2.2.1. Irritability

Irritability was measured dimensionally using an empirically-derived irritability factor scale from the parent-reported Child Behavior Checklist (CBCL) (Stringaris et al., 2012, Wiggins et al., 2014). This scale has been previously validated (Evans et al., 2020, Wiggins et al., 2014) and comprises 3 items (“stubborn, sullen, or irritable,” “sudden changes in mood or feelings,” “temper tantrums or hot temper”), rated on a scale from 0 to 2 (0 = Not true, 1 = Somewhat or Sometimes True, 2 = Very True or Often True) over the past 6 months. Developmentally appropriate versions of the CBCL, ages 6–18 years (Achenbach and Rescorla, 2001) and ages 1.5–5 years (Achenbach and Rescorla, 2000), were used based on age; irritability items were identical across both versions. This subscale has been shown to have acceptable psychometric properties (alpha = 0.73; Evans et al., 2020; Stringaris et al., 2012; Wiggins et al., 2014). Additionally, CBCL irritability subscale scores for ages 6–18 years and ages 1.5–5 years have previously been used to measure changes across time (Wiggins et al., 2014). Irritability was assessed at baseline (alpha = 0.68) and each follow-up. Non-irritable depression and anxiety symptom measures were used to assess potential confounding impact of current symptoms on the observed resting state MRI findings. See Supplemental Materials for more information on additional measures (i.e., non-irritable depression, anxiety).