The Mental Life of Monkeys and Apes: A Study of Ideational Behavior ROBERT M. YERKES Harvard University BEHAVIOR MONOGRAPHS Volume 3, Number 1, 1916 Serial Number 12 Edited by JOHN B. WATSON The Johns Hopkins University WITH SIX PLATES AND FIVE TEXT FIGURES CONTENTS I. Interests, opportunity and materials II. Observational problems and methods III. Results of multiple-choice experiments: 1. Skirrl, _Pithecus irus_ 2. Sobke, _Pithecus rhesus_ 3. Julius, _Pongo pygmaeus_ IV. Results of supplementary tests of ideational behavior: 1. Julius, _Pongo pygmaeus_: Box stacking experiment Box and pole experiment Draw-in experiment Lock and key test 2. Skirrl, _Pithecus irus_: Box stacking experiment Box and pole experiment Draw-in experiment Hammer and nail test Other activities 3. Sobke, _Pithecus rhesus_: Box stacking experiment Draw-in experiment Box and pole experiment Other activities V. Miscellaneous observations: 1. Right- and left-handedness 2. Instinct and emotion: Maternal instinct Fear Sympathy VI. Historical and critical discussion of ideational behavior in monkeys and apes: 1. Evidences of ideation in monkeys 2. Evidences of ideation in apes VII. Provision for the study of the primates and especially the monkeys and anthropoid apes VIII. Bibliography I INTERESTS, OPPORTUNITY AND MATERIALS Two strong interests come to expression in this report: the one in the study of the adaptive or ideational behavior of the monkeys and the apes; and the other in adequate and permanent provision for the thorough study of all aspects of the lives of these animals. The values of these interests and of the tasks which they have led me to undertake are so widely recognized by biologists that I need not pause to justify or define them. I shall, instead, attempt to make a contribution of fact on the score of each interest. While recognizing that the task of prospecting for an anthropoid or primate station may in its outcome prove incomparably more important for the biological and sociological sciences and for human welfare than my experimental study of ideational behavior, I give the latter first place in this report, reserving for the concluding section an account of the situation regarding our knowledge of the monkeys, apes, and other primates, and a description of a plan and program for the thorough-going and long continued study of these organisms in a permanent station or research institute.