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Early reports led to the fear that exposure to cocaine during prenatal life caused brain damage and intellectual and social impairment (l). But more recent evidence has presented a different picture, in which the effects of cocaine are more subtle (2). This view should not inspire complacency. From a public health perspective, these subtle effects can have profound consequences for the success of these children in school and for the cost of special education services.
To investigate the magnitude of these effects, we examined the studies in our Robert Wood Johnson database (3), a collection of the published literature on prenatal cocaine exposure and child outcome. Of the published studies, 101 met methodological inclusion criteria (original research, published in an English-language refereed journal, human subjects with prenatal cocaine exposure, neurobehavioral outcome measures, and inclusion of a control or comparison group with statistical analysis). Only eight of these studies were completed with school-age children (4-11); most were studies of infants and preschoolers. Intelligence quotient (IQ) was measured in five studies, receptive language in four, and expressive language in five, making it possible to get our first systematic glimpse of the long-term effects of prenatal cocaine exposure.
We quantified these cocaine effects using meta-analysis, a statistical procedure in which effects from different studies are pooled to provide a better estimate of the effect size, in this case prenatal cocaine exposure, than can be determined from a single study (12). A list of the 101 studies in the database and key details from each of the eight studies used in the meta-analysis can be found at Science Online at www.sciencemag.org.
In our meta-analysis, for each study a Z value was computed directly from the t statistic derived from mean differences between cocaine-exposed and control cases. These Z values were then weighted by the size of the cocaine-exposed group. Only the exposed group was used in weighting, because one study used a large normative sample as its control group, which would have unduly skewed the meta-analysis results toward that single study. The standard test for the pooled weighted Z's provided the significance criterion for the combined effect size. Effect sizes for each study were computed by taking the difference between the means of the exposed and control groups, divided...