Effect of sex chromosome number variation on attention-deficit/hyperactivity disorder symptoms, executive function, and processing speed.

AIM: To study sex differences in attention-deficit/hyperactivity disorder (ADHD) symptoms, we explored whether X chromosome absence or excess is independently associated with deficits in attention and hyperactivity, executive function, and processing speed. METHOD: We assessed 116 children (ages 3y 10mo-11y 11mo, mean 8y 5mo, SD 1y 11mo) with a variable number of sex chromosomes: 36 females with Turner syndrome (45, X0), 20 males with Klinefelter syndrome (47, XXY), 37 typically developing females (XX), and 23 typically developing males (XY). RESULTS: X chromosome absence was associated with increased attention problems, hyperactivity, and deficits in inhibitory control, compared with female children with XX (all p<0.003). Conversely, X chromosome excess was associated with weakness in working memory (p=0.018) and approached significance for attention problems (p=0.071) but not with hyperactivity, or weakness in inhibitory control relative to male children with XY. Using non-parametric effect size to quantify the clinical effect revealed that X chromosome absence affected attention, hyperactivity, executive function, and processing speed (all r>0.4), while X excess affected in-laboratory as well as parent-reported working memory (all r>0.4). INTERPRETATION: Our observations provide compelling evidence that the absence or excess of an X chromosome distinctly affects cognition and behaviors associated with ADHD.