Kris N. Kirby

Delay-discounting probabilistic rewards: Rates decrease as amounts increase

The independence of delay-discounting rate and monetary reward size was tested by offering subjects (N = 621) a series of choices between immediate rewards and larger, delayed rewards. In contrast to previous studies, in which hypothetical rewards have typically been employed, subjects in the present study were entered into a lottery in which they had a chance of actually receiving one of their choices. The delayed rewards were grouped into small (

Preference Reversals Due to Myopic Discounting of Delayed Reward

30–

One-year temporal stability of delay-discount rates.

35), medium (

BootES: An R package for bootstrap confidence intervals on effect sizes

55–

Making choices in anticipation of similar future choices can increase self-control

65), and large amounts (

Concave utility, transaction costs, and risk in measuring discounting of delayed rewards.

70–

Tenure security, private time preference, and use of natural resources among lowland Bolivian Amerindians

85). Using a novel parameter estimation procedure, we estimated discounting rates for all three reward sizes for each subject on the basis of his/her pattern of choices. The data indicated that the discounting rate is a decreasing function of the size of the delayed reward (p < .0001), whether hyperbolic or exponential discounting functions are assumed. In addition, a reliable gender difference was found (p = .005), with males discounting at higher rates than females, on average.

The present values of delayed rewards are approximately additive.

A basic stationarity axiom of economic theory assumes stable preference between two deferred goods separated by a fixed time To test this assumption, we offered subjects choices between delayed rewards, while manipulating the delays to those rewards Preferences typically reversed with changes in delay, as predicted by hyperbolic discounting models of impulsiveness Of 36 subjects, 34 reversed preference from a larger, later reward to a smaller, earlier reward as the delays to both rewards decreased We conclude that the stationarity axiom is not appropriate in models of human choice

False alarm: a reply to Over and Evans

The temporal stability of delay-discount rates for monetary rewards was assessed using a monetary choice questionnaire (Kirby & Marakovic, 1996). Of 100 undergraduate participants who completed the questionnaire at the initial session, 81 returned 5 weeks later and 46 returned 57 weeks later for subsequent sessions. The 5-week test—retest stability of discount rates was .77 (95% confidence interval 5 .67—.85), the 1-year stability was .71 (.50–.84), and the 57-week stability was .63 (.41—.77). Thus, at least when similar testing situations are reinstated, discount rates as individual differences have 1-year stabilities in the range that is typically obtained for personality traits. Discount rates index an attribute of the person that is relatively stable over time but that is moderated by aspects of the situation, such as reward type and deprivational state.

An Empirical Assessment of the Form of Utility Functions.

Bootstrap Effect Sizes (bootES; Gerlanc & Kirby, 2012) is a free, open-source software package for R (R Development Core Team, 2012), which is a language and environment for statistical computing. BootES computes both unstandardized and standardized effect sizes (such as Cohen’s d, Hedges’s g, and Pearson’s r) and makes easily available for the first time the computation of their bootstrap confidence intervals (CIs). In this article, we illustrate how to use bootES to find effect sizes for contrasts in between-subjects, within-subjects, and mixed factorial designs and to find bootstrap CIs for correlations and differences between correlations. An appendix gives a brief introduction to R that will allow readers to use bootES without having prior knowledge of R.