Evidence for the Need to Renorm the Bayley Scales of Infant Development Based on the Performance of a Population-based Sample of 12-month-old Infants

Assessment on the Bayley Scales of Infant Development of 305 twelve-month-old infants born at full term (>2,500 grams) in rural North Carolina resulted in mean Mental Developmental Indexes (MDI's) of 114 and 109 for whites and nonwhites, respectively, and a mean Psychomotor Developmental Index (PDI) of 110 for each group. Because the sample was systematically drawn from the entire population of an eight-county region, such high scores raised the suspicion that the 1969 Bayley norms are outdated. Exploration of alternative hypotheses suggested that home testing might have positively affected sample scores, but support for the hypothesis that the Bayley norms for 12-month-olds are outdated was derived from published means for other samples of infants born in the 1970s and from recent age placement revisions of items on the Gesell Developmental Examination. A renorming of the Bayley Scales is recommended.