Convergent validity

Bailey et al. acknowledge the well-known observation that DD is largely uncorrelated with measures of impulsivity. While accurate, this view fails to characterize the state of the field. Impulsivity itself is a multifactorial construct showing little correlation among its putative forms (e.g. acting without forethought, response disinhibition, and DD). Decades of research have failed to identify common neurobiological mechanisms among these various ‘impulsivities’, and the effects of behavioral and pharmacological interventions vary across forms. For these and other reasons, some have called for rejecting the superordinate construct of impulsivity in favor of continued, rigorous inquiry of its various component measures (Strickland & Johnson, Reference Strickland and Johnson2021). We agree with this perspective.

Bailey et al. outline additional concerns over convergent validity, arguing that DD has yet to show strong and replicable associations with other cognitive and personality measures. These authors also suggest that, in order to be considered an ‘important’ individual difference, DD should show incremental validity over these other measures when examining associations with substance use disorder (SUD) and other clinical disorders. We agree that DD has shown relatively modest associations with executive function and personality measures, but these findings are replicable at the behavioral and neurobiological levels. Such consistent, but limited, covariance with a variety of measures suggests not necessarily, as Bailey et al. argue, that DD is an unimportant individual difference, but rather that its determinants are multivariate.

Moreover, Bailey et al. disregard emerging evidence of incremental validity. Specifically, using machine-learning approaches to predict SUD, measures of DD outperform the Stroop task, Iowa Gambling task, Continuous Performance task, and other measures of attention, planning, and inhibitory control (e.g. Bickel, Moody, Eddy, & Franck, Reference Bickel, Moody, Eddy and Franck2017). Nonetheless, we agree that the literature would benefit from additional studies examining incremental validity, particularly in predicting health behaviors beyond substance use.

Divergent validity

Bailey et al. note concerns over divergent validity of the DD paradigm, citing evidence that elevated DD is robustly associated with not only SUD but various other psychiatric disorders, including depression, bipolar disorder, and schizophrenia (for a meta-analysis, see Amlung et al., Reference Amlung, Marsden, Holshausen, Morris, Patel, Vedelago and McCabe2019). Thus, as Bailey et al. argue, measures of DD may not differentially predict diagnostic status among patients with varied disorders. This remains an empirical question and more research should examine the specificity of such associations. However, some research provides emerging evidence of divergent validity. For example, SUD and comorbid mood and personality disorders are interactively associated with higher DD compared to SUD alone (e.g. Moody, Franck, & Bickel, Reference Moody, Franck and Bickel2016). Conversely, anorexia nervosa is associated with lower DD compared to healthy controls (for a meta-analysis, see Amlung et al., Reference Amlung, Marsden, Holshausen, Morris, Patel, Vedelago and McCabe2019). Thus, DD rates at opposing ends of a continuum are associated with divergent pathology.

More generally, we note that the well-documented comorbidity between SUD and other psychiatric disorders suggests transdiagnostic risk factors and etiologies. That DD is associated with a variety of clinical endpoints is not necessarily a cause for concern, but may instead be a strength of the paradigm and provides the opportunity to investigate a single decision-making process relevant to a broad range of disorders. However, further research is needed to understand the circumstances under which genetic, neurobiological, or environmental predispositions manifest in divergent phenotypes. Indeed, this pursuit is a focus of the NIH's Research Domain Criteria (RDoC), an area in which DD holds some promise (e.g. Levitt et al., Reference Levitt, Oshri, Amlung, Ray, Sanchez-Roige, Palmer and MacKillop2022).

Generalizability

Bailey et al. argue that little evidence suggests that DD is a generalizable measure of decision-making. In the process, these authors disregard evidence of reliable correlations in measures of discounting across different commodities (e.g. money, food, health, and drugs of abuse; for a systematic review, see Odum et al., Reference Odum, Becker, Haynes, Galizio, Frye, Downey and Perez2020). Thus, contrary to the limited findings reviewed by Bailey et al., knowledge of how an individual discounts one delayed commodity provides information about how they are likely to discount other delayed commodities.

Given this correspondence, the majority of studies on DD preferentially examine discounting of monetary outcomes because, unlike non-monetary outcomes, money is universally valued, easily quantifiable, highly fungible, and its utility curve is approximately linear across a broad range of values. We recognize, however, that discounting of non-monetary, pathology-relevant commodities in some circumstances may provide incremental validity over the study of monetary outcomes (e.g. discounting of food in the study of obesity; for a meta-analysis, see Amlung, Petker, Jackson, Balodis, & MacKillop, Reference Amlung, Petker, Jackson, Balodis and MacKillop2016). As such, future research should continue to examine the relationship between non-monetary discounting, SUD, and other disorders. Moreover, choice arrangements that may more closely model real-world ITC should also be explored (Green & Myerson, Reference Green and Myerson2019). For example, ‘cross-commodity’ discounting tasks arrange choices between qualitatively different immediate and delayed outcomes, which may characterize intertemporal tradeoffs evident in health-related decision-making (e.g. immediate food v. delayed weight loss in the study of obesity).