Teachers' convictions concerning the origins of human nature and development, which are their own psychological conceptions, constitute an element of their knowledge of the reality and a basis for creating notions and assessing interpersonal processes. The presented research was trying to determine the relation between the way teachers understand human nature and development (as growing, as learning, or as an interaction of genes and environment) and the way they react to aggressive students' behaviours. 289 teachers from urban secondary schools took part in the research. The way the respondents understood human nature and development was determined with the use of a questionnaire based on two other diagnostic tools used in research on educational ideologies [1] and on convictions concerning the origins of human nature [2]. The results obtained on scales that correspond with three ways teachers understand human nature and development (growing, learning or interaction of genes and environment) were compared with the allocation of teachers’ interventions in case of aggressive students' behaviours.