To determine models of formal competence explaining and describing educational phenomena or aspects of them it is a permanent challenge in Mathematical Education. Researchers in Mathematical Education watch and observe the theoretical models of other disciplines, and in some opportunities they transfer and adapt them to the educational phenomena. That is the case of the complementarity principle, imported by Steiner (1985) from the quantum mechanics.
The discussion between the ideas that come from the theories of Piaget and Vygotsky deserve the maximum interest in the present time, sometimes, for the divergent consequences that seem to follow from both theories; in other instances, for the possibility of being used as two complementarity theories in the orientation of the educational practice.
In this work we will analyze how the complementarity principle is improperly applied, in different contexts of educational investigation in the Mathematics area, to the theories that come of Piaget and Vygotsky, in particular when dealing with the interaction between ``studient pairs” in Mathematical classrooms.