria for Antisocial Personality Disorder. Biological psychiatry, 71 (3), 247-53 PMID: 21762879
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