From Bill Kessen’s idea of the child as a “cultural invention” (Kessen, 1983) it follows that developmental psychology can-not function fruitfully without historical analysis. Developmentalists should stop “positivistic dreaming” and develop a historical developmental psychology. The history of childhood shows how a historical process of infantilization has tak-en place since Rousseau and the 19th century pedagogical and educational theories and institutions. In the 20th century a new process of de-infantilization took place, caused mainly by the modern mass media (Postman, 1982). It is demonstrat-ed how this led to the “disappearance of childhood”. Babies no longer were considered and studied as “empty-headed” (William James’ conception of the baby experiencing “one great blooming, buzzing confusion”): impressive new research methods and data “filled the baby’s brain” and made the baby much more human than ever before in history. With the narrowing of the gap between childhood and adulthood adolescence as a bridge is less necessary than before. Not only the disappearance of childhood is going on; at the same time there is a correlated disappearance of adolescence. The conclusion must be that the study of cognitive, social and personality development should take into consideration the cultural historical embeddedness.