The essay integrates the psychometric intelligence approach with the cognitive-developmental approach or the stage theory erected by Piaget and his disciples. The latter led to Piagetian Cross-Cultural Psychology and the accumulation of an immense body of data. It shows that different IQ levels are indicative of the peculiar stages of cognitive and personality development that characterize pre-modern and modern societies, that is, the distinction between pre-formal and formal thinking. It reveals the true significance of low IQ scores and the rise of scores, known as Flynn effect, among modern populations. The result is a Historical Anthropology that illuminates social evolution, history, law, economics, politics, morals, etc. This new anthropology contradicts the "official spirit" of the humanities and social sciences of the past decades, both its "cultural relativism" and "universality of rationality". It resurrects the leading pre-war theories, which were based on developmental approaches, and improves, enlarges, and elaborates them.