To clarify the psychological characteristics associated with attention-deficit/hyperactivity disorder (ADHD), an umbrella review was conducted concerning the evidence of differences in behavioral indices between individuals with ADHD and individuals with typical development. Attention measures have large effect sizes and include omission error, commission error, and standard deviation of the reaction time in a continuous performance test. However, in previous research, the indices did not precisely discriminate between individuals with ADHD and individuals with typical development, which suggests that one psychological function cannot explain all characteristics of ADHD. The effect sizes of the other indices were small to medium, although these indices have been associated with the core characteristics of ADHD. In previous studies that used multiple indices, children with ADHD were classified into several groups, based on different psychological characteristics. The findings revealed heterogeneity in the psychological characteristics of ADHD. Future studies that focus on the heterogeneity of ADHD and that examine the psychological functions of ADHD, independently of diagnostic systems, is suggested. Thus, research trends may change the concepts of ADHD.
