Piagetian cross-cultural psychology indicated that humans living in preindustrial nations do not develop the adolescent stage of formal operations but stay in the preoperational or concrete operational stage. The stage of formal operations originated among intellectual elites of early modern times and became a mass phenomenon in the 20th century, many empirical indicators suggest. Racial-biological approaches cannot explain these phenomena, but those that combine developmental psychology with socialization theory can. The culture of modern, industrial nations, with their systems of primary and secondary socialization and their systems of occupation and media, stimulate young brains and psyches to use open developmental windows so strongly that they can reach the formal operational stage successfully. Conversely, premodern cultures, across times and regions, do not stimulate humans to develop beyond the childhood stages. Accordingly, unused developmental windows in youth lead to early arrested development there. Both ethnographic data and Piagetian training studies indicate that premodern adults cannot skip stages, being stuck in the childhood stages throughout their lives. However, children born in premodern cultures can develop the formal operational stage when they move to modern cultures in their early years. Only the theory of (open or closed) developmental windows can explain these remarkable phenomena. Data indicate that every stage or substage described by developmental psychology was once the “developmental age” certain nations or social milieus had actually reached.

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