Studies of lead exposure and the developing central nervous system: a reply to Kaufman

Kaufman's critique of the lead-cognition hypothesis is a reiteration of wel l worn and weary claims raised many times in the past, primarily by spokesp ersons for the lead industry. They have been fully rebutted in the literatu re. The essence of these claims is that those studies showing an associatio n between lead and IQ are flawed by uncontrolled confounding, multiple comp arisons, and errors in measurement. Any effect of the lead, Kaufman asserts , if present, is small. This response examines each of these issues and sho ws that they lack substance. Lead's negative impact on IQ persists in most modern studies after confounding has been controlled in many different stat istical models. At least three metaanalyses have confirmed an effect of lea d at low dose. Animal studies in which lead is given systematically, and th e issue of confounding thereby avoided, demonstrate an unequivocal lead eff ect at similar doses to the human studies. The criticism of multiple compar isons similarly does not withstand examination. Measurement errors are nons ystematic and nondifferential. They are, therefore, null biasing. The actua l size of the lead effect has been shown to be substantial, and to be found most prominently at the ends of the distribution. Kaufman says that lead r equires study of diverse dimensions of intellect, but he restricts his scop e to a sample of studies of lead and IQ, ignoring recent high quality studi es that show a clear lead effect, and in those studies that he critiques he ignores data that contradict his position. His article adds nothing to the dialogue on lead neurotoxicity. (C) 2001 National Academy of Neuropsycholo gy. Published by Elsevier Science Ltd.