Research reportObese women show greater delay discounting than healthy-weight women
Introduction
Obesity is increasing in prevalence and is a risk factor for many serious illnesses (Baskin, Ard, Franklin, & Allison, 2005; Ogden et al., 2006; Wyatt, Winters, & Dubbert, 2006). Recently, some investigators have emphasized the similarities between obesity and drug addiction (Davis, Levitan, Muglia, Bewell, & Kennedy, 2004; Del Parigi, Chen, Salbe, Reiman, & Tataranni, 2003; Pelchat, 2002; Volkow & Wise, 2005). Current neurobiological models have suggested that addiction is accompanied by dysfunctions in two brain circuits, one mediating impulsive behavior stemming from lack of inhibitory control and the other mediating oversensitivity to perceived drug cues (Jentsch & Taylor, 1999; Kolb, 1999; Robinson & Berridge, 2003; Volkow, Fowler, & Wang, 2004; Volkow, Fowler, Wang, & Swanson, 2004). Although obesity undoubtedly has a complex etiology, the same neural circuits dysfunctional in addicted individuals may be dysfunctional in some people who are obese (Alonso-Alonso & Pascual-Leone, 2007; Del Parigi et al., 2003; James, Gold, & Liu, 2004; Pelchat, 2002; Volkow & Wise, 2005; Wang, Vollkow, Thomas, & Fowler, 2004). A number of observations support this view with regard to inhibitory control circuitry. Obese children are more likely to behave impulsively than children of normal weight (Braet, Claus, Verbeken, & Vlierberghe, 2007; Nederkoorn, Braet, Van Eijs, Tanghe, & Jansen, 2006; Sigal & Adler, 1976; but see Bonato & Boland, 1983). A few studies have reported decision-making deficits in obese adults; however, studies using tasks related to impulsivity had possible confounding subject variables which make interpretation problematic (Davis et al., 2004, Gunstad et al., 2007, Pignatti et al., 2006).
A particularly useful task for studying decision-making has been the delay, or temporal, discounting of reward task (e.g., Bickel, Odum, & Madden, 1999; Bickel & Marsch, 2001; Raichlin, Raineri, & Cross, 1991; Vuchinich & Simpson, 1998). Delay discounting (DD) is a measure of being driven by immediate gratification vs. the willingness to wait for longer term or delayed rewards of greater magnitude. Individuals with various substance abuse problems involving cocaine, opiates, or alcohol; smokers; gamblers; individuals with some psychiatric conditions; and children show higher rates of delay discounting, meaning that they prefer smaller, immediate rewards more often than controls do (Alessi & Petry, 2003; Bickel et al., 1999; Crean, de Wit, & Richards, 2000; Kirby, Petry, & Bickel, 1999; Madden, Petry, Badger, & Bickel, 1997; Mitchell, 1999, Petry, 2003; Reynolds, Richards, Horn, & Karraker, 2004; Scheres et al., 2006; Vuchinich & Simpson, 1998).
Some evidence supports a relationship between body mass index (BMI) and preference for sooner vs. later benefits (Smith, Bogin, & Bishai, 2005; Zhang & Rashad, 2008; but see Borghans & Golsteyn, 2006). One study that compared obese and non-obese women on a delay discounting task found no group differences (Nederkoorn, Smulders, Havermans, Roefs, & Jansen, 2006), but did not consider the possible confounding variable of income, which may be an important influence on delay discounting (Green, Myerson, Lichtman, Rosen, & Fry, 1996). It is also possible that differences in IQ (de Wit, Flory, Acheson, McCloskey, & Manuck, 2007) obscured a potential obese and control difference in DD, although the groups did not differ in mean education (Nederkoorn, personal communication). Because of the recently suggested parallels between drug addiction and obesity, we hypothesized that obese individuals would show greater delay discounting than controls in the absence of confounding effects of IQ and income.
Section snippets
Participants
Introductory psychology students (N = 1290) were screened for eligibility during the initial week of classes. Inclusion criteria consisted of BMI ≥30 kg/m2 (obese) or BMI of 18.5–24.9 kg/m2 (controls), and age 18–50 years. Exclusion criteria included being a current or past smoker, having a current or prior substance abuse or gambling problem, having a current or past eating disorder based on the Eating Disorder Diagnostic Scale (EDDS; Stice, Telch, & Rizvi, 2000), having a serious health
Comparison of AUC's
ANOVA on loge AUC's revealed a nonsignificant main effect of BMI (F[1,91] = 2.82, P = .097), but a significant BMI × Gender interaction (F[1,91] = 5.81, P = .018; Fig. 1; see also Fig. 2, Fig. 3). Subsequent simple-effects analyses showed greater discounting in obese women compared to healthy-weight women overall (P = .002) as well as on the individual DD tasks (P = .011 and .002 for DD-Low and DD-High, respectively). No such differences were seen for men (P values >.40). Because DD performance has been
Discussion
Our results supported our hypothesis that obese individuals would display higher rates of delay discounting, or choose more immediate but smaller monetary rewards than controls, but this result was found only for the women. Subsequent analyses indicated that the delay discounting difference between obese and control women was not related to differences in age, income, or IQ. This is the first study to reveal delay discounting deficits in obese individuals and the first study to show
Acknowledgement
We would like to thank Rudy Vuchinich for useful discussions about delay discounting, Mark Bolding for programming the DD task, and Joshua Kean for helping to run subjects.
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