It will be observed, also, that there has been no attempt here to teach introductory psychology. It is assumed that readers of this volume will be acquainted with the vocabulary of elementary psychology. The time has definitely passed when it was either feasible or desirable to present all topics in a single volume. Those who would learn what modern educational psychology has to teach now expect, first of all, to equip themselves by study of a general introductory text. The lists of references are selected, not complete. To present complete bibliographies of all works bearing immediately or remotely upon every topic treated would cumber the volume inexcusably. References have been selected for these lists because they are historically indispensable, because they contain information of fundamental importance, or because they summarize much previous work. I believe that the selection is such that from the books and articles listed it will be possible for the student who wishes to do so, to construct the complete bibliography and history of each topic, up to the present time. The hundreds of teachers who have sat in the lecture room of Professor E. L. Thorndike will see how many guiding suggestions for this volume have come from that source. Professor W. A. McCall has given counsel on certain chapters. Many investigators and publishers have extended courtesies, which are acknowledged through the references, and to which attention is here gratefully directed. I am indebted to Dr. John S. Richards, Medical Superintendent of The Children’s Hospital, Randall’s Island, New York, and to Mr. L. L. Kolburne, student at Teachers College, for assistance in securing illustrative material for Chapter VII. Finally, I have enjoyed the advantage of editorial supervision by Professor M. V. O’Shea. My chief hope for the volume is that it may contribute toward the welfare of school children compelled to attend upon prescribed education, without due regard for their idiosyncrasies of original endowment. LETA S. HOLLINGWORTH TEACHERS COLLEGE COLUMBIA UNIVERSITY May, 1923 TABLE OF CONTENTS PAGE PREFACE vii EDITOR’S INTRODUCTION xvii CHAPTER I. PRELIMINARY DISCUSSION 1 Speculation Concerning the Nature of Ability. Results of Quantitative Investigation.