Interrelations between the brain, psychological stage development, and societal evolution
Brain researchers are required to study Piagetian cross-cultural psychology, the second branch of cross-cultural psychology, following psychometric intelligence research. According to empirical data collected in over one thousand studies conducted throughout the world, pre-modern peoples do not attain the adolescent stage of formal operations typical of modern peoples. The divergent developmental courses concern the whole range of personality development across all logical, physical, social, and moral dimensions. The presented data on divergent psychological developments between pre-modern and modern peoples are relevant for an encompassing understanding of the history of culture, society, religion, philosophy, morals, law, and politics. Links must exist between these divergent developmental paths and the corresponding cerebral structures that carry the respective psychological stages. This paper presents these interrelations to the extent that they are known, such as neocortex development, brain lateralisation, and density and number of nerve cells and synapses. Brain research can deliver decisive contributions to the explanation of both the arrested development of pre-modern populations and the very slow psychological progress during the several phases of societal evolution. The new theory offers new directions for the international brain research that focuses the role of impoverishing and enriching environments on ontogenetic brain developments. While it considerably deepens the milieu theory of brain development, it tends to dwarf those approaches that emphasise the permanent and unchanged state of the brain since the Pleistocene.