Evaluating the combined effects of effort and probability on monetary discounting

Researchers have studied how the effort to obtain a reward affects its value and how effort discounting compares to other reinforcer characteristics (e.g., delay, social, probability). However, many everyday choices are likely influenced by how effort combines with other reinforcer characteristics, which have primarily been studied in isolation. In this experiment, we tested nine models describing how effort combines with probability to influence monetary discounting of $100 using an adjusting-amount task. Overall, participants' (N = 61) choices followed expected patterns: Steeper probability discounting was observed at higher effort levels, and steeper effort discounting was observed at lower probabilities. All nine effort-probability models described group and individual data well. Using AICc, the general theory of discounting model best described most participants' data, with Rachlin's multiplicative hyperboloid model ranking second and a multiplicative combination of Rachlin's hyperboloid for probability and a power-law for effort ranking third. In total, the results from model fitting indicate that probability and effort interact to influence choice. Existing models describe this interaction well, but future research will be needed to determine the conditions under which different models might be favored. These preliminary findings add effort-probability interactions to the literature on the multiple control of choice.