ABSTRACT
Since the dawn of history people have noticed differences in intelligence among individuals and have wondered about the causes of these obvious differences. Intelligence has been described by many different words – brightness, cleverness, reasoning power, judgment, and quickness in learning, in grasping abstract concepts, and in solving problems. Every parent, teacher, and employer has observed differences among children and adults in all these characteristics that we call ‘intelligence’. A few persons appear extremely ‘bright’, a few appear extremely ‘dull’, and the vast majority falls somewhere between these extremes. There is a continuous gradation of mental ability from the one extreme to the other, from idiot to genius. Just as we see a continuous gradation of differences in other characteristics of humans, such as physical stature, so too there is a similar gradation of differences in intellectual ability. Indeed, individual variation is a fundamental aspect of all living things. Without individual variation, biological evolution as we know it could not have occurred.
