Scott E Lukas Revised by Nancy Faerber

BEHAVIORAL ECONOMICS Contemporary conceptualizations view drug dependence as a ''syndrome in which the use of a drug is given a much higher priority than other behaviors that once had higher value'' (Jaffe, 1990). Thus, in order to understand drug dependence, the complex interactions among reinforcers must be understood. Consider, for example, the frequently observed case of polydrug abuse. On some occasions, the drugs may be used simultaneously (e.g., cocaine and heroin), whereas on other occasions one drug may be used in lieu of another (e.g., benzodiazepines and opioids). Understanding reinforcer interactions is also vital to developing effective treatment because treatments, be they pharmacological (e.g., methadone, nicotine gum) or non-pharmacological (e.g., AA meetings, alternative behaviors), often try to supplant drug reinforcers with other more acceptable reinforcers. Although understanding reinforcer interactions is important for understanding drug abuse, no method currently exists to quantify or even categorize these different types of reinforcer interactions.

Behavioral economics provides a means to understand the interactions among qualitatively different reinforcers. The value of behavioral economics derives from its unique ability to quantify the effects of qualitatively different reinforcers and their interactions (Bickel et al., 1992; Bickel, De-Grandpre, & Higgins, 1995; Bickel & Vuchinich, 2000; Hursh, 1980, 1991).

Several concepts from behavioral economics are relevant to this important issue. A central concept is the demand law which stipulates that total consumption decreases as price increases, all else being equal (Allison, 1979). Price can be considered anything (e.g., response requirement, monetary price, delay, changes in the amount of the commodity while holding the monetary or work price constant) that decreases availability of a commodity. Indeed, drug self-administration studies that vary price (response requirement) report results consistent with the demand law; that is, drug consumption decreases as the response requirement decreases (Griffiths, Bigelow, & Henningfield, 1980; Young & Herling, 1986). However, behavioral economics does more than simply restate this observation with a different terminology; it adds by quantitatively characterizing the relation between price and consumption via the economic measure of own-price elasticity (Bickel & DeGrandpre, 1996; Hursh & Bauman, 1987; Samuelson & Nordhaus, 1985). Own-price elasticity measures the proportional change in consumption across price conditions. If consumption of a particular reinforcer decreases proportionally to a large extent as price increases, then the consumption is referred to as elastic. If consumption decreases proportionally to a limited extent as price increases, then the consumption is referred to as inelastic. Elastic and inelastic consumption are quantified by elasticity coefficients greater than 1.0 and less than 1.0, respectively (Hursh, 1980). When examined across a broad range of prices, elasticity of demand for many reinforcers is often mixed: inelastic at low prices and elastic at higher prices.

With this method, then, qualitatively different reinforcers can be compared and distinguished in drug-dependent populations. For example, in a recent study money and cigarettes were compared among cigarette-deprived subjects on progressive ratio schedules (Bickel & Madden, 1999). Response requirements were increased across sessions and the same response requirement was imposed separately for both commodities. At the lowest response requirements, money was self-administered to a greater extent than cigarettes (a greater intensity of demand). As response requirement increased, money was shown to be more sensitive to price than cigarettes. The own-price elasticities of money and cigarettes were 2.1 and 0.9, respectively, with money being 2.3-fold more sensitive to price. Such efforts can be meaningfully extended to clinical settings via simulation technology (Petry & Bickel, 1998; Jacobs & Bickel, 1999). Jacobs and Bickel used questionnaires to assess the reported consumption of cigarettes and heroin singly and concurrently across a range of prices ($0.01 to $1,120) in opioid-dependent smokers undergoing treatment for heroin addiction. Across conditions in which cigarettes and heroin were available alone, or concurrently, demand for heroin was less elastic than for cigarettes. For example, heroin consumption was defended to a greater extent across increases in price than cigarettes. Taken together, these studies indicate that individuals with drug dependence value the primary drug of dependence more than other commodities.

Own-price elasticity, like other behavioral effects, is not an inherent "property" of a drug, but is determined by several variables including the context in which it is measured (Hughes, Higgins, & Bickel, 1988). Indeed, the presence of other rein-forcers can alter own-price elasticity (Hursh, 1984; Bickel et al., 1995). Within an economic framework, reinforcer interactions lie on a continuum that can be quantified with a measure termed cross-price elasticity (i.e., proportional change in consumption of reinforcer A as a function of the price of reinforcer B) (Hursh & Bauman, 1987). At one end of the continuum, reinforcers are substitu-table reinforcers; that is, as the price of one rein-forcer increases (e.g., price of attending a movie theater), consumption of a second reinforcer (i.e., the substitute) will increase (e.g., video rentals). At the other end of the continuum, reinforcers can be complementary; that is, as the cost of one reinforcer increases (e.g., soup), the consumption of a second reinforcer (i.e., the complement) also decreases (e.g., soup crackers). Between these two extremes are independent reinforcers; as the cost of one reinforcer increases (e.g., movie attendance), the consumption of a second reinforcer (e.g., soup crackers) will remain unchanged. The quantification of substitutes, compliments, and independent rein-forcers is measured by cross-price elasticity values greater than zero, less than zero, and equal to zero, respectively (Hursh & Bauman, 1987).

A review and reanalysis of self-administration studies support this continuum (Bickel et al., 1995). Specifically, the results of sixteen drug self-administration studies that employed concurrently available reinforcers were reanalyzed using the eco nomic measure of cross-price elasticity. In that review, price was defined as responses required per milligram of drug per ingestion (i.e., unit price) (Hursh et al., 1988; Bickel et al., 1990). A wide variety of reinforcers (e.g., cocaine, food, heroin, and cigarettes) across several species (e.g., rats, humans, baboons, and rhesus monkeys) were examined. Overall, the results of reanalysis demonstrated that each of the studies demonstrated one of the three types of interactions specified by economics. Extensions can also be made to clinical settings again by simulations. For example, the Petry and Bickel (1998) study described earlier found that heroin prices affected other drug purchases. The cross-price elasticity measure showed that as the price of heroin rose and heroin purchases decreased, valium and cocaine purchases increased. However, increases in the price of valium did not affect heroin purchases. This suggests an asymmetrical relationship between heroin and valium purchases when the price of one drug increases while the price of the other remains constant.

Collectively, these studies suggest that the availability of a concurrent reinforcer can significantly modulate drug intake. Sometimes, reinforcers interact as substitutes, whereas at other times they function as complements. Introducing a substitute tends to decrease drug consumption, while introducing a complement may increase drug consumption. Unfortunately, few of these studies has pro-spectively examined these interactions. Additional research that systematically and parametrically examines reinforcer interactions may facilitate a greater understanding of the ways in which the availability of alternative reinforcers increases or decreases drug consumption.

This approach to studying interactions of any reinforcers may have several important implications. First, this research may have implications for understanding behavioral vulnerability. For example, one issue in drug dependence is why only a few people exposed to a drug of abuse go on to become dependent. Perhaps, vulnerable individuals have limited availability to alternative nondrug substitutes and easy access to complements for drug taking (e.g., Carroll, Lac, & Nygaard, 1989). Second, this research may provide a useful way to characterize the various types of polydrug abuse (e.g., Petry & Bickel, 1998). Third, such research may provide an empirical basis for developing treatment strategies; that is, in treatment it may be worthwhile to decrease the response cost of obtaining other substitutable nondrug reinforcers. With regard to complements, this research suggests that drug-abuse treatment may be more successful if consumption of complements is also decreased. For example, Higgins and colleagues (1993) examined the effects of disulfiram (antabuse) therapy among patients abusing cocaine and alcohol. Disulfiram is a medication that is used to deter alcohol use by inducing nausea and vomiting when alcohol is consumed. This study found that supervised disulfiram therapy was associated with significant decreases in alcohol and cocaine use as measured by breath and urine samples. More recently, Carroll and colleagues (1998) examined the effects of three standardized psychotherapy treatments alone and with disulfiram treatment among a large, diverse sample of individuals who abuse cocaine and alcohol. Their results indicated that disulfiram treatment was associated with significantly better retention in treatment and longer duration of abstinence from alcohol and cocaine use. Fourth, behavioral economic analyses of research may predict conditions in which relapse is likely. Relapse may occur when substitutable reinforcer interactions become unavailable and complements become available (Vuchinch & Tucker, 1988,1996). Thus, the study of reinforcer interactions may increase our understanding of the etiology, maintenance, and treatment of drug dependence as well as relapse.

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