Ecological systems theory is a broad term used to capture the theoretical contributions of developmental psychologist Urie Bronfenbrenner.[1] Bronfenbrenner developed the foundations of the theory throughout his career,[2] published a major statement of the theory in American Psychologist,[3] articulated it in a series of propositions and hypotheses in his most cited book, The Ecology of Human Development and further developing it in The Bioecological Model of Human Development and later writings.[4][5][6] A primary contribution of ecological systems theory was to systemically examine contextual variability in development processes. As the theory evolved, it placed increasing emphasis on the role of the developing person as an active agent in development and on understanding developmental process rather than "social addresses" (e.g., gender, ethnicity) as explanatory mechanisms.[6]