We know now that in the vast majority of cases dull children become dull adults, average children average adults, superior children superior adults. As for the feebleminded, they always remain feeble-minded. All the available scientific evidence points to the conclusion that the child of two years whose mental development is 40 or 50 per cent retarded will, in most cases, continue to be in the neighborhood of 40 or 50 per cent retarded. The child who has superior special abilities does not ordinarily lose them. Even the child's trends of interests are now regarded as having prognostic value. Educational and vocational guidance are thus gradually being placed upon a solid foundation. There is probably nothing in the child's mental life or his behavior that would be found altogether lacking in significance for later development if we knew all the facts. There is reason to believe that both delinquent and psychotic tendencies of adults have their prodromes of symptoms in childhood and that it may sometime be possible to identify in early life a majority of those who are headed for these dangers.

The relation between mental and physical growth has been extensively investigated. Correlations have been made between mental development on the one hand and growth in height, weight, lung capacity, motor ability, and physiological or anatomical maturation on the other. The correlations have usually been found positive but extremely low. There is only the slightest tendency for physical development to parallel mental development. Height, strength, dentition, skeletal ossification, and even pubertal changes bear little relation to mental growth. One may assume a close correlation between mental development and growth changes in the finer neural structures, but neurologists have not yet been able to tell us much about these correlations except in their grosser aspects. Glandular and other chemical influences upon mental growth and behavior patterns offer one of the most promising fields for future research. Cretinism, long known for its disastrous effects upon mental development, is probably only one of many such influences, though the most spectacular one in its obvious outcomes. Chemical correlates of personality types have been mentioned in an earlier section. Already the clinical psychologist and the physiological chemist are working side by side in the study of problem children.

Development is more dependent upon natural maturation than upon formal training. Gesell has shown that if one member of a pair of infant twins is for several weeks given frequent practice in a motor activity, such as climbing stairs, while the other member of the pair is given no practice, the latter, when later given the opportunity to practice, reaches within a few days the level of skill the other has taken weeks to acquire. Much the same seems to be true of mental acquisitions. The nine-year-old of average intelligence who has not been taught to read can be made ready for the third school grade within a few months. The twelve-year-old of average intelligence who has been deprived of opportunity to learn to read can be made ready for high school by the time he is fourteen. Researches in child psychology have shown that much time is wasted in schools by presenting subject matter prematurely. Many a child agonizes for months over long division who, a couple of years later, would be able to learn it in thirty minutes or an hour. The same is true of every type of curriculum material. Refinement of pedagogical method is often resorted to where timeliness of instruction would be far more economical and effective. Experiments have shown that it takes an extraordinary amount of pedagogical ingenuity to teach a strictly average five-year-old to read as well as first grade children are expected to, but that almost any method works fairly well when the same child has reached the mental age of six and a half or seven years. It is not intended to convey the idea that nearly all school instruction is given prematurely. For the average child it is, in the main, fairly well timed; it is the exceptionally dull and the exceptionally bright who suffer. By the lock-step system the dull are promoted beyond their ability to master the subject matter, while the bright are under-promoted. The child of 75 I.Q. cannot master first grade work until he has reached the age of eight or nine years. The child of 125 I.Q. can do so at the age of five; yet by custom, and in many states by legal requirement, both are expected to enter upon first grade work at the age of six. In spite of unpsychological legal enactments, great progress has been made in recent years in adjusting the difficulty of school work to the child's maturation level, and further progress in this direction may be expected.

The question has often been raised whether there is any definite relationship between childhood brightness or dullness and the age when ability ceases to improve. It has been found that mentally defective children usually reach their maximum mental development a little earlier than do average children, idiots earliest of all, and next the imbeciles. However, above the level of defectives it is doubtful whether much correlation exists between grade of mentality and age of reaching maturity, although there is some evidence to suggest that the intellectually gifted tend to continue their mental growth somewhat later than average children do.

The Psychology of Learning. Learning has been one of the favorite fields of psychological investigation since the pioneer work by Ebbinghaus in the eighties of the last century. The greater part of this work has been devoted to investigations of memorizing and relatively little to learning to solve problems. The laws of use and disuse were laid down by James in his "Principles of Psychology," and to these Thorndike and others have added the law of effect. By the law of use is meant that a mode of response is stamped in by repetition; by the law of disuse, that it tends to fade out when repetitions cease. The law of effect refers to the supposed stamping-in effect of the satisfaction or dissatisfaction which the response brings.