Fluid reasoning and reading difficulties among children with ADHD.

Children with attention-deficit/hyperactivity disorder (ADHD) commonly experience difficulties in reading and in fluid reasoning (G). According to Cattell's Investment Theory (1987), G is a causal factor in the development of crystallized knowledge (G) and academic skills; therefore, the co-occurrence of reading and G difficulties within ADHD may not be coincidental. In the present study with children with both ADHD and reading difficulties ( = 187; 61% male;  = 9.2), we utilized mediation analyses to test direct and indirect (through G, phonemic awareness, and rapid automatized naming [RAN]) effects of G on four basic reading skills: untimed word recognition, untimed phonemic decoding, word reading efficiency, and phonemic decoding efficiency. The direct effect of G on all reading skills was nonsignificant; however, significant indirect effects were observed. Specifically, G exerted an effect indirectly onto all reading skills through a serial and joint mechanism comprised of G and phonemic awareness (i.e., G → G → phonemic awareness → reading achievement). G also exerted an effect indirectly onto untimed word recognition and phonemic decoding through phonemic awareness (i.e., G → phonemic awareness → untimed word recognition/untimed phonemic decoding). Results build upon Cattell's Investment Theory by linking G with reading difficulties among children with ADHD, suggesting that such difficulties may arise from weaknesses in G and insufficient investment of G into reading through G and phonemic awareness.