Smedslund, J. The acquisition of conservation of substance and weight in children. II. External reinforcement of conservation of weight and of the operations of addition and subtraction.Scand. J. Psychol., 1961,2, 71–84. —The subjects were 5–7 years old. Group I was allowed to observe empirical conservation of weight on a balance. Group II practised on additions and subtractions of material with controls on the scales. Group III took only the pre‐ and posttests. The data show no appreciable differences in frequency of acquisition of conservation between the three groups. It is concluded that none of the experimental conditions were necessary or sufficient for the acquisition of principles of conservation. The hypothesis is advanced thatcognitive conflictmay be the crucial factor i