JOURNAL OF THE EXPERIMENTAL ANALYSIS OF BEHAVIOR 1993, 59, 483-500 NUMBER 3 (MAY) EFFECTS OF INCOME ON DRUG CHOICE IN HUMANS R. J. DEGRANDPRE, WARREN K. BICKEL, S. ABU TURAB Rizvi, AND JOHN R. HUGHES UNIVERSITY OF VERMONT

The effects of income (money available to spend during the experimental session) on human choice were examined in a concurrent-schedule arrangement. Subjects were 7 nicotine-dependent smokers, and reinforcers were puffs on the subject's usual brand of cigarette ("own") and puffs on a less preferred brand of cigarette with equal nicotine content ("other"). Across sessions, income varied and the price of the two reinforcers was held constant, with the other puffs one fifth the price of the own puffs. As income increased, consumption of own puffs increased while consumption of the less expensive other puffs decreased. These effects of income on choice were highly consistent across subjects. For some subjects, however, income had little effect on total puff consumption. Finally, an additional condition examined whether price and income manipulations would have functionally equivalent effects on choice by repeating an income condition in which the price of the other brand was increased. Although the increased price of the other puffs decreased their consumption in 4 subjects, 2 subjects showed increased consumption of the other puffs at the higher price. The results, when defined in economic terms, indicate that the own puffs were a normal good (consumption and income are directly related), the other puffs were an inferior good (consumption and income are inversely related), and the direct relationship between consumption of the other puffs and their price is defined as a Giffen- good effect. The latter result also suggests that for these 2 subjects, price and income manipulations had equivalent effects on choice. These results extend findings from previous studies that have examined the effects of income on choice responding to human subjects and drug reinforcers, and provide a framework for further experimental tests of the effects of income on human choice behavior. Meth- odological and theoretical implications for the study of choice and for behavioral pharmacology are discussed. Key words: behavioral economics, behavioral pharmacology, choice, demand, drug reinforcers, nic- otine, drug policy, income, lever press, humans

Numerous studies have investigated choice other stimulus. As stated by Katz (1990), the between drug reinforcers and between drug relative frequency of responding for one re- and nondrug reinforcers. Under these proce- inforcer "is used as an indication of the dif- dures, two or more reinforcers are available in ferences in reinforcing effectiveness of the two a concurrent-schedule arrangement. Respond- consequences" (p. 285). ing on one manipulandum is maintained by For example, one study examined choice drug reinforcement, while responding on an- responding for different doses of intravenous other manipulandum is maintained by either cocaine in rhesus monkeys using concurrent a different dose of the same drug, a different variable-interval 1-min schedules (Iglauer & drug, or a nondrug reinforcer. These studies Woods, 1974). Greater responding (and some- are typically used to assess the reinforcing ef- times exclusive responding) occurred for the fectiveness of one stimulus relative to some higher dose, suggesting greater effectiveness of the higher doses. Other studies have examined choice between different drug reinforcers. For This research was conducted when the first author was example, one study examined the effects of a predoctoral student in the Department of Psychology, methylphenidate dose on responding main- University of Vermont. This work is funded by a Sigma one two Xi Grant-in-aid (R.J.D.), National Institute on Drug tained by it and of doses of cocaine Abuse Training Grant T32 DA-07242 (R.J.D.), and (Johanson & Schuster, 1975). Preference for Grants RO1 DA-06626 (W.K.B.), and PHS K02-00109 cocaine decreased as the comparison dose of (J.R.H.). We thank Melissa Jurgens and Michael Layng methylphenidate increased, suggesting, again, for their assistance in preparing the manuscript. Send that the effectiveness of a reinforcer may vary requests for reprints to the first or second author at the Department of Psychiatry, Human Behavioral Pharma- as a function of dose. These and numerous cology Laboratory, 38 Fletcher Place, University of Ver- other studies have shown the utility of choice mont, Burlington, Vermont 05401-1419. procedures for examining relative reinforcing 483 484 R. J. DeGRANDPRE et al. effectiveness of pharmacological and non- above, studies investigating income have de- pharmacological substances (for a review of fined income as the resources available to the studies on choice and reinforcer effectiveness, organism to obtain food during the experi- see Katz, 1990). mental session (i.e., number of trials or rein- Other studies suggest that choice may also forcers per session; Battalio, Kagel, & Kogut, be a function of variables that are independent 1991; Hastjarjo & Silberberg, 1992; Hastjarjo of the reinforcers (e.g., intertrial interval; ITI). et al., 1990). For example, in one study of One such study examined whether choice be- macaque monkeys, ITI was varied as the in- tween heroin and food, at constant magnitudes, come manipulation (i.e., number of reinforced would vary as a function of ITI (Elsmore, trials per session) while session length was Fletcher, Conrad, & Sodetz, 1980). Even held constant (Silberberg et al., 1987, Exper- though an equal number of choices for heroin iment 1). Choice of a large bitter-tasting food and food occurred at low ITIs, heroin choices pellet increased relative to a small normal pel- decreased considerably more than food choices let when income was decreased. In economic as ITI increased. This finding is important in terms, the bitter pellet was an inferior good showing that choice between two reinforcers and the small normal pellet was a normal good can be altered by variables that are indepen- in this context (Silberberg et al., 1987). A sec- dent of the schedule and magnitude of rein- ond study manipulating income (Hastjarjo et forcement for the concurrently available re- al., 1990) replicated the effects of income re- inforcers. More recently, the manipulation ported in the Silberberg et al. study using rats made in the Elsmore et al. study (1980) has as subjects, and a third study (Hastjarjo & been conceptualized in terms of the economic Silberberg, 1992) extended these findings to notion of income and has been further shown show that income could significantly alter to affect choice between concurrently available choice between an immediate and delayed re- different reinforcers (Silberberg, Warren- inforcer in rats. The present study assessed the Boulton, & Asano, 1987). effect of income on drug choice in human cig- Income can be defined as the amount of funds, arette smoking. This study also examined, in goods, or services available to any one indi- a more limited scope, whether these income vidual at any given time (Pearce, 1986). In manipulations might have effects on drug behavioral terms, income manipulations can choice that are functionally equivalent to those be conceptualized as constraints on total re- of price manipulations. inforcement within a session. Microeconomic Although considerable research has been theory considers the relation between income conducted on choice using drug reinforcers, to and consumption and recognizes that increases our knowledge there are no laboratory studies in income can either increase or decrease choice that explicitly manipulated income using drugs of any particular good, depending on the type as reinforcers. Nor, to our knowledge, have of good and the availability of other goods the effects of income been investigated in hu- (Deaton & Muellbauer, 1980; Lea, Tarpy, & mans in a laboratory setting. Webley, 1987). For example, an increase in income might increase seafood consumption while decreasing consumption of hamburger. METHOD The former relationship is defined in economic terms as a normal good (i.e., normal-good ef- Subjects, Apparatus, and Reinforcers fect), and the latter is defined as an inferior One female and 6 male smokers partici- good (i.e., inferior-good effect). Normal goods pated. Subjects ranged from 20 to 38 years old often correspond to the everyday notion of lux- and smoked one or more packs of 0.6- to 1.2- uries, whereas inferior goods are often neces- mg nicotine cigarettes per day. Nicotine self- sities. In recent experimental analyses, the eco- administration via cigarette smoking was used nomic notion of income has been extended to in this study because it is well established as nonhuman choice between nonidentical rein- a potent reinforcer in humans (Henningfield forcers (Hastjarjo & Silberberg, 1992; Hast- & Goldberg, 1983) and because it has many jarjo, Silberberg, & Hursh, 1990; Silberberg features of other drugs of dependence (West et al., 1987). Consistent with the definitions & Grunberg, 1991) that are responsive to en- EFFECTS OF INCOME ON CHOICE 485

Table 1 Subject characteristics. Cigarettes Fagerstrom per day Nicotine/tar Subject Age Gender scores (avg.) Preferred brand (mg/cigarette) BT 35 M 8 25 American Lightss 1.2/16 JH 38 M 9 30 Winstons 1.1/17 JR 22 M 8 30 Marlboro, 1.2/17 KC 20 F 7 20 Camel LightsD 0.7/09 KS 36 M 9 30 Merits 0.6/08 PZ 25 M 7 25 Camel1 filter 1.0/15 WR 20 M 8 20 Marlboros 1.2/17 vironmental variations (Hughes, 1989). Sub- ject was asked to rate the three brands from jects were recruited from newspaper adver- most to least preferred. Following this assess- tisements, were in good health, and reported ment, the least preferred brand was provided no medication usage or drug or alcohol abuse to the subject as the second brand (referred to other than nicotine. Subjects had to score a as "other" brand below). Those subjects who minimum of 7 on the Fagerstrom's Tolerance completed this assessment after the first session Questionnaire (Fagerstrom & Schneider, 1989) were provided with one of these three brands to participate in the study. Subjects were com- (chosen arbitrarily) during the first session. pensated for their participation with monetary payment that was not contingent upon any Procedure particular type of performance in the experi- Subjects participated in a minimum of 11 mental session. Table 1 lists these and other 3-hr sessions (range, 1 1 to 18), one session per relevant characteristics of the subjects. day. The number of sessions varied due to An Apple IIe® microcomputer controlled subject dropouts (Subject PZ) and differences and obtained data from a response console (61 between subjects in the time necessary to reach cm by 30 cm by 46.5 cm) that contained three stability. Sessions were conducted 3 days per Lindsley plungers (Gerbrands No. G6310; week (either Tuesday, Thursday, and Friday centered from left to right on the face of the or Monday, Wednesday, and Friday). Prior response console). Subjects responded on the to each experimental session, the subject was left and right plungers only. Responses made required to abstain from smoking to reach a on the center manipulandum produced no pro- carbon monoxide (CO) level of 50% or lower grammed effect. Sessions were conducted in than his or her nonabstinence level (obtained rooms that contained one response console, during the initial interview). This technique overhead fluorescent lighting, a desk lamp, was used because expired air CO has been several current magazines, the daily local shown to correlate with the number of ciga- newspaper, and a radio. Carbon monoxide rettes smoked (Henningfield, Stitzer, & Grif- from expired air samples from the lungs was fiths, 1980). Subjects were instructed to ab- measured using a MiniCO® Carbon Mon- stain from cigarette smoking for 5 to 6 hr in oxide Breathkit (produced by Catalyst Re- order to meet this criterion. If a subject failed search Corporation). to reduce his or her CO to the required level, Throughout the experiment, each subject the session was canceled without payment and had access to two brands of cigarettes. The was rescheduled. first brand was the subject's own brand. The Next, each subject took one uniform puff on second brand was determined either prior to his or her own cigarette 30 min prior to the or following the first session. The subject was start of the session (cigarettes were provided provided with three brands; each was different by the experimenter). This 30-min presession than his or her own brand but had an equiv- puff was used to equate the time from last alent nicotine rating (based on Federal Trade cigarette exposure across subjects (see Hen- Commission [FTC] ratings, 1991). The sub- ningfield & Griffiths, 1981). 486 R. J. DeGRANDPRE et al.

Instructions. On the 1st day of the experi- design. That is, after establishing a baseline, ment (Session 1), subjects were instructed: the three income conditions were examined with baseline conditions interposed between In this study you can respond on the left and right levers to earn puffs. You will have a cer- them. Each baseline condition was maintained tain amount of money provided to you to spend until choice of the own puffs was stable (i.e., on puffs during the session. On different days plus or minus one self-administration of own the amount of money may vary and so will the puffs over three sessions; JH's data did not cost of the puffs for both types of puffs. Prior meet this stability criterion). The sequence of to beginning each day you will be provided with the three income conditions varied across sub- this sheet, which will inform you about the jects in a mixed order (see Table 2). Each parameters above. When you have earned puffs, condition was completed in a single session the cost of these puffs will be subtracted from (except for baseline sessions). the amount you have remaining, which will be shown on the screen. Also, when you have Following the completion of these income earned puffs you will have 5 minutes to take conditions, some subjects completed additional them. You will be instructed to inhale, hold for conditions. First, additional income conditions 5 seconds, and exhale. The screen will count were used to assess the effects of a more com- down the 300 seconds and will tell you when plete range of income amounts on drug choice you can respond again by displaying "You may (see Table 2). Second, a single session was respond now." conducted in which the price of the other puffs Also prior to the first session and all other was raised from $0.10 to $0.25 (for two puffs) sessions, subjects were given the following daily at one of the income amounts previously tested parameters for that session: (i.e., Conditions B, C, or D). The price of the other puffs was increased to examine whether 1. Your brand (left lever/one response re- the price manipulation would produce an in- quired) crease or decrease in consumption of that good Puffs cost: cents for two puffs. relative to choice at the lower price at the same 2. Other brand (right lever/one response income amount. required) The procedure for the 3 subjects who par- Puffs cost: cents for two puffs. ticipated first was identical to the above pro- 3. Today you have $ to spend on puffs. cedure except that (a) baseline income was an Daily procedure. The sequence of conditions amount that constrained subjects' choice re- varied across subjects because the procedure sponses for their own puffs such that income was modified after the first 3 subjects UH, manipulations consisted of increases in income KS, and WR) completed the study. The in- (instead of manipulations that decreased in- come conditions in the modified procedure were come), (b) subjects obtained one puff per self- more systematic than in the earlier procedures administration (instead of two puffs), (c) the and thus will be described first. income conditions were selected in a less sys- All subjects who completed the modified tematic fashion (i.e., not necessarily one half, procedure completed a minimum of four in- one third, and one sixth of baseline income), come conditions: baseline income (Condition and (d) fewer income manipulations were made A), one half baseline income (Condition B), (see Table 2). one third baseline income (Condition C), and Puffingprocedure. After responding for puffs, one sixth baseline income (Condition D). These the subject lit one of the cigarettes he or she percentages of income were rounded up to the was provided, but did not inhale. The subject nearest $0.50. For each of these income con- was required to take one puff (first 3 subjects) ditions, price of the subject's own and other or two puffs within 5 min according to a uni- puffs was $0.50 and $0.10, respectively, for form-puff procedure in which the subject took two puffs. Income for the initial baseline ses- one uniform puff, inhaled, retained the smoke sion was set at $15.00. Income for all remain- in the lungs for 5 s, and then exhaled (Griffiths, ing baseline sessions (i.e., baseline income) was Henningfield, & Bigelow, 1982). When a sec- that amount of money spent in the first session ond puff was allowed, it was taken 25 s after rounded up to the nearest $0.50. The exper- the subject exhaled the first puff. Also, the imental design for these income-manipulation experimenter observed and recorded the sub- conditions can be described as an ABACAD ject's puff topography (e.g., shallow, normal, EFFECTS OF INCOME ON CHOICE 487

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Fig. 1. Consumption (puffs) of own and other puffs is plotted as a function of income amount for all 7 subjects. Data at the baseline income amount (shown in Table 2) for each subject represent mean consumption across all baseline sessions. An asterisk is shown on the x axis to represent the baseline income. EFFECTS OF INCOME ON CHOICE 491 or deep puffs) during the first session and in- z 100 the not to that . structed subject change topog- 0 raphy. Topography was monitored through- IL. *Em: out the study, and the subject was told of 0 deviations. z * JH 0 U o 10 i U 3B JR ;U' * KC RESULTS o KS (0 U LPZ z o WR Effects of Income 0 When choice of the two reinforcers (own 1. * U puffs and other puffs) is plotted as a function 1 10 100 of income amount (see Figure 1), the magni- INCOME (IN $) tude of the income effect is an orderly function Fig. 2. Consumption (puffs) of own puffs for all 7 of this variable (data are plotted in log-log subjects is plotted together as a function of income amount. coordinates to show proportional change such These data are also shown in Figure 1 separately for each subject. Data at the baseline income amount (shown in that the slope of the line, in economic terms, Table 2) for each subject represent mean consumption equals its elasticity coefficient; see Samuelson across all baseline sessions. & Nordhaus, 1985). In all but two cases, every successive increase in income, from the lowest income amount to the highest income amount, increased choice responses for the own puffs. and increased choice responses for the other In those cases in which an increase did not puffs. In many cases, when the session income occur (low-income conditions for BT and KC), was radically different from the previous ses- no responses were made for the own puffs sion's or the following session's income, the across the two consecutive income amounts (i.e., reinforcer chosen reversed, such that the re- floor effect). A similar function exists in the inforcer preferred in one session was opposite opposite direction for the other puffs, albeit to that of the next session. more variable (i.e., increases in income de- The effects of income on total consumption creased choice responses for the other puffs). were varied. First, the income manipulations Several subjects' data for both own puffs and had little effect on 2 subjects' (BT and WR) other puffs suggest an asymptote in consump- total consumption, even though choice was sig- tion at high and low income amounts, respec- nificantly affected. Note that Subject BT's to- tively (e.g., BT and KC). To illustrate the tal consumption was affected at very low and across-subject similarity of this function for the very high incomes. Also, Subject WR's base- own-puff data, these data for all 7 subjects line levels of consumption shifted in the middle shown in Figure 1 are collapsed into a single of the experiment and returned to his original function in Figure 2. The effects of income on levels toward the end of the study. Second, 3 consumption of subjects' own puffs are mark- subjects' (JH, KC, and PZ) total consumption edly similar across subjects. generally decreased as income decreased. In order to illustrate better the control ex- However, total consumption for 2 of these sub- erted by the income manipulation over choice jects (KC and PZ) was generally constant ex- responding across sessions for each subject, the cept when income was so low that baseline data plotted in Figure 1 are plotted again in levels of consumption were not mathematically Figures 3 and 4 as a function of session order. possible. Subject JH, whose choice responding Total puffs (own puffs + other puffs) are also did not meet stability requirements after the plotted for each subject. Recall that Subjects first income manipulation (one third condi- JR, KS, and WR participated in the original tion), showed gradual decreases in consump- procedure. tion as the experiment progressed. Finally, for Changes in income across sessions signifi- Subjects JR and KS, the variability in total cantly altered choice responding. Again, de- puffs produced by the income manipulations creases in income from the baseline amount is similar in magnitude to the variability in decreased choice responses for their own puffs total puffs across the baseline conditions. This 492 R. J. DeGRANDPRE et al.

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0 2 4 6 8 10 12 14 16 18 20 (M 2 4 6 8 10 12 0 2 4 6 8 10 12 14 16 18 20 SESSION Fig. 4. Consumption (puffs) of the two reinforcers (own and other puffs) is plotted as a function of session for the 3 subjects who participated under the original procedure (top panels). The income amount for each session is indicated in dollar amounts just above one data point for each session. Also, total puffs (own + other puffs) are plotted as a function of session (lower panel). The data for sessions in which price was varied are not shown. variability makes it difficult to interpret the overall data showed preferences for the more effects of income on total consumption. expensive brand (i.e., increases in own puff Another aspect of the total-consumption data consumption with increases in income). This is the finding that subjects sometimes did not occurred especially when the first session had spend their entire income even in sessions in a $15.00 income (see data from Subjects BT, which they responded for both types of puffs JH, KC, and PZ). The most striking example (see "$ not spent" and "distribution within of this pattern of responding is shown by Sub- session" in Table 2). This result appeared to ject BT's data; he responded exclusively for occur primarily because subjects conserved their own puffs when income was very high. This income to ensure that they could obtain puffs tactic is also indicated by the distribution of throughout the session. As a result of this tac- responding by many subjects that alternated tic, subjects frequently ran out of time in the between the more expensive puffs (own puffs) session prior to spending their allotted income. and the less expensive puffs (other puffs), es- For example, even when subjects were allotted pecially toward the end of the session (see Ta- an income that was almost (or more than) ble 2). enough to respond exclusively for the more expensive puffs (i.e., own puffs), they still of- Effect of Price ten purchased some or many of the cheaper Table 3 shows the effects of repeating an brand (i.e., other puffs), even though their income condition when the price of the other

Fig. 3. Consumption (puffs) of own and other puffs is plotted as a function of session for the 4 subjects who participated under the modified procedure (top panels). The income amount for each session is indicated in dollar amounts just above one data point for each session. Also, total puffs (own + other puffs) are plotted as a function of session (lower panel). The data for sessions in which price was varied are not shown. 494 R. J. DeGRANDPRE et al.

Table 3 using postsession ratings of "how much they Results of price manipulation. liked" their own puffs and the other puffs. This was done by asking subjects to respond Relative Quantity quantity to this question on two 100-mm visual ana- consumed consumeda logue scales (VAS; 100 = "very much") that to brands of puffs. These Low High Low High corresponded the two Subject Brand price price price price data were collected only for the 4 subjects who participated under the modified procedure. The BT own 20b 12 54b 33 mean "liking" scores across all sessions for the other 17b 24 46b 67 JH own 12 12 37.5 60 own and other puffs, respectively, were 81.2/ other 20 8 62.5 40 57.5 for Subject BT (who showed an increase KC own 14 12 58.3 60 in consumption of the other puffs), 82.8/0.0 other 10 8 41.7 40 for Subject JH, 89.9/1.83 for Subject KC, and KS own 3b 1 37.6b 10 for Subject PZ (all of whom showed other 8.13b 9 62.4b 90 87.6/0.88 PZ own 6 6 37.5 60 decreased consumption of the other puffs). other 10 4 62.5 40 When liking scores for the own or other puffs WR own 5.67b 3 30.2b 23 are removed for those sessions in which own other 11.44b 10 69.8b 77 or other puffs were not chosen, these results a Percentage of total puffs. do not change. That is, subjects reported very bMean, based on multiple sessions. similar liking scores regardless of whether they smoked a particular brand during the session. Finally, to examine the possibility that an puffs was increased from $0.10 to $0.25 (ex- increase in price may be functionally equiva- cept Subject KS, whose other-puffs price in- lent to a decrease in income, a post-hoc analysis creased to $0.20; Subject JR did not complete was done that compared consumption during the price condition). The effect of the price the high-price condition (shown in Table 2) increase is shown in absolute terms (con- to the consumption predicted from subjects' sumption of own and other puffs) and in rel- income functions (shown in Figure 1). To do ative terms (percentage of total consumption this, the increase in price was transformed into for own and other puffs). If multiple sessions a decrease in income by using the observed of the income condition were completed at the consumption of both brands at the higher price low price (i.e., if baseline income was used), to determine the income that would have been the data for the low price represent a mean. spent in the price condition if the other good Increasing the price of the other puffs in- was at its previous price of $0. 10 per two puffs. creased the absolute consumption of the other For example, BT spent $3.00 on both brands puffs by Subjects BT and KS and relative con- during the price condition at an income of sumption by Subjects BT, KS, and WR. For $6.00. Thus, if the other puffs had cost their example, BT increased consumption of the normal price ($0.10 per two puffs), BT would other puffs from a mean of 17 puffs to 24 puffs; have required an income of only $4.20 to pro- WR increased relative consumption of the other duce this result. Using this amount ($4.20), if puffs from 69.8% to 77%. Importantly, the the price and income manipulation were func- absolute or relative consumption at the high tionally equivalent for this subject, consump- price does not appear to be related simply to tion at $6.00 income (at the high price for other the absolute or relative consumption at the low puffs) should equal consumption at $4.20 at price. That is, although Subjects KS and WR- the low price. To test this, we can plot BT's who showed relative increases in consumption data from the price condition at an income of of the other puffs at the higher price-chose $4.20 along with the income data shown in more of the other puffs at the low price, so did Figure 1. 2 of the 3 subjects (JH and PZ) who did not For the 3 subjects who showed relative in- show relative or absolute increases (see Table creases in consumption for the other puffs dur- 3). ing the price condition (BT, KS, and WR), To examine these individual differences, the consumption was consistent with the respective acceptability of the other puffs was assessed income functions for own and other puffs (see EFFECTS OF INCOME ON CHOICE 495

* own (low price) O other (low price) M own (high price) Q other (hlgh price) BT 100' 100 JH . . . o U O O !0 - 10 10 o0 U

I * - 0 0 I - 0 0 1 10 100 1 10 100

100 KC 100 KS

n O 0i- 0 U-= 10- 0 00m~~ 10 UOoos

1'- 1 . 1 10 50 1 10 100 WR 100 - PZ 100

U 10 0 00 10 0 0 U-

1 10 100 1 10 100 INCOME (IN $)

Fig. 5. The same data plotted in Figure 1 are shown for the subjects who completed the price condition (small symbols) along with the data from the price manipulation (large symbols). The latter data are plotted at the income amount that would have been spent if puffs were only $0.10. Own-puff and other-puff data at the baseline income amounts are means across all baseline sessions.

Figure 5). However, data from the 3 subjects 3 subjects' consumption of own puffs was who showed decreases in relative consumption greater than predicted by their income function of the other puffs UH, KC, and PZ) were not for those puffs, whereas these subjects' con- consistent with their respective income func- sumption of other puffs was clearly lower than tions for the own and other puffs. Instead, all predicted by the function for the other puffs. 496 R. J. DeGRANDPRE et al.

DISCUSSION exchange procedure was used in which sub- The main results can be summarized as fol- jects were provided with some income to spend lows: First, as income increased, consumption throughout the session via a discrete response of own puffs increased while consumption of for one or both of two differently priced re- the less expensive other puffs decreased. The inforcers. Despite these differences, at least magnitude of these within-subject effects was four other studies have demonstrated normal an orderly function of income. For some sub- and inferior goods using a concurrent-operant jects, these effects of income on choice had little arrangement (Battalio et al., 1991; Hastjarjo effect on total drug consumption. Second, in- & Silberberg, 1992; Hastjarjo et al., 1990; Sil- creases in the price of the other puffs resulted berberg et al., 1987). in a decrease of other puff consumption in 4 By demonstrating similar effects with a new subjects, but resulted in an increase in other species (humans) and a new and important puff consumption in the other 2 subjects who class of reinforcers (drug), this study further completed this condition. demonstrates the utility of the income concep- As outlined in the introduction, the effects tualization in the study of choice. Concerning of income and price manipulations have been species generality, replicating the effects of in- conceptualized in economic terms in previous come on choice with humans is of obvious im- studies (e.g., Silberberg et al., 1987). Goods portance. The present study not only shows whose consumption increases as income in- the effects of income previously shown in an- creases are typically defined as normal goods imals but goes further in demonstrating more (i.e., goods for which income and consumption complete functional relations between income, are directly related), whereas goods whose con- price, and choice using a medium-of-exchange sumption decreases as income increases are procedure common to everyday human envi- typically defined as inferior goods (i.e., goods ronments. Concerning reinforcer generality, for which income and consumption are in- the finding that drug reinforcers are affected versely related). Consequently, own puffs in by income in much the same way as food re- the present study can be defined as a normal inforcers are indicates that these effects of in- good and other puffs as an inferior good for come are general and appear to be applicable all 7 subjects. As shown in Figures 1 and 2, to most or all primary reinforcers. Finally, the the effects of income on choice can be plotted different effects of income on concurrently so as to depict the nature of these goods. These available reinforcers were well described by functions, in which consumption is plotted as income functions (i.e., Engel curves) and the a function of income, are defined in economic characterization of inferior or normal goods. terms as Engel curves. Moreover, the effect of This conceptualization is useful in that (a) it increasing consumption of an inferior good by provides a framework to organize the differ- increasing its price is termed a Giffen-good ef- ential effects of income across a range of in- fect. That is, when the slope of a demand (price) come magnitudes that produce complete para- function is positive-as opposed to the negative metric functions, and (b) the two functions for slope predicted by the law of demand-the normal and inferior goods provide an exper- good is acting as a Giffen good (Lea et al., imental-economic model or baseline for fur- 1987; cf. Gilley & Karels, 1991). Such an effect ther complex analyses, such as the interaction was shown here in 2 of 6 subjects. Note that between income and other more traditional when the slope of a demand (price) function factors shown to influence choice (discussed is negative, the good is acting as an ordinary below). good. The implications of these effects and The functions for normal and inferior goods conceptualizations are discussed below. provide an important demonstration that a re- inforcer is not endowed with inherent prop- Income Manipulations erties independent of the historical and current The income manipulation employed in pre- context in which it controls behavior. For ex- vious research on the effects of income on choice ample, in the present study, income reversed has been trials per session (Hastjarjo & Sil- preference between the two reinforcers such berberg, 1992; Hastjarjo et al., 1990; Silber- that reinforcer effectiveness, conventionally berg et al., 1987). The income manipulation defined (see Katz, 1990), depended on income in this study differed in that a medium-of- and not on any aspects of the reinforcers per EFFECTS OF INCOME ON CHOICE 497 se (e.g., within-session rate or magnitude of ever, when income decreased, preference re- reinforcement). Given that the availability of versed, thus altering the assessment of abuse a drug is almost always constrained in drug liability. self-administration studies, the choice observed In terms of drug taking and drug policy, between the concurrently available reinforcers these findings suggest that decreases in the drug may be due, in part, to the prevailing income. user's income (e.g., unemployment) may not This is not to say, however, that qualitative affect drug intake (i.e., decrease it) but rather and quantitative aspects of the reinforcer and may affect drug choice (e.g., switching from reinforcement schedule are not important. cocaine to crack cocaine). This finding also Rather, the effect a reinforcer has on respond- suggests that the development and production ing is determined by a confluence of these en- of lower priced drug substitutes (e.g., crack vironmental variables. cocaine) may increase as a result of increases This notion of intrinsic properties also ap- in price of the already available drug, decreases plies to the behavioral-economic terms of in- in the supply of the latter, or decreases in ferior, normal, and Giffen goods. Although income. useful in categorizing reinforcers relative to one another across different situations, these Price Manipulations terms describe a relationship among choice, The analysis of the effects of price in the price, and income with respect to a particular present experiment is very preliminary, yet it context and thus do not describe inherent prop- is an initial step toward addressing the inter- erties of the reinforcer per se (cf. Hastjarjo et action between price and income factors in al., 1990). For example, although the other determining drug consumption and the con- puffs were an inferior good for all 7 subjects ditions under which price and income manip- in the present study, this good would likely be ulations are functionally equivalent. The Gif- a normal good in a concurrent arrangement in fen-good effect shown in 2 subjects has been which these puffs were available along with shown in previous experimental analyses. For puffs with no nicotine content. example, in Experiment 2 of Silberberg et al. The effects of income on choice also have (1987), price was manipulated by varying the relevance to issues concerning the above lia- probability of reinforcement for responding on bility of drugs. Abuse liability refers to the like- the lever producing the bitter pellet. By in- lihood that any given pharmacological agent creasing the price of the bitter pellet, choices will be self-administered by humans and will for the bitter pellet increased, thus demon- be used in a nonprescribed fashion for strating a Giffen-good effect. Similar effects nonmedical purposes. The drug self-admin- were also shown in rats by Hastjarjo et al. istration preparation is one method for ascer- (1990) and Battalio et al. (1991). taining abuse liability, based on the finding in As already stated, Giffen-good effects are behavioral pharmacology that drugs of abuse paradoxical in that they are defined by posi- can serve as positive reinforcers and are self- tively sloping demand (price) curves, opposite administered by human and nonhuman ani- in direction to those predicted by the law of mals in laboratory settings (Griffiths, Bigelow, demand (Allison, 1983; Lea et al., 1987). That & Henningfield, 1980; Young & Herling, increases in price of a reinforcer can increase 1986). The present data are relevant to this its consumption is also perplexing in behavior- issue because they demonstrate important ef- analytic terms, because decreases in rate of fects of a variable not widely shown to affect reinforcement are assumed to decrease, not in- drug consumption in laboratory settings (viz. crease, response rate or strength (Battalio et income). By showing that decreases in income al., 1991; Williams, 1988). Despite these ap- in some contexts may actually increase choice parent paradoxes, however, the present study of an inferior good relative to a normal good, suggests that the Giffen-good effect is quan- these data suggest that laboratory analyses of tifiable and that an experimental analysis of abuse liability may need to examine liability such dynamic relations between price and in- in a variety of choice contexts. For example, come is possible. For example, the price ma- at high income levels in the present study, the nipulation was shown here to be functionally other puffs appeared to have low abuse lia- equivalent to an income manipulation for 3 bility relative to the subjects' own puffs; how- subjects (2 of which showed Giffen-good ef- 498 R. J. DeGRANDPRE et al. fects). This conclusion is based on two find- own puffs to eight puffs at a cost of $2.00 (8/2 ings. First, when price of the other puffs in- x $0.50 = $2.00) and increase consumption creased, Subjects BT, KS, and WR increased of other puffs to 32 puffs at a cost of $4.00 the proportion of money spent on other puffs (32/2 x $0.25 = $4.00). In actuality, BT did (46% - 67%, 62.4% - 90%, 69.8% - 77%), decrease own puffs (to 12 puffs) and increased whereas Subjects JH, KC, and PZ decreased consumption of other puffs (to 24 puffs). the proportion spent (62.5% - 40%, 41.7% - Although Subject BT showed a clear Gif- 40%, 62.5% - 40%). Interestingly, all 3 of the fen-good effect, the 2 other subjects (KS and latter subjects had a 60% - 40% distribution WR) who showed functional equivalence be- between own and other puffs at the high price tween income and price did not show as robust (see Table 3). an effect of price on choice. One interpretation Second, the data from the price condition of this finding is that Giffen-good effects rep- were consistent with the income functions for resent one end of a continuum in which sub- the 3 subjects who showed an increase in their jects respond in a manner that maintains a relative consumption of the other puffs. The constant overall intake of something provided other 3 subjects' outcomes, however, were in- by both reinforcers (e.g., nicotine). At the other consistent with their respective income func- end of this continuum, subjects respond in a tions. One factor that was associated with manner that maintains a constant intake of the whether subjects showed functional equiva- normal good. According to this continuum, the lence (or the Giffen-good effect) was their self- 6 subjects who completed this condition could reported acceptability of the other puffs. The be ranked according to relative consumption 1 subject (BT) who completed the VAS scales of the other puffs across the low and high price and showed functional equivalence reported as follows: KS, BT, WR, KC, JH, and PZ that the other good was more acceptable than (see Table 3). the 3 remaining subjects who did not show This discussion highlights the complexity of functional equivalence or Giffen-good effects. the interaction of price and income. Clearly, Below is an account of why subjects' relative however, more laboratory research on the ef- or absolute preferences might increase as the fects of price might help to better determine price of that reinforcer increases. the conditions under which Giffen-good effects Economic theory suggests that Giffen-good occur. Such research might, for example, ex- effects are most likely to occur when a signif- amine whether there is a window of prices and icant proportion of one's income is allocated incomes in which a Giffen-good effect would to an essential but inferior good (Lea et al., occur and whether that window is correlated 1987). A Giffen good is observed when, as the with other phenomena (e.g., price at which price of the inferior good increases, the con- choice between the two reinforcers is at equi- sumer compensates for the reduced purchasing librium). power by reducing consumption of a more ex- pensive normal good in favor of purchases of Economic Factors in the Study of Choice the (still cheaper) inferior good; this substi- The income manipulation in this and pre- tution might occur because the normal good vious studies is interesting because the manip- and the inferior good share some of the same ulation does not alter the rate, magnitude, or essential features. Consider, for example, Sub- delay of reinforcement of either of the con- ject BT in the present study. This subject con- currently available reinforcers. This is similar sumed about 20 puffs of both the own ($0.50 to the procedure of open and closed economies, for two puffs) and other puffs ($0.10 for two in that the variable of openness does not di- puffs) at the $6.00 income. Thus, he spent rectly alter any quantitative or qualitative as- about $5.00 on own puffs and $1.00 on other pects of the reinforcers (Hursh & Bauman, puffs. Now, if the price of the other puffs is 1987). Despite this fact, income reversed con- raised to $0.25 at this income amount, one can sumption in the present study such that the mathematically determine choice between the reinforcer that would be considered to have the two brands if he is to maintain a total of 40 greatest effectiveness depended on income and puffs while simultaneously maximizing con- not on any aspects of the reinforcers per se. sumption of the normal good. According to this This finding thus suggests that the degree to analysis, BT should decrease consumption of which a stimulus can function as a reinforcer EFFECTS OF INCOME ON CHOICE 499 cannot be completely predicted by considering predicting (functional equivalence between only these traditional characteristics of rein- price and income) the effects of a complex forcers (e.g., rate, magnitude, delay). array of manipulations. Future research will Moreover, the finding that a price manip- need to assess the generality of these claims, ulation can have very different effects on choice especially those related to the functional equiv- across subjects (i.e., increase or decrease pro- alence between price and income. 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