There has been much controversy concerning the mental mechanisms involved in the processing of complex words, especially between the dual mechanism theory and the single mechanism theory over inflectional morphology. In this article we present a new set of data from Japanese causatives drawn from the experiments on aphasic patients, which show that two different types of causatives may involve two different mechanisms of rule and associative memory, thus demonstrating the validity of the dual mechanism theory over the processes of derivational morphology.