Changing views led to a more positive reconsideration of the Iowa research (Skeels and Dye, 1939; Wellman, 1940; Skodak and Skeels, 1949; Skeels, 1966) and other studies (Spitz, 1945; Spitz and Wolfe, 1946). Theoretical support came from scholars who built on the work of Hebb (1949) and, later, Piaget. New work by Kirk (1958), Hunt (1961, 1964), and Bruner (1962) provided more support for a renewed emphasis on environmental intervention in the early years. Perhaps no one pushed the environmentalist view further than Bloom (1964), who argued that development was most sensitive to the influences of environment during periods of rapid growth and that half of adult intelligence was developed by age 5.

The preschool programs developed for disadvantaged children in the 1960s and 1970s not only built on this new work but also incorporated views of theory and practice from a wide variety of traditions in psychology and education. Despite the programs’ emphasis on their potential cognitive benefits, most sought to enhance the development and well-being of the whole child (Day and Parker, 1977). Especially in the early years, they had to address concerns that preschool programs might negatively affect social and emotional development by separating children from their mothers (Caldwell and Smith, 1968). Researchers developed “model” programs specifically to investigate the potential for preschool education to influence the learning and development of economically disadvantaged children. Much of what is known about the nature and magnitude of preschool education’s influences derives from rigorous studies of these model programs. Such studies also provide considerable information about the characteristics of highly effective programs.

Over the past four decades, many studies have been conducted of the immediate and short-term (one or two years) effects of programs on the learning and development of children from low-income families. Both quantitative research syntheses (that pool estimates across studies and apply statistical tests) and traditional best-evidence reviews have found that such programs produced meaningful gains in cognitive, social, and emotional development during the preschool years (White and Casto, 1985; McKey et al., 1985; Ramey et al., 1985). Although the studies of Head Start and public preschool programs have tended to em-



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