Demographic and cognitive characteristics of the sample groups.

Values are mean ± standard deviation. Significance was set at p<0.05. All p-values refer to ANOVAs, except for gender where the p-value refers to a chi-square test. Where ANOVA’s returned a significant result, post-hoc Tukey Honest Significant Difference (Tukey HSD) tests were performed. US - Con, refers to a pairwise comparison between unaffected siblings and controls. ADHD-Con, refers to a a pairwise comparison between ADHD and controls. ADHD - US, refers to a pairwise comparison between ADHD and unaffected siblings. Four subscales of the Children’s Social and Behavioural Questionnaire (CSBQ) which probe ASD spectrum symptoms are shown in this table: Social Interest, Social Understanding (Understanding), Stereotypy and Resistance to Change. ASD-total is calculated as a sum of these four subscales. ADHD-total scores are calculated according to the algorithm described in detail in the supplementary information (Questionnaire Items S1).
Abbreviations: Gender (m/f), Gender (male/female); Handedness (r/l/a), Handedness (right, left, ambidextrous).