Urie Bronfenbrenner, was a co-founder of the US national Head Start program and widely regarded as one of the world's leading scholars in the interdisciplinary domain of developmental psychology, child-rearing and human ecology. At his death in 2005, he was the Jacob Gould Schurman Professor Emeritus of Human Development and of Psychology at Cornell University, where he spent most of his professional career.
A leading scholar in the field of developmental psychology, Bronfenbrenner developed an Ecological Systems Theory, in which he delineated four types of nested social systems: the microsystem (such as the family or classroom); the mesosystem (microsystems in interaction); the exosystem (external environments which indirectly influence development), and the macrosystem (the larger socio-cultural context). To these he later added a fifth, the Chronosystem (the evolution of the external systems over time). The major statement of this theory The Ecology of Human Development (1979), has influenced how psychologists and other social scientists approach the study of human beings and their environments.