Cognitive performance in memory tasks, as measured by the N-back task, shows large differences between attention defi- ciency disorder (ADD) patients and controls. Recent findings indicate fewer anatomical differences, which, in turn, makes a cognitive modeling of the inherent information processes pos- sible. In this article, we investigate these aspects for 0-back and 1-back problems and introduce a new cognitive model explaining differences between patients and controls. The ACT-R model explains the results by differences in the util- ity and reward function. The presented methodology – using cognitive modeling for controls and patient groups – works to- ward two goals: (1) A potential explanation of a source of dif- ferences for a medically relevant group and (2) to present and evaluate a new approach using cognitive modeling in psychi- atric research areas to identify more detailed descriptions of mental differences.