At the beginning of the 21st century, looking back at the past century of psychological theory and practice, we observe what could probably be called a century of defining and redefining intelligence, as researchers across nations and disciplines addressed the commonalities and differences in cognitive skills of humans. Throughout this period, numerous approaches emerged to account for the variations in human abilities. The majority of these approaches have focused on individual differences. They set themselves apart from the theoretical streams focusing on the explanation of cognitive functioning. These different perspectives have found expression in the idiographic and nomothetic orientations, the so-called two disciplines of scientific psychology, put forth by Cronbach (1957) and before him, Allport (1937), who ascribed the distinction to the German philosopher Windelband. Only recently, we have begun to observe attempts at combining the two cultures of psychology to arrive at a unified approach to human intelligence, recognizing that the sources of differences were the exact same sources that gave humans the type of cognitive processing system shared by members of the same species.

The last century has also demonstrated a great shift from Europe to America of psychological inquiry into intelligence. From its modest beginnings in the attempt by Simon and Binet to address the practical need to establish which students would best benefit from education in France, the field moved in great strides to large-scale testing of thousands of children, adolescents and adults in the United States.