Bronfenbrenner's ecological framework for human development was first introduced in the 1970s as a conceptual model and became a theoretical model in the 1980s. Two distinct phases of the theory can be identified. Bronfenbrenner stated that "it is useful to distinguish two periods: the first ending with the publication of the Ecology of Human Development (1979), and the second characterized by a series of papers that called the original model into question." Bronfenbrenner's initial theory illustrated the importance of place to aspects of the context, and in the revision, he engaged in self-criticism for discounting the role a person plays in his or her own development while focusing too much on the context. Although revised, altered, and extended, the heart of Bronfenbrenner's theory remains the ecological-stressing person-context interrelatedness.
