To the Editor.
—Dr Needleman and colleagues1 reported an association between bone lead levels and antisocial and delinquent behavior in a sample of boys aged 7 to 11 years. If true, this is an important finding with profound social, political, and medical implications. The article has already attracted public attention.2 Thus, it is important to evaluate the validity of the study in terms of the generalizability of the findings to the whole population of US children. Epidemiologists call this "external validity." Needleman et al present evidence in support of the "internal validity" (ie, generalizability of the findings to the studied sample), but they fail to consider its external validity. In fact, examination of their data indicates that the examined sample may not have been unbiased.In the authors' Table 1, data for a number of covariables used in the multivariate analyses of the examined sample data are shown