A study of this frequency surface shows a tendency for people to be grouped about the central tendency or average. There are many people of average weight or nearly so, but few people who deviate widely from the average weight. If we measure people with reference to any other physical characteristic, or any mental characteristic, we get a similar result, we find them grouped about an average or central tendency. =Mental Differences.= Just as we differ physically, so also we differ mentally, and in the various aspects of our behavior. The accompanying diagram (Free Association) shows the distribution of a large number of men and women with respect to the speed of their flow of ideas. When men and women are measured with respect to any mental function, a similar distribution is found. [Illustration: FIGURE VI--FREQUENCY SURFACE--FREE ASSOCIATION Solid line, men; broken line, women. The numbers below the base represent the number of words written in the Free Association test, and the numbers at the left represent the number of people making the respective scores.] An interesting question is whether our mental differences have any relation or connection with one another. If one mental characteristic is of high order, are all the others of high order also? Does a good memory indicate a high order of attention, of association, of imagination, of learning capacity? Experiments show that mental characteristics have at least some degree of independence. But the rule is that they generally go together, a high order of ability in one mental function indicating a high order of ability in at least some others, and a low order of ability in one function indicating a low order in other functions. However, it seems that abilities that are very much specialized, such as musical ability, artistic ability, etc., may exist in high order while other mental functions may be only mediocre. It is a common thing for a musical person to be of rather poor ability otherwise. To the extent that special abilities require specialized differences in the structure of brain, nervous system, or sense organ, they can exist in some degree of independence of other functions. Musical ability to some extent does require some such differences and may therefore be found either with a high or a low degree of ability in other characteristics.