Association Between the Probability of Autism Spectrum Disorder and Normative Sex-Related Phenotypic Diversity in Brain Structure
Authors: Christine Ecker, Derek S. Andrews, Christina M. Gudbrandsen, Andre F. Marquand, Cedric E. Ginestet, Eileen M. Daly, Clodagh M. Murphy, Meng-Chuan Lai, Michael V. Lombardo, Amber N. V. Ruigrok, Edward T. Bullmore, John Suckling, Steven C. R. Williams, Simon Baron-Cohen, Michael C. Craig, Declan G. M. Murphy
Journal: JAMA Psychiatry (2017)

<h3>Importance</h3> Autism spectrum disorder (ASD) is 2 to 5 times more common in male individuals than in female individuals. While the male preponderant prevalence of ASD might partially be explained by sex differences in clinical symptoms, etiological models suggest that the biological male phenotype carries a higher intrinsic risk for ASD than the female phenotype. To our knowledge, this hypothesis has never been tested directly, and the neurobiological mechanisms that modulate ASD risk in male individuals and female individuals remain elusive. <h3>Objectives</h3> To examine the probability of ASD as a function of normative sex-related phenotypic diversity in brain structure and to identify the patterns of sex-related neuroanatomical variability associated with low or high probability of ASD. <h3>Design, Setting, and Participants</h3> This study examined a cross-sectional sample of 98 right-handed, high-functioning adults with ASD and 98 matched neurotypical control individuals aged 18 to 42 years. A multivariate probabilistic classification approach was used to develop a predictive model of biological sex based on cortical thickness measures assessed via magnetic resonance imaging in neurotypical controls. This normative model was subsequently applied to individuals with ASD. The study dates were June 2005 to October 2009, and this analysis was conducted between June 2015 and July 2016. <h3>Main Outcomes and Measures</h3> Sample and population ASD probability estimates as a function of normative sex-related diversity in brain structure, as well as neuroanatomical patterns associated with ASD probability in male individuals and female individuals. <h3>Results</h3> Among the 98 individuals with ASD, 49 were male and 49 female, with a mean (SD) age of 26.88 (7.18) years. Among the 98 controls, 51 were male and 47 female, with a mean (SD) age of 27.39 (6.44) years. The sample probability of ASD did not increase significantly with predictive probabilities for the male neuroanatomical brain phenotype. For example, biological female individuals with a more male-typic pattern of brain anatomy were equally likely to have ASD than biological female individuals with a characteristically female brain phenotype (<i>P</i> = .40 vs .55, respectively; χ<sup>2</sup><sub>1</sub> = 1.11;<i>P</i> &gt; .05; difference in<i>P </i>values, −.15; 95% CI, −.10 to .40). This finding translates to an estimated variability in population prevalence from 0.3% to 0.6%, respectively. Moreover, the patterns of sex-related neuroanatomical variability associated with ASD probability were sex specific (eg, in inferior temporal regions, where ASD has different neurobiological underpinnings in male individuals and female individuals). <h3>Conclusions and Relevance</h3> These findings imply that the male neuroanatomical phenotype does not carry a higher intrinsic risk for ASD than the female neurophenotype and provide important novel insights into the neurobiological mechanisms mediating sex differences in ASD prevalence.