Sources of error in the Piagetian water-level task

Task materials were devised which allowed two different explanations of the Piagetian water-level task to be distinguished. Two experiments are reported with 5-6 and 7-8 year olds in which both a predictive and a perceptual task were used. The results support Ibbotson and Bryant's explanation that the performance of young children is affected by a perpendicular bias. This effect was weaker, particularly among 7-8 year olds, in the perceptual task.