To 469 subjects of age 7 to 16 from primary and secondary schools in Switzerland we individually described ten possible tasks and asked for each of them whether they felt able to perform them successfully. Immediately afterwards the subjects were asked why he or she thought so. The justifications given by the subjects were reliably classified into ten categories. The observed frequencies varied with respect to tasks and to age (no interaction). Control-beliefs in social tasks as compared to non-social tasks were more often 'justified' with reference to actions strategies and to the control share of other participant persons. Referring to dipositions (self-schemata), to emotions or motives, and to rules increased with age, while referring to action strategies and to critical characteristics of the task decreased with age. References to autobiographical episodes was more frequent in case of belief in having control over a given task than in case of belief in lacking control over a given task; the reverse was true for the reference to task participants.