To determine whether the level of cognitive function, as measured by IQ, was an important covariate for heterogeneity, we split probands according to IQ into four groups: (i) those with IQ > 80; (ii) those with 80 ≥ IQ ≥ 70; (iii) those with 70 > IQ > 25; and those with IQ ≤ 25. For GWA analyses, we used only Groups (i) and (iii), which had the largest sample size. IQ was measured in various ways by the different recruitment centers, but for our purposes we used verbal, non-verbal (performance) and full-scale IQ assessments. If an individual's score was >80 for any of these three measures, the proband was classified into the above 80 group; otherwise, providing IQ was evaluated on at least two measures and none were ≥70, the proband was classified into one of the below 70 groups. Sample sizes for principal analyses are given in Table 1.
