BEHAVIORAL AND BRAIN SCIENCES (2013) 36, 661–726 doi:10.1017/S0140525X12003196 An opportunity cost model of subjective effort and task performance Robert Kurzban Department of Psychology, University of Pennsylvania, Philadelphia, PA 19104 [email protected] https://sites.google.com/site/pleeplab/ Angela Duckworth Department of Psychology, University of Pennsylvania, Philadelphia, PA 19104 [email protected] https://sites.sas.upenn.edu/duckworth Joseph W. Kable Department of Psychology, University of Pennsylvania, Philadelphia, PA 19104 [email protected] http://www.psych.upenn.edu/kable_lab/Joes_Homepage/Home.html Justus Myers Department of Psychology, University of Pennsylvania, Philadelphia, PA 19104 [email protected] Abstract: Why does performing certain tasks cause the aversive experience of mental effort and concomitant deterioration in task performance? One explanation posits a physical resource that is depleted over time. We propose an alternative explanation that centers on mental representations of the costs and benefits associated with task performance. Specifically, certain computational mechanisms, especially those associated with executive function, can be deployed for only a limited number of simultaneous tasks at any given moment. Consequently, the deployment of these computational mechanisms carries an opportunity cost – that is, the next-best use to which these systems might be put. We argue that the phenomenology of effort can be understood as the felt output of these cost/benefit computations. In turn, the subjective experience of effort motivates reduced deployment of these computational mechanisms in the service of the present task. These opportunity cost representations, then, together with other cost/benefit calculations, determine effort expended and, everything else equal, result in performance reductions. In making our case for this position, we review alternative explanations for both the phenomenology of effort associated with these tasks and for performance reductions over time. Likewise, we review the broad range of relevant empirical results from across sub-disciplines, especially psychology and neuroscience. We hope that our proposal will help to build links among the diverse fields that have been addressing similar questions from different perspectives, and we emphasize ways in which alternative models might be empirically distinguished. Keywords: evolutionary psychology; mental effort; neuroeconomics; phenomenology; self-control I have no expectation that the laws of mental fatigue will be V1 to object recognition systems, and so on, more or less formulated in the immediate future. continuously during waking hours. The operation of these — Raymond Dodge (1917, p. 89) systems carries no phenomenology of effort, and perform- ance reductions, if any, are slight. These observations imply Remarkably, given that fatigue has been studied formally for that at least some of the brain’s functions can continue over well over 100 years, there is still no scientifically mature theory of its origins and functions. sustained periods with minimal reduction in performance — G. Robert J. Hockey (2011, p. 167) and without any conscious sensation of effort. In contrast, other mental tasks (e.g., scanning a display for infrequent, subtle signals, doing mental arithmetic, etc.) give rise to 1. Introduction the conscious sensation of effort and seem difficult to execute continuously over time (Ackerman 2011). For some of the brain’s functions, such as the regulation of Why are some, but not all, mental operations performed body temperature and heart rate, performance is main- without the sensation of effort and without performance tained without noticeable impairment over time. Similarly, loss? Our goal here is to sketch a computational explanation the visual system executes its functions, from the retina to for both the subjective phenomenology of mental effort © Cambridge University Press 2013 0140-525X/13 $40.00 661 Kurzban et al.: An opportunity cost model of subjective effort and task performance and the associated behavioral performance reductions. Our marathon of solving multiplication problems continuously, interest ranges broadly, from tasks such as the Stroop 12 hours per day, observing that it took her longer to (Webb & Sheeran 2003), to math problems (Arai 1912), solve problems over each successive day’s session and con- to complex decision-making (Masicampo & Baumeister cluding that “difficult and disagreeable continued work 2008; Vohs et al. 2008). We propose that both phenomen- brings about a decrease in the efficiency of the function ology and performance in these mental tasks rest on a exercised” (p. 114). In 1946 Huxtable et al. replicated common foundation: computations of their benefits and Arai’s experiment with three graduate student participants. costs relative to other operations to which the same pro- Performance decrements over the course of each day were cesses might be applied. Subjective effort, on this view, is measurable but slight in magnitude and not as consistent as the conscious, experienced measurement of the costs – participants’ reports of extreme weariness, restlessness, and especially the opportunity cost – of continuing the task. boredom. In retrospect, one participant commented The subjective experience of mental effort, which is gener- that she “[w]ould not repeat these four days for $10,000” ally aversive, in turn motivates reallocation of compu- (Huxtable et al. 1946, p. 52). tational processes to relatively more valuable tasks. Our explanation contrasts with proposals that attribute perform- 1.1.1. Within-task performance reductions and associ- ance reductions to depletion of a resource or to “willpower” ated phenomenology. More recently, vigilance tasks, (e.g., Gailliot & Baumeister 2007). which require monitoring visual displays or auditory streams for infrequent signals (e.g., Mackworth 1948), have been shown to reliably induce decrements in perform- 1.1. Phenomena to be explained ance over time and concomitant increases in perceived In one of the earliest studies of mental effort, Arai (1912) mental effort (Scerbo 2001; Warm et al. 2008). Likewise, practiced multiplying pairs of four-digit numbers in her after long periods of time in flight simulators, pilots are head until, after several months, she had reached a more easily distracted by non-critical signals and less able plateau in performance. She then completed a four-day to detect critical signals (Warm et al. 2008). Ratings of boredom in vigilance tasks increase rapidly above pre-task levels typically (Scerbo & Holcomb 1993), but the increase in boredom can be delayed by minor variations in task ROBERT KURZBAN is an Associate Professor at the Uni- parameters, such as increasing stimulus variety (Scerbo versity of Pennsylvania in the Department of Psychol- 2001). ogy. Drawing on his background in evolutionary Performance reductions have also been observed in a psychology and behavioral economics, he has published variety of other tasks that require sustained attention. In numerous journal articles on an array of topics, includ- “fl ” ing morality, cooperation, friendship, mate choice, anker tasks, for example, subjects are asked to supernatural beliefs, modularity, and self-control. He respond to a central target stimulus (e.g., to indicate the serves as the Editor-in-Chief of Evolution and Human direction of an arrow), while adjacent stimuli with incon- Behavior, and in 2008 he won the inaugural Early gruent information (e.g., arrows that point in the opposite Career Award for Distinguished Scientific Contribution direction from the target) make the task more difficult. from the Human Behavior and Evolution Society. His In one version of the task, where the central target is a fi rst book, Why Everyone (Else) is a Hypocrite: Evol- letter and flanking stimuli are other letters, performance ution and the Modular Mind, was published in 2010 generally worsens after 90 minutes (Lorist et al. 2005). by Princeton University Press. Likewise, performance (as measured by reaction time “ ” ANGELA DUCKWORTH is an Associate Professor of Psy- and accuracy) decreases over time in task-switching para- chology at the University of Pennsylvania, where she digms, in which subjects are asked to respond to different studies non-IQ competencies, including self-control features of the stimulus (e.g., the color or the size) depend- and grit, which predict achievement. She has been ing on the trial (Lorist et al. 2000). Similarly, in a data entry awarded by the National Institute on Aging for her task intended to induce fatigue, Healy et al. (2004) found study of self-control from the perspectives of economics that accuracy declined over time. and psychology. Broadly, tasks that engage executive functions show performance decrements over time (Holding 1983; van JOSEPH W. KABLE is the Baird Term Assistant Professor der Linden et al. 2003). Notably, rewards improve per- of Psychology at the University of Pennsylvania. He formance in executive function tasks (e.g., Krebs et al. studies the cognitive and neurophysiological mechanisms of human decision making, using an integrated empirical 2010), suggesting that performance reductions are not approach that borrows from economics, the psychology mandatory, as one might expect if reductions were due to of judgment and decision making, and social and cogni- processes akin to mechanical breakdowns. tive neuroscience. Prior to joining the Department of Psychology at Penn, he was a