On the Spectrum

Cortical morphology in children with autistic traits
Two hundred fourteen children were excluded based on missing information on autistic
traits and 105 children were excluded due to poor image quality (See Supplementary
information). For 15 children, a gyrification output could not be constructed. Furthermore,
for each sibling or twin pair, one sibling was excluded (n=17). Two children were excluded
based on major incidental findings. The final study sample consisted of 717 6-to-10 year old
children (Supplementary Figure 1).
Informed consent and assent were obtained from parents and children, respectively, after
providing them with a complete description of the study. All procedures were approved by
the Medical Ethics Committee of the Erasmus Medical Center.
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Social Responsiveness Scale
Around age 6 years (range 4.89-8.90 years, see Table 1), the Social Responsiveness Scale
was administered to obtain a measure of autistic traits (Constantino 2002). The Social
Responsiveness Scale provides a valid quantitative measure of subclinical and clinical autistic
traits (Constantino, Davis, Todd, Schindler, Gross et al. 2003). We utilized the 18-item shortform
of the Social Responsiveness Scale, which shows correlations ranging from 0.93 and 0.99
with the full scale in three different large studies (see Supplementary section for additional
information). The authors recommend cut-offs for screening in population-based settings
(consistent with weighted Social Responsiveness Scale scores of 1.078 for boys 1.000 for
girls) (Constantino 2002).
At approximately 7 years of age, children who scored on the top 15 th percentile of the Child
Behavior Checklist 1½-5 total score and those who scored on the top 2 nd percentile of the PDP
scale underwent a screening procedure for ASD, using the Social Communication Questionnaire
(SCQ) a 40-item parent-reported screening instrument to assess characteristic autistic behavior.
SCQ scores 15 or above are considered positive for screening (Berument, Rutter, Lord, Pickles
and Bailey 1999). Children with scores above this threshold were invited for an Autism Diagnostic
Observation Scale (ADOS) (Lord, Risi, Lambrecht, Cook, Leventhal et al. 2000) and their mothers
for an Autism Diagnostic Interview–Revised (ADI-R) (Lord, Rutter and Lecouteur 1994).
Magnetic Resonance Imaging
All children were first familiarized with the MRI scanning environment during a mock scanning
session. Structural MRI scans were obtained on a 3-Tesla scanner (Discovery MR750, GE
Worldwide, Milwaukee, USA). Cortical reconstruction and volumetric segmentation were
performed with the FreeSurfer image analysis suite version 5.1 (http://surfer.nmr.mgh.
harvard.edu/). Details can be found in the Supplementary material.
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