She saw human behavior as guided by universal, innate characteristics in human psychology which her son and collaborator Mario Montessori identified as "human tendencies" in 1957. In addition, she observed four distinct periods, or "planes", in human development, extending from birth to six years, from six to twelve, from twelve to eighteen, and from eighteen to twenty-four. She saw different characteristics, learning modes, and developmental imperatives active in each of these planes, and called for educational approaches specific to each period.
