ABSTRACT
Object-concept and its development is probably the most studied Piagetian cognitive category in the literature about non-human primate cognition. In spite of the wide variability of the testing procedures used in various studies, ranging from the Wisconsin General Testing Apparatus to the Uzgiris and Hunt scale, they all agree in finding successful attainment of stage 5 object-concept in all species tested. The first might easily occur when the testing procedure is a standardization of some kind of displacement task repeated with no or few variations over a large number of trials. One way of protecting oneself against this danger is to vary, as much as compatible with due requirements of standardization, the context and the materials of the task, as Piaget himself always did, in order to prevent as much as possible the development of habits specific to the task.
