Discounting of Delayed Rewards as an Endophenotype

Abstract

The article by Anokhin et al. (1) in this issue of Biological Psychiatry provides strong evidence of the heritability of the neurobehavioral decision process referred to as the discounting of delayed rewards, or temporal discounting (i.e., the devaluation of a reward as a function of delay to its delivery) (2). Using a standard psychophysical titration procedure to assess discounting, Anokhin et al. tested twins (N 5 560) at both 16 years of age and 18 years of age. Use of more than one time point allows estimates of the stability and strength of genetic influence over time. Overall, Anokhin et al. estimated heritability of temporal discounting to be 57%, with strength of genetic influence increasing across time. This study extends a previous finding by these authors (3) in which evidence of similar heritability was observed using a less quantitative measure of discounting and younger twins (12 years old and 14 years old). The most crucial aspects of these results are revealed when they are considered as part of the extant evidence about temporal discounting, including how discounting relates to various maladaptive and psychopathologic behaviors. Excessive temporal discounting is evident across multiple disorders and has been appropriately identified as a transdisease/diagnostic process (4). Excessive discounting has been demonstrated in most forms of substance dependence disorders, including alcohol, nicotine, opioids, and stimulants. Children with attention-deficit/hyperactivity disorder and adults with schizophrenia, major depressive disorder, problem gambling, and obesity also discount delayed rewards more than control subjects (4). Additionally, high rates of temporal discounting have been associated with a variety of poor health behaviors, including failure to check blood pressure, obtain cholesterol testing, attend dental visits, exercise, receive flu shots, wear a seat belt, engage in safe sexual behavior, or be medically adherent (4–6). The discounting of delayed rewards also has been identified as a candidate behavioral marker for drug dependence (7); that is, evidence supports that discounting tracks disease progression and treatment outcomes, suggesting novel therapeutic approaches and identifying potential mechanisms underlying drug dependence. For example, excessive discounting is predictive of drug use initiation, is correlated with the amount of substance used, is predictive of therapeutic outcomes when the treatment is moderately efficacious, and improves after highly effective treatment. Estimates of discounting for individuals in recovery from drug dependence are near or within a normative range. Mechanistically, discounting is thought by most investigators to involve dual neurobehavioral processes, and those mechanisms have suggested novel interventions (8). Whether discounting can serve as a candidate marker for the diseases and disorders or behaviors listed in the preceding paragraph remains to be determined.