The paper contains results of an experimental study of the capacity for mental behavior in 7-17 years old schoolchildren. Main components of the latter have been established (mental analysis of conditions of a problem, finding out and designating relations between objects, planning of the decision, and comprehending corresponding ways and methods), as well as the levels of its development (initial, partial, total). A procedure called by the author "a play of repetition", designed as a means to reveal the level of development of the capacity in a group experiment, is described in sufficient detail. It has been established that the capacity develops in three stages: the first one ends by 9 years, the second occupies the period from 9 to 12 years, the third ends by 12 years.