Successful child-focused intervention programs for economically disadvantaged groups are designed to provide children with cognitively stimulating environments that they are presumed to be less likely to experience at home. Such programs typically offer rich, school-based learning curricula, often in combination with a wide variety of developmentally enhancing activities in a classroom setting. Several recent comprehensive reviews of such interventions have attempted to discern patterns of impact across programs (Bryant and Maxwell, 1997; Farran, 1990, 2000; Yoshikawa, 1994, 1995). Unfortunately, despite a plethora of investigations, most conclude that it is difficult to draw clear conclusions about the effectiveness of any of a variety of specific intervention approaches.

The limitations of this literature are due largely to basic problems in research design (e.g., lack of random assignment, lack of comparable comparison groups) that make the findings of individual studies less reliable and difficult to compare with each other. A more fundamental barrier to comparisons across studies, however, is the considerable variability among intervention programs on a number of important dimensions, such as the age of the children at time of entry, the characteristics of the target population, the nature of the program components, the intensity and duration of service delivery, issues regarding comparison or control conditions, and the nature of the staff and their training. Consequently, it is not possible to be certain that differences in outcomes, when they are found, are due to any one (or a combination) of these factors. Generally speaking, programs that have demonstrated the largest and longest-lasting cognitive gains have been administered to children with multiple risks and have offered the most intensive and longest-lasting services. For example, the largest initial IQ gains were documented in the Milwaukee Project, which targeted low-income, black mothers with intellectual limitations and offered full-day infant and preschool child care for the first five years of life, as well as parent education and job training (Garber, 1988). The association between the intensity or duration of service and child outcomes, however, has not been a consistent finding in other studies.

In contrast to the extensive attention paid to cognitive performance, relatively few evaluations of child-focused interventions for low-income children have provided short-term outcome data on social adjustment. Those studies that have reported such information generally have not found much evidence of either reduced problems or increased positive behavior. Nevertheless, some researchers have argued that the subsequent documentation of differences in progress through school and into adulthood (as illustrated by differential rates of welfare dependence and criminal behavior) reflect a social rather than a cognitive impact (Barnett, 1995; Yoshikawa, 1995).

In addition to the broad array of child-focused programs that have



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