This article presents the results of two studies investigating increased intra-individual variability (IIV) in the performance of individuals with attention deficit hyperactivity disorder (ADHD), in two cognitive domains: numerosity judgments and quantitative and verbal reasoning.

Study 1, a pre-registered experiment, involved approximately 200 participants (42.66% female; mean age: 36.86; standard deviation of age: 10.70) making numerical judgments at two time-points. ADHD-symptom severity was assessed on a continuous scale. In Study 2, we collected the data of approximately 3000 examinees who had taken a high-stakes admissions test for higher education (assessing quantitative and verbal reasoning). The sample comprised only people formally diagnosed with ADHD. The control group consisted of approximately 200 000 examinees, none of whom presented with ADHD.

The results of Study 1 revealed a positive correlation between IIV (distance between judgments at the two time-points) and ADHD symptom severity. The results of Study 2 demonstrated that IIV (distance between the scores on two test chapters assessing the same type of reasoning) was greater among examinees diagnosed with ADHD. In both studies, the findings persisted even after controlling for performance level.

The results indicate that individuals with ADHD, v. those without, exhibit less consistent numerosity judgments and greater fluctuation in performance on verbal and quantitative reasoning. The measurement of the same psychological constructs appears to be less precise among individuals with ADHD compared to those without. We discuss the theoretical contributions and practical implications of our results for two fields: judgment and decision-making, and assessment.