PSYCHOLOGY: R. M. YERKES 
639 
IDEATIONAL BEHAVIOR OF MONKEYS AND APES 
By Robert M. Yerkes 
PSYCHOLOGICAL LABORATORY, HARVARD UNIVERSITY 
Received by ihe Academy. October 20, 1916 
The reactive tendencies of two monkeys and a young orang utan 
have been studied by means of the method of multiple choices described 
in a previous communication to the Academy^ and by certain supple- 
mentary methods. 
Four multiple choice problems were presented: (1) the problem of 
choosing from among any group of mechanisms the one at the left; 
(2) the second from the right end; (3) alternately, the first at the left 
end and the first at the right end; (4) the middle mechanism. 
Each of the three primates in question solved problem 1. One 
monkey (P. rhesus) required 70 trials; the other (P. irus), 132 trials. 
The ape succeeded only after 290 trials. The behavior of these three 
animals was most interesting and illuminating. P. irus was erratic, 
easily fatigued or discouraged, and apparently of low grade intelligence. 
P. rhesus, on the contrary, was alert, businesslike, intent on his task, 
and direct in his attack on experimental devices. The orang utan was 
childlike in his desire for assistance, as also in his resentment of annoy- 
ances or disappointments. Pie was given to settling down to a simple 
routine. 
The accompanying figure 1 presents the curves of error for these ani- 
mals as constructed from the data in connection with problem 1 . 
P. rhesus (Sobke) quickly and regularly eliminated mistakes and 
completely solved the problem. His disposition as well as his achieve- 
ment, is pictured by the curve of errors. 
P. irus (Skirrl) exhibited marked irregularities of performance, and 
the curve indicates his variable attention and effort as well as his slow 
progress toward success. 
The orang utan (Juhus) reacted uniquely, as his curve suggests. At 
the very outset, he developed a definite habit of response which, as it 
happened, was inadequate for the solution of the problem but yielded 
constantly 60% of correct first choices. The habit or reactive tendency 
was that of choosing each time the box nearest to the starting point. ^ 
Julius continued to use this metliod without variation for eight suc- 
cessive days. Then a break occurred, but after a few days he settled 
back into the old rut. At the end of 230 trials, it was decided to try to 
destroy the ape's unprofitable habit. This attempt was made by using 
