NATURE VOL. 308 1 MARCH 1984

L£TIERSTO'Insight' in the pigeon: antecedents
and determinants ofan intelligent performance

a410WP

270WP

269WP

406WP

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R. Epstein*, C. E. Kirshnit, R. P. Lanza& L. C. Rubin
Department of Psychology and Social Relations, Harvard University,Cambridge, Massachusetts 02138, USA

In 1917 Wolfgang Kohler reported some rather extraordinary
instances of problem solving by a number of chimpanzeesl , andhis observations have been the subject of controversy eversince2 ,3. The period of quiescence that sometimes preceded tbe
solution, its sudden onset, and its smooth, continuous emergence
were proffered as evidence that (1) contrary to suggestions oflearning theorists of the day, problem solving was not necessarilya trial-and-error process, and (2) constructs such as 'insight'
were necessary for an adequate acconntl,4-(i. Here, in an attempt
to shed further light on these issues, we have replicated withpigeons a classic problem withwhichKohlerconfronted his chim­panzees. Pigeons that had acquired relevant skills solved theproblem in a remarkably chimpanzee-like (and, perforce,human-like) fashion. The possible contributions of ditlerent
experiences were determined by varying the training histories
of different birds. We offer a tentative moment-to-momentaccount of a successful performance.
Kohler placed a banana out of reach in one corner of a room
and a small wooden crate about 2.5 m from the position on thefloor beneath it. After a number of fruitless attempts by all six
chimpanzees in the room to jump for the banana, one of them
paced for several minutes, then suddenly moved the box haH ametre from the position of the banana "and springing upwards
with all his force, tore down the banana"l. Both research 1 andtheory8 suggest that chimpanzees will not solve problems of thissort if they have not first had certain experiences. We speculated
that two behaviours had to have been acquired: pushing objects
towards targets and climbing on objects to reach other objects.
Since a pigeon normally does neither, it seemed an ideal candi­date to test the contribution that previous learning might maketo success in this problem.
Eleven adult male pigeons served as subjects. Each was main­tained at about 80% of the weight it would achieve given freeaccess to food. Most had had a variety of laboratory experience,but none had ever been used in a problem solving experiment.
Birds 269WP and 270WP were racing Homers; the others werewhite Carneaux. All sessions were conducted in a cylindricalwire-mesh chamber 69 cm in diameter, except those of birds
1l0YP, 233WP and 274WP, which were conducted in smaller
rectangular chambers. A cardboard box, 8 cm high and with abase 10 cm2 , was used in some conditions, as was a small facsimileof a banana, 7 em in length. A standard grain dispenser wasattached to the base of each chamber as shown in Fig. 1.
The following history yielded successful performances withall of the birds we tested: (1) A repertoire of 'direCtional pushing'
was established. Each bird was trained to push the box towards
a green spot, 4 cm in diameter, which was placed at random
positions along the base of the chamber wall(s}. Pushing wasextinguished in the absence of the green spot. Major training
steps included reinforcing aimless pushes; reinforcing pecks tothe spot; reinforcing sighting the spot and pushing the box
towards it with the movement of the box constrained by a thinwire; reinforcing sight-and-push behaviour with the wire
removed and the box close to the spot; and gradually increasing
the distance between the box and the spot 9 • Proficient perform­
ances were established in 8, 1 and 4 weeks, respectively, for the• Present address: Cambridge Center for Behavioral Studies, 11 Ware Street, Cambridge,
Massachusetts 02138, USA.

bc274WPFig. 1Birds that had been trained to climb and peck but neverto push did not push the box in the test situation (a). Birds that
had been trained (i) to climb and peck and (ti) to push the box
aimlessly for long periods of time pushed the box over much of
the floor space of the chamber. The birds rarely looked up whilepushing. One of the birds stopped pushing in the appropriate placeand climbed and pecked the banana after having pushed for morethan 14 min (b). Birds that had been trained (i) to climb and peckand (ii) to push the box towards a green spot placed at random
positions along the base of the chamber solved the problem
efficiently and in a manner suggestive of human problem-solving
behaviour (c). Other controls are described in the text. The times
given are in minutes and seconds. A boxed time is the time tosolution.

subjects whose performances are shown in Fig. 1e. The banana
was never present during this training. (2) Concurrently, eachbird was trained to climb onto the box and peck the banana,
which was suspended overhead. The box was fixed in place
during this condition, and pecking it was never reinforced. The
position of box and banana was changed repeatedly. In thepresence of box and banana, the bird would reliably climb onto
the box and peck the banana. In the absence of the banana andin the presence of the spot, the bird would push the box towards
the spot. (3) Each bird was occasionally placed alone with thebanana until the bird neither flew nor jumped towards it.
The following test situation was arranged. The banana wassuspended out of reach (41 cm from the floor) at a point (deter­
mined by a random number) near an edge of the chamber, andthe box was placed elsewhere in the chamber. All test sessions
for these and all other subjects were filmed or videotaped.
The performances for the first three SUbjects were remarkably
similar. At first each pigeon appeared to be 'confused'; itstretched and turned beneath the banana, looked back and forthfrom banana to box, and so on. Then each subject began rathersuddenly to push the box in what was clearly the direction of
the banana (Fig. 1e). Each subject sighted the banana as it
pushed and readjusted the box as necessary to move it towards
the banana. Each subject stopped pushing in the appropriate
place, climbed and pecked the banana.
A fourth bird (233WP) solved the problem after 24 min. The
performance was disrupted by 1,000 W of lighting which had
been added to facilitate filming. When, after 20 min, the lighting
was reduced, the bird solved the problem in just under 4 min.

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We conducted four variations on this training with otherpigeons. Two birds (294WP and 273WP) were trained to peckthe banana but not to climb. Jumping and flying were extin­
guished, and the birds were placed alone with the box until they
showed no signs of discomfort in its presence. Then the banana
was suspended out of reach above it. Each bird stretchedrepeatedly towards the banana at first. Subject 273WP stumbled
onto the box and then fell off. After the first few minutes ofeach session, attempts to reach the banana ceased. We termin­
ated each session after 10 min. We concluded that the establish­
ment of climbing was probably critical to the solution.
Two birds (270WP and 410WP) were trained to climb andpeck but not to push. Jumping and flying were extinguished.
Neither bird pushed the box when given the test (Fig. la). Twobirds (409WP and 282WP) were trained to climb and peck andto push the box around the chamber for long periods of time.
They were never trained to push towards a target, nor to pushin straight lines. Jumping and flying were extinguished. Thebirds pushed apparently aimlessly when given the test (Fig. Ib),
We concluded that a repertoire of directional pushing was prob­
ably critical to an efficient solution.
With one bird (llOYP) we established directional pushingand climbing-and-pecking but did not extinguish brute force
attempts to reach the banana. Like Kohler's chimpanzee, thebird jumped and flew repeatedly towards the banana for several
minutes, then pushed the box towards the banana, climbed andpecked. The solution appeared after about 7 min.
Based on these and other experiments, a tentative, moment­
to-moment account of a successful performance can be given.
At first stimuli were present which controlled both behaviour
with respect to the banana and behaviour with respect to thebox. The behaviour we interpreted as a sign of perplexity wasprobably the result of competition between these behaviours.
Behaviour with respect to the banana quickly disappeared,
probably because of the recent history of extinction of jumping
and flying when the banana was out of reach (compare theperformance of bird 11OYP). The birds may have begun to push
because, as behaviour with respect to the box increased inrelative frequency, the birds faced the box more directly, which
was very nearly the stimulus in the presence of which pushing
had been reinforced (the green spot was absent). Why theanimals pushed towards the banana is unclear and still underinvestigation. A process similar to what some call 'functional
generalization,lO (as opposed to generalization based solely oncommon physical characteristics) seems to be involved. Birds
that were trained to push towards the spot but not to peck thebanana did not push towards the banana in the test situation
but did push towards the banana when subsequently trained topeck it. In other words, the birds pushed towards the banana
apparently for the 'right reasons'-because they had learned
directional pushing and because some history of reinforcement
had made the banana 'important'. Directional performances
may also have been produced by a summal'ion of prevailingresponses: banana-directed pecks may have strengthened
banana-directed pushes (N. E. Miller, personal communication).
The birds stopped pushing in the right place because of aphenomenon called 'automatic chaining': in the course of push­ing towards the banana, they set up for themselves a stimulus(box-under-banana) that controlled other behaviour (climbingand pecking).
We appear to have in hand an instance of 'insightful' problem
solving. The suddenness, directness, and continuousness of theperformances satisfy Kohler's criteria for 'genuine' or 'insightful'solutions1,1l, and people viewing the tapes have liberally
attributed a wide range of human emotions and thoughts to thepigeons. A surprisingly common comment was, "Did the pigeon
really do that?" We may also have in hand an account of similar
performances in chimpanzees and children, for the experiences
we provided are ones that they have probably had before
they are successful in similar situations, and the behaviouralprocesses we have invoked are fairly general in the animal
kingdom12.

We emphasize that we did not train the birds to push the boxtowards the banana; that, except during very early stages oftraining, behaviour with respect to the box was never reinforcedin the absence of the green spot and that such behaviour wasdeliberately extinguished; that pushing the box was never rein­
forced in the presence of the banana and that such behaviour
was deliberately extinguished; and that the spot was absent
during the test. The successful performances must consequently
be regarded as genuinely novel.
The work was supported in part by a NSF grant to Harvard
University and a NIH grant to the Foundation for Research on
the Nervous System. We thank J. Cerelia, J. E. Mazur and B. F.
Skinner for suggestions and E. Stoddard for assistance. The datareported here are based on an analysis of videotapes and films
by an independent observer. Some preliminary results werereported elsewhere l2 before the tapes and films had been
examined.
Received 23 August; aecepted 8 December 1983.
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