 Montessori continued to develop her pedagogy and her model of human development as she expanded her work and extended it to older children.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.