Discounting of money and sex: Effects of commodity and temporal position in stimulant-dependent men and women
Introduction
When individuals choose between immediate and larger-yet-delayed rewards, tradeoffs between the size and the immediacy of the rewards occur. Specifically, the subjective value of the delayed commodity decreases (i.e., is discounted) as the delay increases. This phenomenon, called delay discounting, is often quantified via Mazur's (1987) hyperbolic discounting equation:which describes how the value (V) of some amount (A) of the commodity decreases at a given rate (k) as it is delayed (D). Because the discounting rate (i.e., k) is the only free parameter in this equation, k is said to quantify this behavioral process.
Substance-dependent individuals discount delayed rewards at higher rates than controls (see Bickel, Jarmolowicz, Mueller, Koffarnus, & Gatchalian, 2012b, for a review). Moreover, they discount consumable commodities like cocaine (Bickel et al., 2011, Coffey et al., 2003), heroin (Madden et al., 1999, Madden et al., 1997), or food (Estle et al., 2007, Odum and Baumann, 2007) at steeper rates than money. This suggests that addicted individuals exhibit a reduced ability to value future events (Bickel et al., 2012a, Bickel and Yi, 2008), which manifests as impulsive choices, particularly for consumable commodities. Although these studies typically present choices between hypothetical outcomes, studies have demonstrated that both choice (e.g., Johnson and Bickel, 2002, Kirby and Marakovic, 1996, Madden et al., 2003) and patterns of neural activation (Bickel, Pitcock, Yi, & Angtuaco, 2009) are consistent when participants choose between real or between hypothetical outcomes.
Addicted individuals' poor valuation of future events may manifest as impulsive choices for sexual activity (Jarmolowicz et al., 2013, Johnson and Bruner, 2011). For example, Jarmolowicz et al. compared delay discounting rates for money and for sexual activity in samples of alcohol-dependent (n = 20) and control participants (n = 21). After determining how many sex acts would be subjectively equivalent to receiving $1000, participants completed delay discounting assessments for money and for sexual acts. Discounting rates for sexual activity were higher than for money in all participants (cf. Lawyer, Williams, Prihodova, Rollins, & Lester, 2010). Moreover, discounting rates for sexual activity were higher in the alcohol dependent group than in the control group. Hence, like other consumable commodities, delayed sexual activity is discounted at higher rates than delayed money (Lawyer et al., 2010), and addicted individuals discounted delayed sex more steeply than controls.
Impulsive sexual choices may be particularly relevant to cocaine dependence. Cocaine users often make seemingly impulsive sexual choices such as sex with multiple partners, inconsistent condom use, and trading sex for other commodities (typically drugs, food, housing, and/or money; Farley, 2006, Fleming et al., 2006). The risk of spreading sexually transmitted infections (STI) and the potential for unwanted pregnancies, make these behaviors a major public health concern (Centers for Disease Control and Prevention, 1998, Center for disease control and prevention, 2014). Understanding the ways which cocaine users devalue delayed sexual activity may inform efforts to curb this seemingly impulsive sexual activity. Using a similar procedure, Johnson and Bruner (2011) examined the discounting of delayed safe sex relative to immediate unprotected sex in sixty cocaine-dependent individuals. After choosing pictures of individuals that they would have sex with, participants indicated the likelihood that they could wait various amounts of time (from 1 h to 3 months) for a condom to have sex with the person 1) they most wanted to have sex with, 2) they least wanted to have sex with, 3) they felt was most likely to have an STI, and 4) they felt was least likely to have an STI. Individuals' ability to wait for a condom decreased as the delays increased, particularly for the individual they most wanted to have sex with, and the individual least likely to have an STI.
As noted above, discounting rates for consumables (e.g., drugs, sex, etc.) tend to be higher than for money. Studies comparing the discounting rates across commodities, however, almost exclusively present choices between some amounts of Commodity A now vs. after a delay. This single-commodity discounting (SCD) may not represent the complex choices encountered outside of the laboratory. In life we often choose between Commodity A now and Commodity B after a delay (e.g., drugs now vs. paycheck later), choices which we will refer to as cross-commodity discounting (CCD).
By presenting the entire array of SCD and CCD conditions, delay discounting studies can isolate the relative roles of the immediate and delayed commodities. For example, Bickel et al. (2011) examined SCD for money and cocaine as well as CCD for both cocaine now vs. money later and money now vs. cocaine later. Consistent with previous findings (Coffey et al., 2003), Bickel et al. found SCD for cocaine was steeper than SCD for money. Novel to their experiment, however, CCD rates for cocaine now vs. money later were intermediate to the two SCD rates, yet rates of CCD for money now vs. cocaine later were higher than SCD for cocaine. Bickel et al.'s findings suggest a strong role of the delayed commodity (i.e., delayed cocaine did not retain its value) and lesser effects of the immediate commodity (i.e., immediate cocaine is very appealing).
The present study built from Bickel et al.'s (2011) and Jarmolowicz et al.'s (2013) findings to examine the relative roles of immediate and delayed sex or money by comparing the SCD and CCD for money and sexual activity in treatment-seeking cocaine-dependent individuals.
Section snippets
Participants
The data in this study were taken from a subset in a larger study of a novel treatment for cocaine and methamphetamine use. Recruitment for the original study consisted of placing fliers in the local community, and referrals from other study participants. We selected participants who met DSM-IV (American Psychiatric Association, 2000) criteria for cocaine dependence and indicated that cocaine was their drug of choice. Exclusion criteria for the study included dependence on other drugs (other
Results
Data from the M–M (median R2 = 0.89; IQR = 0.76–0.95), S–S (median R2 = 0.89; IQR = 0.80, 0.96), M–S (median R2 = 0.55; IQR = 0.16, 0.76), and S–M (median R2 = 0.87; IQR = 0.81, 0.90) conditions were well described by the hyperbolic discounting equation (cf. M–S). Fig. 1 shows the ln(k) in each condition for each individual, as well as the mean ln(k) (± 1 standard error) for each condition. Log transformed discounting rates, ln(k), were highest in the M–S condition (2.7926), followed by the S–S (− 1.4394), M–M (−
Discussion
Patterns of discounting for sexual outcomes bore the same quantitative signature as the discounting of other commodities like money or drugs (Bickel et al., 2012b), further demonstrating the generality of delay discounting processes. Consistent with previous studies, delay discounting rates were higher for sex than money (Jarmolowicz et al., 2013, Lawyer et al., 2010). Moreover, consistent with Bickel et al. (2011), the CCD procedures provided insight into the relative roles of the immediate
Role of the funding source
The current research was supported by the National Institute on Drug Abuse grant R01DA030241 to Warren K. Bickel. NIDA had no role in the study design, collection, analysis or interpretation of the data, writing the manuscript, or the decision to submit the paper for publication. This manuscript has been approved by all authors.
Contributors
All authors contributed substantially to this work through the design (Warren Bickel, Darren Christensen, Bryan Jones, and Richard Yi), conduct (Lisa Jackson), analysis (Reid Landes), interpretation (David Jarmolowicz, Warren Bickel) and writing of the final manuscript (all authors).
Conflict of interest
None of the authors have any conflicts of interest to report.
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