Introduction

Since Wolfgang Köhler carried out his pioneering experiments on the intelligence of apes (Köhler, 1917), investigators have commonly tested the intelligence of anthropoids by confronting animals with situations in which a desirable goal is not directly available. The animal's task is to find an indirect way to get to the goal. In the simplest case, the solution consists of approaching the goal through a detour pathway; in more complex cases, the ape must use a tool to reach the goal (e.g., a box is placed under a goal hanging from the ceiling, or a stick is used to obtain an out-of-reach object) (Bingham, 1929; Guillaume & Meyerson, 1930; Köhler, 1917; Yerkes, 1927). Apes are able to solve these problems with remarkable efficiency, demonstrating, from a human Piagetian perspective, higher levels of sensorimotor or practical intelligence (Piaget, 1936).

Typically, in these experiments, the investigator keeps the animal isolated in the cage while the human observers watch from outside. Köhler's original experiments, however, did not actually conform to this arrangement. In many (if not most) experimental situations, Köhler himself and/or other humans were inside the animals' rooms, without any barrier separating them from their anthropoid subjects. This design allowed Köhler to observe what he considered “a most remarkable procedure” of problem solving. After failing to produce the orthodox solution (pushing a box) in the classical problem of the goal hanging from the ceiling, one of Köhler's chimpanzees developed the habit of approaching the human observer, taking him by the hand, and bringing him under the goal, where the animal would try to reach the goal by climbing on the observer.