SummaryAttention-deficit/hyperactivity disorder (ADHD) is characterized by developmentally inappropriate levels of inattention, impulsiveness and hyperactivity. The dynamic developmental theory of ADHD (DDT) suggests that altered effects of reinforcement combined with a deficient extinction may be the main mechanisms for the development of the various symptoms observed in ADHD. Due to the combined effect of a shorter and steeper delay-of-reinforcement gradient and deficient extinction, the DDT predicts that it takes more time to build chains of predictable behavior in children with ADHD compared to other children. The primary aim of this study was to explore this prediction in a group with ADHD-C and ADHD-PI compared to a group of children with other psychiatric problems. The present study is part of a collaborative study with Rosemary Tannock at the Hospital for Sick Children in Toronto, Canada.The sample consisted of 45 children aged 9-12 years, 31 boys and 14 girls. These children were divided into four groups based on DSM-IV diagnoses: ADHD-C, ADHD-PI, ADHD-HI, and a control group of children with other psychiatric problems. The children completed a computerized game-like task called Feed the Animal . Mouse clicks within a specified area were associated with reinforcement. Behavior was measured as responses by the computer mouse under two reinforcement contingencies. Reinforcers were set up according to a random interval schedule (RI 15s) and a random ratio schedule (RR 5). Autocorrelations of consecutive responses was analyzed to investigate predictability of responding. Low predictability in responding would imply greater behavioral variability.We found that the responding in the ADHD-combined group was significantly less predictable than the two other groups during infrequent reinforcement (RI 15s). Thus, the findings that the children with ADHD-combined have more difficulties learning long sequences of behavior than the children with ADHD-PI and psychiatric controls, support the prediction from DDT that there may be different underlying mechanisms in children with ADHD-C and ADHD-PI.The testing and collection of data was conducted by Rosemary Tannock and Anne-Claude Bedard at the Hospital for Sick Children in Toronto, Canada.