Genetic and environmental influences underlying the relationship between autistic traits and temperament and character dimensions in adulthood Angelo Picardi a, ⁎ ,1 , Corrado Fagnani b,1 , Emanuela Medda b , Virgilia Toccaceli b , Paolo Brambilla c,d , Maria Antonietta Stazi b a Mental Health Unit, National Centre of Epidemiology, Surveillance and Health Promotion, Italian National Institute of Health, Rome, Italy b Genetic Epidemiology Unit, National Centre of Epidemiology, Surveillance and Health Promotion, Italian National Institute of Health, Rome, Italy c DISM, InterUniversity Center for Behavioural Neurosciences (ICBN), University of Udine, Udine, Italy d IRCCS "E. Medea” Scientific Institute, UDGEE, Udine, Italy Abstract Background: In recent years, several twin studies adopted a dimensional approach to Autism Spectrum Disorders (ASD) and estimated the contribution of genetic and environmental influences to variation in autistic traits. However, no study was performed on adults over 18 years of age and all but two studies were based on parent or teacher ratings. Also, the genetic and environmental contributions to the interplay between autistic traits and adult personality dimensions have not been investigated. Methods: A sample of 266 complete twin pairs (30% males, mean age 40 ± 12 years) drawn from the population-based Italian Twin Register was administered the Autism-Spectrum Quotient, Temperament and Character Inventory (TCI-125), and General Health Questionnaire (GHQ-12). Genetic structural equation modelling was performed with the Mx program. Estimates were adjusted for gender, age, and GHQ-12 score. Results: Genetic factors accounted for 44% and 20%–49% of individual differences in autistic traits and TCI dimensions, respectively. Unshared environmental factors explained the remaining proportion of variance. Consistently with the notion of a personality profile in ASD characterised by obsessive temperament, autistic traits showed significant phenotypic correlations with several TCI dimensions (positive: HA; negative: NS, RD, SD, C). Genetic and unshared environmental correlations between AQ and these TCI dimensions were significant. The degree of genetic overlap was generally greater than the degree of environmental overlap. Conclusions: Despite some limitations, this study suggests that genetic factors contribute substantially to individual differences in autistic traits in adults, with unshared environmental influences also playing an important role. It also suggests that autistic traits and the majority of temperament and character dimensions share common genetic and environmental aetiological factors. © 2014 Elsevier Inc. All rights reserved. 1. Introduction Autism Spectrum Disorders (ASD) are a set of pheno- typically heterogeneous neurodevelopmental syndromes characterised by difficulties in social communication and social interaction, and unusually restricted, repetitive behaviours and interests [1]. These disorders are considered to be highly heritable. In twin studies, the concordance in monozygotic twins is much higher than that in dizygotic twins [2]; also, the individual risk of ASD was found to increase with increasing genetic relatedness [3]. Rather than treating ASD as discrete entities with a categorical approach based on a distinct boundary between normality and pathology, several authors [4,5] have suggested a dimensional approach to ASD, which concep- tualises these disorders as the upper extreme of a constellation of deficits in social adaptation and communi- cation that may be continuously distributed in the popula- tion. There are several lines of evidence for this notion: autistic traits measured in the general population show a smooth distribution throughout the normal range to the clinical extreme [6,7]; relatives of patients with ASD display high levels of autistic traits [8,9]; factor analytic approaches Available online at www.sciencedirect.com ScienceDirect Comprehensive Psychiatry xx (2015) xxx – xxx www.elsevier.com/locate/comppsych ⁎ Corresponding author at: Italian National Institute of Health, Centre of Epidemiology, Surveillance and Health Promotion, Mental Health Unit, Viale Regina Elena, 299-00161 Rome, Italy. Tel.: +39 06 49904200; fax: +39 06 49904182. E-mail address: angelo.picardi@iss.it (A. Picardi). 1 These authors contributed equally to the paper. http://dx.doi.org/10.1016/j.comppsych.2014.12.018 0010-440X/© 2014 Elsevier Inc. All rights reserved.