SNOW Final draft pigeon

Running Head: PREFERENCE REVERSAL IN A PIGEON 1
Preference as a Function of Reinforcer Size and Delay in a Pigeon
Valerie Snow
University of Florida

PREFERENCE REVERSAL IN A PIGEON 2
Abstract
Previous experiments measuring the value of reinforcers in a self-control paradigm have
shown similar results of which option would be chosen between the smaller sooner and
larger later reinforcers. This experiment was conducted in order to contribute to the
continuing practice. A pigeon was given the choice between a smaller, immediate reward
or a larger, delayed reward. In the baseline phase, the pigeon chose the smaller immediate
over the larger delayed. In the experimental manipulation phase, a delay was added to
both rewards, and the pigeon’s preference reversed such that he preferred the larger,
delayed reward over the smaller, immediate one. When the original baseline
contingencies were re-implemented, the pigeon consistently chose the smaller, immediate
reward over the larger, delayed one. The purpose of this experiment is to identify the
variables that control which reinforcer is perceived as more valuable; at what point, if
any, will the pigeon reverse his preference to the more self-controlled choice? The
results of this experiment were consistent with similar experiments reported previously
which further proves that preference-reversal in pigeons is successful with delays that
change the value of the reinforcers.

Preference as a Function of Reinforcer Size and Delay in a Pigeon
If you, as a human being, were given the option between getting a brand new car
immediately, or getting a brand new car in 10 years, which would you choose? When two
options of the same amount differ in delay, the individual is more likely to find the
immediate option more valuable. Conversely, if you had the option of a piece of candy on
Halloween or a bag-full of candy on Halloween, which would you choose? When two
options of the same delay differ in amount, the larger amount will most likely be more
valuable. When the amount of reward and size of the delay differ, both factors will play
a role in each option’s value as a whole. The options compete because one is more
valuable due to immediacy and the other more valuable due to magnitude.
Most subjects, given the option, would choose the smaller more immediate
reward due to impulsivity, but the choice really depends on how the amounts and delays
interact. Self-control can be defined as preference for a larger, delayed reinforcer over a
smaller, immediate one (Green, Fisher, Perlow, and Sherman, 1981). For example,
lottery winners often choose the larger (less-taxed) later prize, rather than the more
immediate, small (more-taxed) prize. Conversely, Odum (2011) strictly defines
impulsivity as the smaller sooner reward over the larger later. For example, a child given
the option between a short trip to the playground today or a week-long trip to Disney
World in a month, will make the impulsive decision and chose the trip to the playground.
Similarly, if a rat is given the option between 2 s of food immediately, or 4 s of food after
a delay, the rat will choose the impulsive choice: the immediate reward. Impulsive or
self-controlled responses depend on which consequences have the greatest subjective
present value at the time the choice is made. If the smaller sooner option is chosen, the

immediacy is probably to blame. If the larger later option is chosen, it is probably
because of the large amount compared to the small amount.
Odum (2011) was interested in studying delay discounting by comparing previous
experiments, synthesizing their data and analyzing their results. According to Odum,
delay discounting is the decrease in present value of a reinforcer after a delay. With
shorter delays, the value of the reward declined steeply. With longer delays, the value
declined shallowly. Both the amount of the reward, and the length of the delay, help
control choice. Odum discussed the value of the delayed outcome and how this variable
was dependent upon the duration of the delay to the larger later reward. She concluded
that the degree of discounting delay was used to summarize experiments’ results by
sensitivity to delayed rewards. In her figures, the point at which the subject’s preference
was reversed was called the indifference point, which she says represents the value of the
delayed outcome.
It has also been shown that the rate of discounting decreases in relation to the
amount of the reward (Green, Fristoe, & Myerson, 1994). That is, larger rewards are
more steeply discounted than smaller rewards. Furthermore, Green et al. showed that if
equal delays were added to both the smaller and larger rewards, preference reversed from
the smaller sooner to the larger delayed reward. They used equations to further predict
preference reversal. If it can be predicted, the shape of the discount function is hyperbolic
and the equation would show that
V= /(1+ kD).
They also mention that their experiment was under conditions, which the delay
and reward amounts are within a range that could be involved in everyday decision-

making. For example, if that child from the previous example is told they have to wait
until next week to go to the playground or wait an extra week on top of the month they
already have to wait, they might chose to reverse their preference for the Disney trip.
Similarly, if the rat from the previous example had a 30 s delay added to both the smaller
sooner and larger later options, he is likely to choose the larger later reward. If two
different sized reinforcers lose value at the same rate, no change in preference reversal
can be predicted, the shape of the discount function is exponential such that
V= A* exp(-kD).
The following equation predicts preference reversal based on amount and delay:
VSS/VLL= (ASS/ALL)(DLL/DSS).
This equation is a derivative of the matching law. The overall value is equal to the SS
amount over the LL amount times the LL delays over the SS delay. This determined
value makes an apparent change at a certain point by varying the sizes of rewards and
delays to reverse the preference.
Rachlin & Green (1972) were interested in studying choice and commitment.
Specifically, they aimed to assess the emergence of preference reversals and how
preference reversals are a function of delay to reinforcement. To do so, they arranged an
experiment in which five pigeons completed daily sessions consisting of 50 trials. Each
free-choice trial had two links. In the first link, both keys were illuminated white. In order
to get to the next link, the pigeon had to complete a fixed-ratio (FR) schedule of 25 pecks
distributed to either of the two keys. If the 25th peck was on the right key, there was a
blackout for T seconds followed by the illumination of both a green key and a red key
simultaneously. A peck to the right key, which was the smaller sooner option, produced

immediate 2 s of food access followed by a 6-s blackout. A peck to the green key, the
larger later option, produced a 4-s delay followed by 4-s access to food. However, if
during the first link the 25th peck was on the left key, there was a blackout for T seconds
followed by the illumination of only the green key (i.e., the other remained dark). If the
green key was pecked, a 4-s delay would occur followed by 4-s access to food. Rachlin
and Green then varied T across conditions. Additional delays ranging between .5 s and
16 s were added to the original delays of both choices. Their findings showed that at
short delays pigeons responded by pecking the right key in the initial link, causing both
keys (i.e., red and green), to be illuminated. In this case, the pigeons always pecked the
red key and there was an exclusive preference for the small immediate reinforcer. At long
delays, however, the pigeons responded by pecking the left key in the initial link. In this
case, the pigeons were forced to peck this green key because it was the only option
presented. Overall these results suggest that as T increases, preference shifted from the
smaller, immediate to the larger, delayed reinforcer because delay alters the value of a
reinforcer at the time a choice is made.
Ainslie (1974) also was interested in studying the effects of delay to
reinforcement on impulsivity. He aimed to assess whether pigeons could learn a response
that would enable them to pre-commit to a large, delayed reinforcer and curb their
impulsive behavior. To do so, Ainslie used 3 pigeons that underwent an experimental
condition along with 3 control conditions. In the experimental procedure, a green key was
illuminated for 7.5 s. If the pigeon pecked it, the subject would receive 4 s of access to
food at the end of the trial (i.e., after a delay). If the pigeon did not peck it, there was a
4.5 s blackout followed by 3 s of illumination of the red key. If the pigeon pecked the red

key, he earned immediate access to only 2 s of grain. Condition I was the same as the
experimental condition except pecking the green key did not prevent the red key from
illuminating thus removing the pre-commitment effect. Condition II also removed the
pre-commitment effect from the experimental condition by the pigeon receiving the
larger later reward regardless of if the red key was pecked or not. Condition III was the
same as the experimental condition except that the peck of the green key was required in
order for the red key to be illuminated later in the trial. The purpose of this condition was
to see if there was a discrepancy of choice while the green key was illuminated causing
the pigeon to peck even though the alternate choice was preferred. Therefore, in the
experimental condition, the pigeons were given an opportunity to peck the green key, and
making this response inactivated the smaller, sooner option. The findings during the
screening condition showed that the subjects preferred the smaller sooner reward
regardless of the pre-commitment response availability. Overall these results suggested
that the value of a reinforcer was altered by delay because the pre-commitment response
was available only at the start of a trial. It is important to note that the pigeons never
waited until the end of the 7.5 s to pre-commit. In fact, they did so early into the 7.5 s
which shows the value of the reinforcer in relation to the delay.
Logue, Rodriguez, Pena-Correal, & Mauro (1984) were interested in pinpointing
individual differences within the self-control paradigm. They conducted two experiments,
which provided data proving that there are experienced-based differences between
reinforcers of different reward amounts and delays. They concluded that varying degrees
of fading procedures increased preference for the larger delayed reward.

Green et al., (1981) stressed that preferences of varying sizes and delays reverse
as a function of time and that discount functions are hyperbolic. They express that the
exact interaction between reinforcer amount and delay had yet to be determined, but their
results have given them a better understanding of it.
Green & Estle (2003) went a few steps beyond by instead of using just food as a
reward and incorporated water as an alternative reinforcer and they also incorporated rats
as subjects for the first time. They did this because in order to consider the preference-
reversal paradigm as an animal model, it would have to be extended by both an avian
species and a mammalian species. Their results showed that differences in self-control
might be of degree, not kind. They successfully extended the generality to the paradigm
and to different reinforcers. These results further proves that the preference-reversal
animal model can be extended to qualitatively different reinforcers.
The purpose of the present experiment was to examine the effects of different size
reinforcers and delays on the preference of a pigeon. The experiment consisted of 3
phases: baseline, added delay, and return to baseline. We expected to replicate and
extend the findings reported previously by demonstrating that a pigeon’s preference
would reverse from the smaller sooner reinforcer in the baseline condition to the larger
later reinforcer when the added delay was introduced during phase 2. Furthermore, we
used the A-B-A design because we expected that the preference would return back to the
smaller sooner over the larger later choice. Further understanding preference reversals as
a function of reward amount and delay is important because it will help in identifying
variables to control behavior at the time a decision is made.
Method

Subject
The subject was an experimentally-naïve Racing Homer pigeon purchased from
Double T Farms in Glenwood, Iowa. He was a young adult male and approximately a
year old when the experiment began. The subject was maintained at 385 grams, which
was 85% of his free-feeding weight. The pigeon was weighed before and after each
session. During sessions, the subject received a mixture of sterilized hemp seed,
buckwheat, and milo as the reinforcer. The pigeon was fed the difference between his
post-session weight and his 85% free-feeding weight, in grams of food. Post-session
feeding consisted of pellets and mixed grain in equal proportions. Outside of sessions, the
subject lived in a temperature and humidity controlled colony room but was housed in an
individual homecage cage with free access to water and health grit.
Apparatus
The subject was placed into an operant-conditioning chamber (30.9 x 35.4 x 36.5
cm) for the duration of every session. The houselight, which provided general
illumination of the chamber, was 1.5 cm from the ceiling. There were three keys within
the chamber: a green on the left, a white in the center, and red on the right. Each key was
4.5 cm below the houselight, 8.2 cm from the ceiling, and the side keys were 8 cm from
the side walls. The keys were 5.7 cm apart with a diameter of 2.5 cm each. The left key
(green) required a minimum force of 0.2255 N to be activated and the right key (red)
required 0.2354 N. A food aperture, measuring 5 cm in height and 5.7 cm in length, was
9 cm below the center key and 11 cm from the floor. The aperture allowed the pigeon to
access grain from the raised, illuminated hopper during reinforcer presentations. An
observing window, used for the experimenters to view the subject, was 21.5 cm in height

and 9 cm in length. A computer running MED-PC IV software was connected to the
operant chamber by the interface and was used to control experimental events and record
the data.
Procedure
Pre-training. First, the subject was hopper trained. This was achieved by the
pigeon reliably approaching and eating from the raised hopper. Responses to the center
key (illuminated white) were hand-shaped via reinforcing successive approximations. A
peck to the center key was reinforced with 3 s of access to food. Next, when key pecking
was established, the following session consisted of a fixed-ratio 1 schedule with the
center key illuminated white and ended when the subject earned 60 reinforcers. Then, to
arrange a 3-component multiple schedule, we introduced two additional keys— left green
and right red. This schedule lasted for two sessions and each session lasted 40 trials.
After the key was chosen in the first trial, the colors would alternate one at a time: green
on the left and red on the right. If any key was pecked during this session, the subject
received 3 s of access to grain. There were two sessions of forced trials only. A forced
trial is when just one color lights up (randomly) and there is no option as to which color
can be chosen. After that, free choice was allowed.
General procedure. Each session consisted of 40 food-outcome trials. The
experiment is comprised of 3 phases: baseline, added delay, and return to baseline. All
sessions consisted of five free-choice trials followed by two forced-choice trials. This
pattern repeated until the 40 trials were reached.
Baseline. For forced-choice and free-choice trial, if the pigeon pecked the green
key, then he earned 2 s of food immediately. These trials were followed by a 6-s blackout

called the inter-trial interval. If the pigeon pecked the red key, then he received 4 s of
food following a 4-s delay. There was no inter-trial interval; instead, the next trial just
began.
Added delay. A 15-s delay was added to the original delays of both choices.
Thus, if the pigeon pecked the green key, there would be a 15-s blackout following the
same 2 s of access to food. However, if he pecked the red key, there would be a 19-s
delay followed by the same 4 s of access to food.
Return to baseline. The 15-s was removed. In other words, the contingencies
were identical to those in the original baseline phase. These three phases in succession
make the experiment have an ABA single-subject design, allowing results to show how
the individual subject’s preference was affected before, during and after the added delay.
Results
Figure 1 shows the proportion of smaller sooner free-choice trials during the
baseline, added delay, and return to baseline. The proportion of SS choices were
presented on the y-axis and the 30 sessions on the x-axis. Each data point was calculated
by dividing the total number of smaller sooner choices by 30 (i.e., the total number of
free-choice trials). The baseline, the added delay, and the return to baseline phases are
labeled and separated by phase change lines. Both baseline phases show very high
proportions of smaller sooner choices. The line in both of these phases trends from
medium proportions of the smaller sooner choice in the very beginning of the phase to
high proportions for the remainder of the phase with the exception of a slight decrease at
points 6 and 29. The added delay phase, however, shows that the proportions of smaller
sooner choices with the added delay are much lower and the data points vary more. The

line in this phase trends downward with an increase again at session 15, where it then
continues the downward trend again.
Figure 2 shows the CDC method applied to the data from the original baseline and
added-delay phases (as seen in Fig. 1). The CDC method, Conservative Dual-Criterion
Method, was used to improve the visual analysis of graphed data in order to determine
systematic changes in the pattern of data. The two red lines above the data points in the
added delay phase represent the level line and the trend line, both calculated using
baseline data. These lines are overlapping in this figure meaning that the level line, which
includes the mean data, is equal to the trend line. Eight out of ten criterion points need to
fall under both lines for there to be considered a systematic change. All ten of the data
points in the added-delay phase fell below the trend line.
Figure 3 shows the number of switches from smaller sooner to larger later
reinforcer in free-choice trials on the y-axis and sessions across the x-axis. The baseline,
the added delay, and the return to baseline phases are labeled and separated by phase
change lines. In the baseline phase, it began with a few switches, which quickly
decreased to no switches with the exception of an increase during session 6 but then
continues to decrease back to no switches by session 8 through 10. Overall, the subject
did not tend to switch to the larger delayed reinforcer. With the added delay, however, the
subject’s switches varied greatly, and the number of switches was random and sporadic.
During the first session (11), there were a medium amount of switches followed by a
slight increase for a couple sessions. Then, there was a dramatic decrease during session
14 followed by an immediate slight increase, which continues drastically up to the high
point of the graph during session 17. The data then decreases steeply for 2 sessions until

stabling for the last 3 sessions of the phase. The number of switches to larger later during
the added delay phase seemed substantially greater than those in the baseline phases, with
the exception of sessions 1, 2, 6 & 7 in the original baseline phase. This exception might
have occurred because it was at the beginning of the experiment when the subject was
still learning and developing his preference. In the return to baseline phase, there was a
stable trend of no switches for the first six trials until it has a slight increase to only one
switch for the next two sessions until the last two sessions where it returns back to none.
Figure 4 shows the number of switches between the larger later and smaller
sooner choices during the original baseline, the treatment, and the return to baseline
phases. The data was very similar to that of Figure 3. As these data show in both baseline
groups, the subject did not tend to switch to the smaller sooner reward after choosing the
larger later reward. During the duration of the treatment phase, however, the figure shows
that the subject’s choices varied greatly and the number of switches was again, random
and sporadic. The data points during the treatment phase were all significantly higher
than those in the baseline phases, excluding data collected in the original baseline phase
immediately following hopper training. In the return to baseline phase, there was a stable
trend of no switches for the first six trials until it has a slight increase to only one switch
for the last three sessions until the last session where it returns back to none.
Figure 5 shows the total number of smaller sooner and larger later selections in
free-choice trials (y-axis) immediately after a smaller sooner or larger later forced-choice
trials (x-axis) in the baseline phase. The white bars represent the smaller sooner free-
choice, and the shaded bars represent the larger later free-choice. After a smaller
sooner forced-choice trial, the smaller sooner free-choice occurred almost always with

the exception of one larger later choice. After a larger later forced-choice trial, the
smaller sooner free-choice also occurred almost always with the exception of one larger
later choice.
Figure 6 shows the choices the subject made after forced-choice trials in the
treatment phase with an added delay. The white bars represent the smaller sooner
free-choice, and the shaded bars represent the larger later free-choice. After a
smaller sooner forced-choice trial, the smaller sooner free-choice was chosen almost
twice as much as the larger later free-choice response. After a larger later forced-choice
trial, the larger later free-choice response was chosen more than double the amount of
times the smaller sooner free-choice was chosen.
Figure 7 shows the choices the subject made after forced-choice trials in the
returning to baseline phase. The white bars represent the smaller sooner free-choice,
and the shaded bars represent the larger later free-choice. After a smaller sooner
forced-choice trial, the smaller sooner free-choice response was chosen every time. After
a larger later forced-choice trial, the smaller sooner free-choice response was chosen the
majority of the time with the exception of a few larger later free-choice responses.
Figure 8 shows the total number of larger later free-choice responses after a
smaller sooner forced-choice trial (y-axis) in the original baseline, the added delay, and
the return to baseline phases (x-axis). During the original baseline phase, larger later free-
choice responses after a smaller sooner forced-choice trial occurred less often than during
the added-delay. During the added delay, there were 9 choices, which was a much larger
amount. Finally, during the return to baseline phase, there were no choices for the larger
later free-choice response after smaller sooner forced-choice trials. Overall, the fact that

the added delay phase had a significantly higher amount of choices of larger later free-
choice responses after smaller sooner forced-choice trials shows that the delays greatly
affected the pigeon’s preference.
Discussion
The purpose of this experiment was to gain a further overall understanding of the
preference reversal of a pigeon as a function of different amounts of reward and varying
delays. To do so, we arranged an experiment consisting of three phases: the baseline, the
added delay, and the return to baseline. A preference reversal from the smaller sooner
free-choice to the larger later free-choice occurred with an added delay. Overall, the
hypothesis that the preference would be reversed has been supported by the findings.
The results supported the original premise that the impulsive choice of the smaller
immediate reward would be chosen during baseline but the self-controlled choice is
chosen when equal delays were added during the added delay phase. Consistent with
Green et al. (1981), self-control is shown to be a complex behavior, but with each study
provides a further understanding of such a complex behavior. It is a behavior that cannot
be given a simple concrete definition, but rather it is a behavior that requires being
continuously studied and analyzed to be understood. Although this experiment followed
similar results to that of Odum’s (2011), one difference between these two experiments
was the limitation of this particular experiment that there was only one treatment phase of
added delay. Like in Green et al. (1994), the discount function of this experiment proved
to be hyperbolic meaning that the preference reversal could be predicted. Similar to
Rachlin and Green (1972), preference of the subject was reversed as T increased because
the value of the reinforcers was altered at the time the choices were made.

This experiment differed from Ainslie’s (1974) in the sense that the subject was
not given a pre-commitment to the larger delayed reinforcer but was consistent with the
fact that in both experiments the preference in the return to baseline phase reverted back
to the smaller immediate reinforcer. Green and Estle (2003) went further than the scope
of this experiment to include an alternative reinforcer but the results of their experiment
just focusing upon food as a reinforcer was consistent with the results of this one. Logue
et. al (1984) had consistent results showing that the larger later reward was preferred after
a delay was added but this experiment had limited varying degrees of fading to evaluate
the strength of this preference.
Based on the results, this experiment has been consistent with the previous
theories such that preference can be reversed to a larger delayed reinforcer from a smaller
immediate one once a significant delay is added to both options.
Figure 1 shows that during the original baseline phase the subject undoubtedly
preferred the smaller immediate reward, as expected. The return to baseline phase
showed the same preference. The added delay in the treatment phase cause the preference
of the subject to switch to the larger delayed reinforcer.
Figure 2 shows that based on the CDC method we can conclude that the treatment
was effective. Because the data exceeded the criterion by all 10 points being below the
trend line shows that a significant reduction occurred in the smaller sooner choices and
this was obviously due to the delay. This indicated that the treatment phase successfully
decreased impulsivity of the subject.
As briefly mentioned previously, the results from Figures 3 & 4 are interesting. It
would be reasonable to believe that results of switches from smaller sooner to larger later

are shown as predicted but it was expected that there would be far less switches from
larger later to smaller sooner possibly to even show an inverted graph. This was not the
case. Both graphs showing very similar results thus seem peculiar. One possible reason
for this is since the subject preferred the smaller immediate reinforcer during baselines,
the delay added during the treatment phase caused the subject to chose either reinforcer
regardless to which reinforcer preceded it. This reason also resolves why the data points
of both graphs within the treatment phase were random and sporadic. This also could
mean that switches from smaller sooner to larger later do not reflect preference.
Figure 5 shows that regardless of which forced-choice response was given, the
subject greatly preferred the smaller sooner reward during the baseline phase. This is also
shown in Figure 7, which shows very similar results but during the return to baseline
phase rather than the original baseline phase. This proves that the subject’s preference
was not influenced by which forced-choice response was given during the baseline
phases.
Figure 6, however, proves that during the treatment phase with an added delay
that the subject’s preference was influenced by which forced-choice response was
presented. Whichever forced-choice trial was chosen, the subject was about twice as
likely to choose that same choice next.
Figure 8 shows that the preference for the larger later reward after a smaller
sooner forced-choice during baselines was extremely weak. During treatments, it was
stronger but still not as strong as the preference for it after a larger later forced-choice (as
seen in Figure 6). Figures 5 through 8 collectively show the subject’s preferences and
what influenced them.

Although these data collectively support the original hypothesis and are consistent
with earlier published research and experiments of similar design, there are some
limitations such as those stated previously. More subjects could have been used to
provide more data and it would make the results more accurate. Also, more sessions with
longer phases would only further the evidence that the experiment is consistent with
previous ones.
There was only one type of reinforcer being used which also limits what can be
concluded. Had there been more treatment phases using varying delay sizes and reward
amounts, there would be more data to find possible indifference points and to further
evaluate delay discounting. Another variable could have been added such as water as an
alternative reinforcer like Green and Estle (2003).
Future research should be conducted with more variance in delay and reward size.
The green (left) key would remain being the smaller sooner option but there would be 2 s
of access to food followed by a 2 s interruption followed by another 2 s of food with a 2 s
blackout before a new trial began. The red (right) key would also remain the larger later
option but there would be a 4 s delay after being pecked followed by 4 s of access to food
with no blackout before beginning a new trial. This additional treatment phase will
further evaluate how impulsivity and self-control are affected as a function of reward
amount and delay. What the results show will determine if further changes need to be
made to delays and reward sizes in order to come to an opposing conclusion than that of
this experiment.
Overall, we demonstrated that preference reversal has been effectively proven
possible by the results of this replicated experiment. The importance of this reversal

validates our hypothesis based on the results of previous studies. Moreover, these results
demonstrate that impulsivity can be successfully decreased by means of an added delay,
thus promoting self-control within subjects. Successful replication of results reported in
previous studies with similar results not only further proves the validity and accuracy of
the previous studies but also gives us a deeper understanding and appreciation for the
complex behavior of self-control in a pigeon and how to manipulate his preference.

References
Ainslie, G. W. (1974). Impulse control in pigeons. Journal of the Experimental Analysis
of Behavior, 21, 485-489.
Green, L., Fisher, E. B., Perlow, S., & Sherman, L. (1981). Preference reversal and self-
control: Choice as a function of reward amount and delay. Behavior Analysis
Letters, 1, 43-51.
Green, L., Fristoe, N., & Myerson, J. (1994). Temporal discounting and preference
reversals in choice between delayed outcomes. Psychonomic Bulliten & Review,
1, 383-389.
Green, L., & Estle, S. J. (2003). Preference reversals with food and water in rats.
Journal of the Experimental Analysis of Behavior, 79, 233-242.
Logue, A. W., Rodriguez, M. L., Pena-Correal, T. E., & Mauro, B. C. (1984). Choice in
a self-control paradigm: Quantification of experience-based differences. Journal
of the Experimental Analysis of Behavior, 41, 53-67.
Odum, A. L. (2011). Delay discounting: I'm a k, you're a k. Journal of the Experimental
Analysis of Behavior, 96, 427-439.
Rachlin, H., & Green, L. (1972). Commitment, choice and self-control. Journal of
the Experimental Analysis of Behavior, 17, 15-22.

Figure Captions
Fig. 1. Proportion of SS free-choice trials during baseline, added delay, and return to
baseline phases.
Fig. 2. CDC Method of Fig. 1. The red line represents the trend line at which if any data
point falls below it, it is considered a criterion to showing a reduction in impulsivity; 8
out of 10 need to be below this trend line to be considered significant.
Fig. 3. Number of switches from SS to LL in free-choice trials during the baseline,
added delay, and return to baseline phases.
Fig. 4. Number of switches from LL to SS in free-choice trials during the baseline, added
delay, and return to baseline phases.
Fig. 5. Total number of SS and LL choices in free-choice trials immediately after SS
or LL forced-choice trials in baseline. White bars represent the smaller sooner free-
choice, and the shaded bars represent the larger later free-choice.
Fig. 6. Total number of SS and LL choices in free-choice trials immediately after SS or
LL forced-choice trials in added delay. White bars represent the smaller sooner free-
Fig. 7. Total number of SS and LL choices in free-choice trials immediately after SS or
LL forced-choice trials in return to baseline. White bars represent the smaller sooner free-
Fig. 8. Number of LL selections in free-choice choice trials immediately after SS forced-
choice trials (shown on the y-axis) in the baseline, added delay, and return to baseline
phases (shown on the x-axis).

Figure 1

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ProportionofSSchoices
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