Sex differences in pediatric EEG inter-subject correlation during naturalistic movie watching: a large-scale characterization of the Healthy Brain Network EEG dataset

Inter-subject correlation (ISC) of EEG during naturalistic viewing is increasingly used as a candidate pediatric biomarker, yet its behavior at cohort scale remains uncharacterized. We characterize ISC in 1143 children and adolescents (765 male, 378 female, ages 5 to 21) from the Healthy Brain Network EEG dataset viewing four naturalistic film clips. Three results follow. First, ISC separates narrative from abstract stimuli by 1.7 to 2.4 fold and exceeds a resting-state baseline by 42 to 99 fold, and the previously reported developmental decline replicates throughout. Second, ISC is sex-dependent: males exceed females in every movie (t = 9.7 to 13.5; Cohen's d = 0.61 to 0.85), concentrated in delta and theta, peaking at a frontocentral cluster and at ages 11 to 14. The direction matches an earlier report where the effect was marginal, and we resolve structure that sample could not. It survives three demographic and clinical robustness analyses, same-sex and size-matched templates, ocular component removal, and exclusion of the releases overlapping that report, and is attenuated, by approximately 10 percent adjusting for interpolation count and by 23 percent in recordings free of cluster interpolation, but not explained by differential channel interpolation. Evoked response magnitude, previously proposed as an alternative account, does differ by sex here (d = 0.55 to 0.60) but mediates only 8.6 to 18.1 percent of the effect. Third, ISC predicts none of four CBCL bifactor dimensions after FDR correction at any feature resolution: zero of 16 grand-mean, 2064 per-channel and 16 cluster tests, with confidence intervals excluding standardized associations beyond |{beta}| = 0.106.