Discounting of Reinforcers as a Function of Self-Control

Running head: DISCOUNTING OF PRIMARY VS. GENERALIZED REINFORCERS

Discounting of Primary vs. Generalized Reinforcers as a Function of Self-Control

Randi Hovey

Utah State University

1

2DISCOUNTING OF PRIMARY VS. GENERALIZED REINFORCERS

Abstract

The phenomenon of delay discounting involves the decrease in value of a reinforcement

due to the delay of its presentation. Individuals were asked to choose between hypothetical

reinforcers presented at different values and delay intervals using money as a generalized

reinforcer in one session and food as a primary reinforcer in a second session. The present study

examines delay discounting as a function of impulsivity in relation to the type of reinforcer

within the general population. Data were analyzed using area under the curve and the results

support previous findings which show that generalized reinforcers are less-steeply discounted

than primary reinforcers.


Discounting of Primary vs. Generalized Reinforcers as a Function of Self-Control

Delay discounting, a decision-making phenomenon which has often been associated with

levels of impulsivity. It involves the devaluation of reinforcers due to delayed delivery of the

reward. According to Ainslie (1974), an impulse is defined by the choice of smaller, immediate

rewards at the expense of a large, long-term loss. Discounting is commonly measured by an

indifference point. Essentially this is the point at which a participants’ preference changes from

the one reward to the other. Many maladaptive behaviors such as substance abuse and over-

eating, may perpetuate as a result of discounting (Myerson, 2001; Odum and Rainaud, 2003).

Theoretically, an individual’s lack of self-control results in decreased value of rewarding

outcomes as a function of time (Ainsle, 1974). Enhanced understanding of this phenomenon and

its relationship with various reinforcers may lead to more successful treatment options for some

of society’s most insuperable fallacies. Furthermore, increased evidence of a correlational

relationship between impulsiveness and discounting may prove valuable in the construction of

preventative treatment procedures.

Previous research findings have suggested notable variation in the amount of discounting

between primary reinforcers (i.e. food, water, sex, drugs) and generalized conditioned reinforcers

such as money. Madden et al. (1997) conducted research on the rate of discounting in monetary

rewards compared to that of substance rewards in heroin addicts. Addicts were shown to

discount heroin reinforcers more steeply than money. While these within-subjects results do

suggest primary reinforcers increase impulsive behavior, heroin addicts also showed increased

rate of discounting when compared to a control group on monetary rewards alone. These results

suggest the degree of discounting is a complex combination involving trait characteristics and

type of reinforcer.


In hope of clarifying the distinct role type of reinforcer plays in the discounting

phenomenon, Odum and Rainaud (2003) examined delay discounting of food, alcohol, and

money in participants who were previously screened for addictive disorders. By controlling for

addiction, the experiment could more accurately discern variation in degree of discounting for

primary reinforcers as opposed to conditioned reinforcers without intervening effects caused by

addictive behaviors. Using hypothetical rewards in dollar amounts and equivalent units of

alcohol and food, they found that overall money was less steeply discounted than the primary

reinforcers. Results also showed no significant difference in degree of discounting between food

and alcohol (Odum and Rainaud, 2003). Although these findings show significance, no

replication has been reported to confirm the external validity of these results.

The present experiment expands on previous findings regarding the discounting rates of

different types of reinforcers. Unscreened participants were asked to make a series of choices in

two separate sets of questions that measured discounting for money and food. The goal was to

determine whether hypothetical primary reinforcers tend to be discounted more steeply than

hypothetical generalized conditioned reinforcers. Data was used to determine any correlational

relationship between type of reinforcer, and amount of self-control.

Method

Participants

Participants were 40 online PSY 3400 students from Utah State University. Subjects were

all 18 years of age or older and originated from various geographical locations. Over seventy

percent of respondents were female (72.5%), 27.5% were male. Due to the nature of subject

selection, no socioeconomic information was included in the process of selection. Subjects were


awarded 5 points toward their total grade in the class in return for participation in experimental

sessions which took an average of 60 minutes to complete. There was no pre-screening done,

allowing results to be more indicative of general population parameters.

Materials

In order to examine discounting using similar techniques as previous studies, hypothetical

rewards were used. Hypothetical food reinforcers were presented in units which were determined

by the cost per unit of food. Food and money reinforcers began at $100 or equivalent units and

decreased down to $1. Data was collected using Qualtrics Research Suite.

Procedures

Participants were asked to choose between immediate or delayed money or food rewards

in two separate sessions which were counterbalanced for type of reinforcer as well as delay. The

delayed reward for each instance was always $100 or the equivalent units of participant’s

favorite food. Immediate rewards also began at $100 value and were decreased at specific

intervals over the course of the session. Instructions given prior to participation made the

hypothetical nature of rewards clear and were as follows:

You will be asked to make some decisions about which of two (monetary/food) rewards

you prefer. You will not receive the rewards that you choose, but I want you to make

your decisions as though you were really going to receive these rewards you choose. The

possible rewards will be displayed to you in the form of a multiple-choice survey

question. The options on your left display a reward that you can get today. The options on

the right display the reward that you can get after the specified amount of time. So now

you are being asked to choose between an immediate amount delivered today versus a


delayed amount that you would get after waiting the delay interval. Select the reward that

you would prefer. The choices you make are completely up to you. Please select the

option that you prefer, not what you think I want you to prefer. I do not expect you to

choose one particular reward over the other. Just choose the reward you really want.

Prior to beginning the food discounting task, participants were asked what their favorite

food was and how much they would typically spend on one unit of this food. This information

was then used to compute rewards equivalent to amounts given as reward choices in the

monetary session. Reward intervals for immediate amounts were presented in the following

intervals; $100, 99, 97.50, 95, 92.50, 90, 85, 80, 75, 65, 60, 55, 50, 45, 40, 35, 30, 25, 20, 15, 10,

7.50, 5, 2.50, and 1. Amount of delay also varied between 1 week, 2 weeks, 1 month, 6 months,

1 year, 5 years, and 25 years. Participants were asked simply to select the reward that they would

prefer.

Data Analysis

In order to assess the differences of outcome between discounting of primary vs.

generalized reinforcers, individual indifference points were normalized and calculated using the

area under the curve (Myerson et al., 2001). This method was used due to its theoretically neutral

measurement of discounting. To normalize the data, each value and delay is calculated as a

proportion of maximum value and maximum delay respectively. AUC is then calculated using

the sum of the results of the following equation: (x2 – x1) [(y1 + y2)/2] applied to each successive

delay and the corresponding values where x1, x2 refer to successive delays and y1, y2 refer to the


corresponding indifference points. AUC value is then interpreted within a range from 1 (no

discounting) to 0 (maximum discounting) (Odum and Rainaud, 2003).

Results

Median indifference points for money and food are presented in Figure 1, showing the

rate of discounting for food (red data path) and money (blue data path). Outcome for both food

and money values were included for all 40 participants with no missing values. Both primary and

generalized reinforcers lost value as delay increased. However, congruent with previous research

findings, money was less steeply discounted than food reinforcers expressed in equivalent

amounts. For example, the discounted rate of $100 after a 30-day delay was $85 while the

equivalent amount of food was discounted to $57.5 after a 30-day delay as shown in table 1.

Figure 2 shows the mean value of AUC for money (blue bar) in comparison to the mean

value of AUC for food (red bar). AUC for money (.27) was higher than AUC for food (.14).

Discussion

Findings in the present study further support previous research on the degree of

discounting using consumable vs. non-consumable reinforcers. Kirby and Guastello (2001)

found that monetary rewards were discounted at a smaller degree when presented to college

students than when an equivalent amount of pizza was offered on the same delay scale.

Reinforcers in this study were not entirely hypothetical; participants did receive one randomly

chosen specified outcome after completing the trial (Odum and Rainaud, 2003). Johnson and

Bickel (2002) conducted a similar study using only monetary rewards as hypothetical reinforcers

in one experiment, and real rewards in another and determined no significant difference in the


degree of discounting between samples; further justifying the use of hypothetical rewards in

present research.

Results obtained in this experiment further confirm increased impulsivity in relation to

primary reinforcers, and extend these findings to a general population in the absence of control

for certain attributes, socioeconomic status, or behavioral traits. Future studies should involve

further statistical analyses to determine the significance of such findings within a sample from

the general population. If the degree of discounting primary rewards vs. generalized rewards

within general samples proves statistically significant, findings may have important implications

for determining levels of impulsivity which may put individuals at higher risk of developing

addictive behaviors and could contribute to better preventative measures for substance abuse,

over-eating habits, and possibly inclination to criminal behavior in general.


References

Ainslie, G., 1974. Impulse control in pigeons. Journal of the Experimental Analysis of Behavior.

21, 485-489.

Johnson, Matthew W., Bickel, Warren K., 2002. Within-subject comparison of real and

hypothetical money rewards in delay discounting. Journal of the Experimental Analysis

of Behavior. 77, 129-146.

Kirby, K.N., Guastello, B., 2001. Making choices in anticipation of similar future choices can

increase self-control. J. Exp. Psychol. Appl. 7, 154–164.

Myerson, Joel et al., 2001. Area Under the curve as a measure of discounting. Journal of the

Experimental Analysis of Behavior. 76, 235-243.

Odum, Amy L., Rainaud, Carla P., 2003. Discounting of delayed hypothetical money, alcohol,

and food. Behavioural Processes. 64, 305-313.


Table 1. Numerical indifference points for money and food observed for the specific delay

intervals (in days).

Days 7 14 30 180 365 1825 9125Money 97.5 90 85 65 55 35 7.5Food 77.5 70 57.5 47.5 27.5 20 1.75

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 100000

102030405060708090

100

Median Indifference Points

MoneyFood

Delay (days)

Subj

ectiv

e Va

lue

Fig. 1. Indifference points for money (blue data path) and food (red data path) as a function of

delay.


10

0.05

0.1

0.15

0.2

0.25

0.3

AUC

MoneyFood

Reinforcer Type

AUC

Fig. 2. Mean AUC values for money (blue bar) and food (red data path).

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Discounting of Reinforcers as a Function of Self-Control