This chapter presents the data primarily from two communities: the village of Kokwet in rural Kenya and the town of Duxbury in metropolitan America. Traditional studies of clustering in American children, for reasons that probably have more to do with logistics and economics than a failure of theory, have not examined year-to-year changes in clustering strategy. Changes in elementary visual analysis and construction, in the clustering of items in free recall, and in the rate and type of illusions in the Verbal Transformation Effect are highly canalized growth patterns of middle childhood. Psychological theories of cognitive development are generally vertical. By examining the way subjects of different ages deal with a particular set of tasks, psychologists have derived generalizations about how thinking changes as children mature. A central theme in psychological theories about thinking contrasts organization based on abstracted structural categories with organization based on physical features or function.