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Thomas R. Zentall and Jessica Stagner decision-making behaviour)and possible mechanism for gambling (sub-optimal human Maladaptive choice behaviour by pigeons: an animal analogue

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Maladaptive choice behaviour by pigeons:an animal analogue and possible

mechanism for gambling (sub-optimalhuman decision-making behaviour)

Thomas R. Zentall* and Jessica Stagner

Department of Psychology, University of Kentucky, Lexington, KY 40506-0044, USA

Consistent with human gambling behaviour but contrary to optimal foraging theory, pigeons showedmaladaptive choice behaviour in experiment 1 by choosing an alternative that provided on average twofood pellets over an alternative that provided a certain three food pellets. On 20 per cent of the trials,choice of the two-pellet alternative resulted in a stimulus that always predicted ten food pellets; on theremaining 80 per cent of the trials, the two-pellet alternative resulted in a different stimulus thatalways predicted zero food pellets. Choice of the three-pellet alternative always resulted in three food pel-lets. This choice behaviour mimics human monetary gambling in which the infrequent occurrence of astimulus signalling the winning event (10 pellets) is overemphasized and the more frequent occurrenceof a stimulus signalling the losing event (zero pellets) is underemphasized, compared with the certainoutcome associated with not gambling (the signal for three pellets). In experiment 2, choice of thetwo-pellet alternative resulted in ten pellets with a probability of 20 per cent following presentation ofeither stimulus. Choice of the three-pellet alternative continued to result in three food pellets. In thiscase, the pigeons reliably chose the alternative that provided a certain three pellets over the alternativethat provided an average of two pellets. Thus, in experiment 1, the pigeons were responding to obtainthe discriminative stimuli signalling reinforcement and the absence of reinforcement, rather than toobtain the variability in reinforcement.

Keywords: gambling; choice behaviour; signalling reinforcement; pigeons; humans

1. INTRODUCTIONMaladaptive human decision-making (e.g. buying lotterytickets or playing slot machines) has two important charac-teristics: on average, the net return is less than the amountwagered; and having sufficient experience with the game tolearn about the odds of winning does not appear to reduceits frequency (paradoxically, it may actually increase thetendency to gamble). Those who gamble tend to overvaluethe winning outcomes and undervalue the losing outcomes[1–3]. This bias may be related to the availability heuristic[4], the fact that wins are more salient than losses [5]. Inaddition, the extra value of winning may be strengthenedby the social attention (reinforcement) that often comeswith winning. Gamblers may also believe superstitiouslythat they have some control over the outcome of agamble [6] and continue wagering in the face of a stringof losses because they may not understand the principleof randomness or the independence of events ([7]; e.g.the gambler’s fallacy).

Although many people gamble occasionally for itsentertainment value, others become addicted and aredebilitated by it [8]. In some cases, pathological gamblinghas been associated with substance abuse and withgeneral psychiatric problems including suicide [9].

By contrast, non-human animals should be less sus-ceptible to the attraction of a poor gamble because it is

likely to affect their survival [10]. According to optimalforaging theory, animals should make more ‘rational’choices because evolution should have favoured the survi-val of animals that do [11]. Given appropriate experience,non-human animals are presumed to be sensitive tothe relative amounts of food obtained from differentalternatives (or patches).

But there is also evidence that animals prefer alterna-tives that lead to signals for reinforcement, even if onother trials those alternatives lead to signals for the absenceof reinforcement [12,13]. For example, pigeons prefer analternative that sometimes produces a green light thatalways predicts reinforcement and equally often producesa red light that predicts the absence of reinforcement (dis-criminative stimuli) over an alternative that produces ablue or yellow light, each of which predicts reinforcement50 per cent of the time (non-discriminative stimuli; seefigure 1). In this case, the preference for the ‘informative’red or green light over the ‘uninformative’ blue or yellowlight incurs no added cost to the animal because theprobability of reinforcement associated with the twoalternatives is equal (50%). But would the pigeon preferthe informative alternative if there was a substantial costinvolved in the form of loss of food?

Recently, when choice of one alternative provided 75per cent reinforcement and the other only 50 per centreinforcement but was followed by discriminative stimuli(one followed by 100% reinforcement, the other by 0%reinforcement), pigeons showed a reliable preference for* Author for correspondence (zentall@uky.edu).

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the 50 per cent reinforcement alternative [14]. Stagner &Zentall [15] found similar results when pigeons chose a20 per cent reinforcement alternative with discriminativestimuli over a 50 per cent reinforcement alternative withnon-discriminative stimuli (see also [16–18]).

When humans gamble, they choose between a certainoutcome (keeping the money in their pocket) and anuncertain outcome (the gamble). In both the studiesmentioned above [14,15], however, the pigeons chosebetween uncertain outcomes. In each case the higher prob-ability outcome was still uncertain (75% in one case, 50%in the other). Thus, unlike human gambling behaviour, thepigeons may have been avoiding the stimuli associated withthe uncertain (and unsignalled) outcome by choosingdiscriminative over non-discriminative stimuli.

Thus, the purpose of these experiments was to deter-mine if the results of earlier research resulted frompigeons’ avoidance of the uncertainty of reinforcementassociated with the higher probability of reinforcementalternative or from the preference for the possible appear-ance of a stimulus that predicted a high probability(or high magnitude) of reinforcement.

2. EXPERIMENT 1A better analogy to human gambling would be to givepigeons a choice between one alternative associated witha certain outcome (analogous to money in one’s pocket)and a second alternative associated with a low-probability,high-payoff stimulus on some trials or a high-probability,zero-payoff stimulus on others. To be analogous to mal-adaptive human gambling, the average payoff for thefirst alternative should be greater than that of thesecond alternative. The question is whether pigeons willprefer the first alternative with the greater net payoff(which they should, according to optimal foragingtheory) or the second alternative involving low probabilityand high payoff (which they may if they behave likehuman gamblers).

3. METHOD(a) Subjects

The subjects were eight unsexed white carneau pigeons ran-

ging from 5 to 8 years of age purchased from the

Palmetto Pigeon Plant, Sumter, South Carolina. The

pigeons were kept on a 12 L : 12D cycle and were

maintained at 80 per cent of their free feeding body weight.

They had free access to grit and water, and were cared for

in accordance with the University of Kentucky’s animal

care guidelines.

(b) Apparatus

A Med Associates (St Albans, VT) ENV–008 modular oper-

ant test cage was used. The response panel in the chamber

had a horizontal row of three response keys. Behind each

key was a 12-stimulus inline projector (Industrial Electronics

Engineering, Van Nuys, CA) that projected vertical and

horizontal lines as well as red, yellow, blue and green hues.

Reinforcement was delivered by a pellet dispenser mounted

behind the response panel (Med Associates ENV–45).

A 28 V, 0.1 A houselight was centred above the response

panel. A microcomputer in the adjacent room controlled

the experiment.

(c) Procedure

(i) Pretraining

All pigeons received two pretraining sessions in which they

were required to peck once for reinforcement on each side

key to one of the four hues (yellow, red, green and blue)

and to one of the two line orientations (horizontal and

vertical), as well as to a triangle stimulus on the centre key.

There were 78 stimulus presentations per session: six invol-

ving each hue and line stimulus on each of the side keys

and six of the triangle stimulus on the centre key. Each

response was reinforced with three food pellets.

(ii) Training

The design of the training phase appears in figure 2 (exper-

iment 1). All trials were initiated by presentation of a

triangle stimulus on the centre key. A single peck to the tri-

angle turned it off. On forced trials (40 per session), either

a vertical or horizontal line was presented on one of the

side keys. Each line orientation could appear equally often

on the left or right. One peck to this stimulus initiated a

coloured stimulus on the same key for 10 s. For half of the

pigeons, if a vertical line had been presented (20 trials per

session), 20 per cent of the time the colour would be, for

example, red (four trials per session) and after 10 s, 10 pel-

lets would be presented. The other 80 per cent of the time

that a vertical line appeared, the colour that followed would

be, for example, green (16 trials per session) and after 10 s no

pellets would be presented. If a horizontal line had been pre-

sented (20 trials per session), a peck would change the

stimulus to, for example, blue 20 per cent of the time (four

trials per session) and to, for example, yellow 80 per cent

of the time (16 trials per session). In each case, after 10 s,

three pellets always would be presented. The colours that sig-

nalled 10, three and zero pellets were counterbalanced over

subjects, as were the line orientations that signalled the dis-

criminative stimuli and the non-discriminative stimuli.

Each session also included 20 choice trials. Choice trials

were also initiated by pecking the triangle stimulus. When

pecked, both vertical and horizontal lines were presented

simultaneously on the left and right keys. A single peck to

either line orientation stimulus resulted in presentation of

one of the colours with the same probability as on forced

trials. The unchosen key was darkened. The choice trials

were presented randomly among the forced trials. All pigeons

were trained with this procedure for 40 sessions.

choice

(a) (b)

p = 0.50 p = 0.50 p = 0.50 p = 0.50

red green blue yellow

p(rf) = 1.0 p(rf) = 0 p(rf) = 0.5 p(rf) = 0.5

white white

or or

Figure 1. Choice trials in which one alternative (a) led to asignal for reinforcement on 50% of the trials or a signal forthe absence of reinforcement on the remaining trials, andthe other alternative (b) led to either of two signals, each ofwhich was a signal for reinforcement on 50% of those trials.

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4. RESULTSThe pigeons developed a strong preference for the alterna-tive associated with a mean of two pellets of reinforcementper trial (10 pellets with a probability of 20%) over thealternative associated with a consistent three pellets pertrial. Acquisition of the preference for the alternative withthe discriminative stimuli is presented in figure 3. Whenthe preferences were pooled over the last five training

sessions, the preference was 82.2 per cent and significantlydifferent from chance (as determined by a single-sampletwo-tailed t-test: t(7) ! 4.23, p ! 0.004, effect size r !0.83). Of the eight pigeons in this experiment, sixshowed a clear preference for the alternative with the discri-minative stimuli (more than 92%) over the last fivesessions, whereas two pigeons were relatively indifferent(45% and 50% choice of the discriminative stimuli).

choice initial link

terminal link

p = 0.20 p = 0.80 p = 0.20 p = 0.80

green red blue yelloworor

exp. 1

exp. 2

10 pellets

10 pellets 10 pellets

0 pellets 3 pellets 3 pellets

3 pellets3 pellets

(p = 1.0)

(p = 0.20) (p = 0.20)

(p = 1.0) (p = 1.0)

(p = 1.0) (p = 1.0)

(p = 1.0)

mean = 2 pellets mean = 3 pellets

Figure 2. Experiment 1: choice trials in which one alternative led to a signal for 10 pellets of reinforcement on 20% of the trialsor a signal for the absence of reinforcement on 80% of those trials (overall mean reinforcement per trial ! two pellets) and theother alternative led to one signal that occurred on 20% of the trials and the other that occurred on 80% of the trials but bothsignals were always followed by three pellets. Experiment 2: choice trials in which one alternative led to either of two signals for10 pellets of reinforcement on 20% of the trials. One signal occurred on 20% of the trials and the other occurred on 80% of thetrials (again the overall mean reinforcement per trial ! two pellets), and the other alternative led to one signal that occurred on20% of the trials and the other that occurred on 80% of the trials, and both signals were always followed by three pellets.

100

90

80

70

60

50

40

30

20

10

0

perc

enta

ge c

hoic

e of

the

2-pe

llet a

ltern

ativ

e

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39

session

discriminative stimuli (exp. 1)

non-discriminative stimuli (exp. 2)

Figure 3. Experiment 1 (discriminative stimuli): acquisition of the preference for a mean of two pellets over a mean of threepellets with discriminative stimuli signalling 10 pellets on 20% of the trials and zero pellets on 80% of the trials over the pre-ference for a constant three pellets. Experiment 2 (non-discriminative stimuli): acquisition of the preference for a mean of threepellets over a mean of two pellets with the absence of discriminative stimuli; both stimuli signalled 10 pellets on 20% of thetrials and zero pellets on 80% of the trials. Error bars represent+ s.e. of the mean.

An animal analogue of human gambling T. R. Zentall & J. Stagner 3

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5. DISCUSSIONThe results of experiment 1 indicate that pigeons preferan alternative providing discriminative stimuli thatsignal an occasional large reinforcement (20% of thetrials) or its absence (80% of the trials) over an alternativeproviding non-discriminative stimuli that always signal asmaller reinforcement, in spite of the fact that the alterna-tive associated with the consistent smaller reinforcementprovided 50 per cent more reinforcement than the otheralternative.

That two of the eight pigeons did not prefer the prob-abilistic 10 pellets over certain three pellets suggests thatthere may be interesting individual differences in thisform of gambling behaviour. The small number ofpigeons that avoided the lure of the large probabilisticreinforcement precluded further differentiation of theirbehaviour but it should be noted that none of the pigeonspreferred the greater reward associated with the certain3-pellet alternative.

6. EXPERIMENT 2To what extent could the preferences found in experiment1 be due to a preference for a variable magnitudeof reinforcement (zero or 10 pellets) over a fixedmagnitude of reinforcement (three pellets) independentof the stimuli that signalled those magnitudes [19]? Inexperiment 2, the probability of reinforcement associatedwith the two colours that had been associated with 10and zero pellets was equated. That is, if in experiment 1the red stimulus had been associated with 10 pellets andthe green stimulus had been associated with zero pellets,then in experiment 2 presentation of either colour was fol-lowed by 10 pellets 20 per cent of the time. Thus, theprobability of receiving 10 pellets associated with the initialchoice was still 20 per cent, as it was in experiment 1, butthe large magnitude of reinforcement (as well as theabsence of reinforcement) was no longer signalled.

7. METHOD(a) Subjects and apparatus

The subjects were seven new unsexed white carneau pigeons

similar to those used in experiment 1. The apparatus was the

same as that used in experiment 1.

(b) Procedure

The procedure was the same as that of experiment 1 with the

following exceptions. Choice of the variable-outcome

alternative resulted in the presentation of one of two colours

(one on 20 per cent of the trials and the other on 80 per cent

of the trials), but 10 pellets followed either colour on 20 per

cent of the trials. Thus, those colours were no longer discrimi-

native of reinforcement but the probability of 10 pellets of

reinforcement for choice of that alternative continued to be

20 per cent (see figure 2, experiment 2). That is, in each

session, for the variable-outcome alternative there were 20

forced trial presentations of the more frequently presented

colour, 16 of which were followed by zero pellets of

reinforcement and four of which were followed by 10 pellets

of reinforcement. For that alternative, there were also five

forced trial presentations of the less frequently presented

colour, four of which were followed by zero pellets of

reinforcement and one of which was followed by 10 pellets

of reinforcement. Thus, there were 25 forced trials to the

alternative associatedwith a constant three pellets of reinforce-

ment (as in experiment 1). Therewere also 25 choice trials, for

a total of 75 trials per session. The change from experiment 1

in the number of trials of each type and the number of total

trials was necessitated by the change in the probability of

reinforcement associated with the colours that signalled 20

per cent reinforcement (from 100 to 20% reinforcement and

from 0% to 20% reinforcement). In all other respects, the pro-

cedure was the same as in experiment 1. The experiment

consisted of 40 sessions of training.

8. RESULTSWhen the two discriminative colours associated with 10and zero pellets were equated at 20 per cent probabilityof obtaining 10 pellets, in contrast to the pigeons in exper-iment 1, these pigeons showed a strong preference for thealternative associated with a consistent three pellets. Whenpooled over the last five sessions of training, the meanchoice of the alternative associated with a consistentthree pellets was 79.9 per cent. This difference was signifi-cantly different from chance (t(6) ! 3.32, p ! 0.02, effectsize r ! 0.78). Of the seven pigeons, six showed a strongpreference for the three-pellet alternative over the two-pellet alternative. Thus, when reinforcement was not dif-ferentially signalled, the pigeons preferred the alternativeassociated with a higher mean number of pellets per trial.Acquisition of the preference for the alternative associatedwith a consistent three pellets appears in figure 3.

9. DISCUSSIONThe results of experiment 2 indicated that the preferencefor an average of two pellets over a consistent three pelletsfound in experiment 1 did not depend on the variability ofreinforcement associated with the two-pellet alternative.Instead the results indicate that the preference for thetwo-pellet alternative depended on the signalling valueof the stimulus that differentially predicted that 10 pelletswould be coming.

10. GENERAL DISCUSSIONThe results of these experiments suggest that pigeonsshow a tendency to make maladaptive decisions similarto those of humans. That is, pigeons prefer a signal fora low-probability, high-payoff alternative over a signalfor a certain low-payoff alternative that on average pro-vides 50 per cent more reinforcement. The results ofexperiment 2 suggest that it was not the variability inreinforcement that was responsible for the preferencefor less over more reinforcement in experiment 1 butthe preference for the discriminative stimuli that reliablysignalled the presence and the absence of reinforcementto follow.

The results of experiment 2 may have additionalimplications for human gambling. The importance ofthe conditioned reinforcer in demonstrating this maladap-tive choice by pigeons suggests that similar processes maybe affecting the behaviour of human gamblers (e.g. [20]).For example, matching a winning lottery ticket comeswell before obtaining the reward, and watching thewinning pattern as it comes up on a slot machine comesbefore the money is received. Would gamblers wager asoften as they do if there were no signals for the occasions

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of their winning (e.g. slot machine gamblers could not seethe three wheels, roulette players could not see the ballskipping over the slots in the wheel)?

The results of these experiments complement theresults of earlier research [14] which show that pigeonsprefer an alternative that provides 50 per cent reinforce-ment (when followed by a stimulus that predicts 100%reinforcement and equally often by a stimulus thatpredicts 0% reinforcement) over an alternative thatprovides 75 per cent reinforcement (independent of thestimulus that follows). Similarly, the results of theseexperiments are consistent with research that found thatpigeons prefer an alternative that provides 20 per centreinforcement (when followed on 20% of the trials by astimulus that predicts 100% reinforcement and on 80%of the trials by a stimulus that predicts 0% reinforcement)over an alternative that provides 50 per centreinforcement (independent of the stimulus that follows).

Behavioural ecologists might argue that this kind ofchoice (and those involved in most human games ofchance) does not occur in nature. Although animals innature may confront differential probabilistic outcomesof different magnitudes associated with their choices, itis often the case that approaching a low-probability,high-value outcome will increase the probability of itsoccurrence by bringing the animal into closer proximityto a high-value patch. Thus, what may induce animals(including humans) to pursue a large low-probabilityreward is that, in nature (but not typically in the labora-tory or casino), such behaviour often increases theprobability of the large reward.

One could also argue that to the degree that the pigeonsdo not get sufficient food during the experimental session,they will eventually be provided with additional food in thehome cage, and this may have caused them to be willing toforgo food during the experimental sessions. However,research on delay discounting in which pigeons are givena choice between a small immediate reinforcer and a largedelayed reinforcer suggests that reinforcement that isdelayed by as much as 5 s has very little value to a pigeon[21]. That is, pigeons have a very short time horizon andgenerally will choose a small amount of food right awayover a much larger amount of food at a later time [22].

Inone sense, the present results are different fromhumangambling decisions because the pigeon does not yet have thereward prior to making its decision, whereas the human hasto give up already earned money. However, this differenceshould make the likelihood of the wager even less probablebecause humans are forgoing a present (immediate)reward (the money in their pocket) over the (howeverunlikely) possibility of a larger future gain [23].

We believe that the mechanism responsible for thismaladaptive behaviour is the contrast between the expec-tation of reinforcement associated with the alternativeprior to its selection and the expectation of reinforcementassociated with the stimulus that appears after thealternative is chosen (see [24]). Thus, in the presentexperiment 1, for the two-pellet alternative, when thestimulus that is associated with 10 pellets appears, thereis a large amount of positive contrast. However, whenthe stimulus that is associated with zero pellets appearsthe resulting negative contrast is relatively small. Thus,the net effect will tend to be positive contrast. For thethree-pellet alternative there should be no contrast

because on every trial three pellets are expected andthree pellets are obtained. The net positive contrast thusfavours the two-pellet alternative. In experiment 2,without discriminative stimuli, the expectation ofreinforcement associated with the two-pellet alternativeis on average two pellets, but because both of the stimulithat follow that choice are also associated with on averagetwo pellets (a 20% chance of obtaining 10 pellets) thereshould be no contrast and thus the pigeons shouldchoose the alternative associated with the greateramount of food (the consistent three-pellet alternative).

A similar contrast effect may account for human gam-bling. As the expectation of a win is quite small, there islarge positive contrast when there is a win but there isvery small negative contrast when there is a loss. Morespecifically, human gamblers tend to overvalue their winsand undervalue their considerably more frequent losses[5] (see also prospect theory [25]).

This research was supported by the National Institute ofChild Health and Human Development Grant HD060 996.We thank Mikael Molet and Jennifer Laude for theircomments on an earlier draft of this manuscript.

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