Results and Discussion We hypothesized that theories about willpower would influence participants’ ability to sustain learning over time. Thus, we predicted that the two groups would demonstrate comparable accuracy and improvement initially but that participants in the non-limited condition would show sustained improvement over the full task. To test these predictions, we examined the effect of limited vs. non-limited condition on a two-piece linear growth model (e.g., [6]). We used a two-piece model because we hypothesized that participants in both groups would show improved performance in the beginning of the task but over time the performance of participants in the limited condition would level off or drop whereas that of participants in the non-limited condition would continue to improve. We simplified the model by dividing the 540 trials into eight equal-size blocks and modeled growth across the first four blocks and the second four blocks using Mplus 6.1 (the findings did not differ when alternative numbers of blocks were tested; see Text S2 for further information regarding this analysis method). We found no effect of condition on the intercept (b = .066, SE = .052, p = .206), indicating that participants in both conditions were equally accurate initially. In the first half of trials, participants in both the limited and non-limited groups demonstrated significant improvements in performance (limited: b = .045, SE = .007, p,.001; non-limited: b = .042, SE = .009, p,.001). Thus, the two groups did not differ in learning early in the task (b = 2.004, SE = .012, p = .777). However, we did find a significant effect of condition on growth across the second half of the trials (b = .010, SE = .005, p = .040). Participants led to view willpower as limited did not improve during second half of trials (b = .003, SE = .003 p = .412), whereas participants who led to view willpower as non-limited continued to increase in accuracy (b = .013, SE = .003, p,.001). Only participants in the non- Theories of Willpower Affect Sustained Learning limited willpower condition sustained learning for the entire duration of the task (Figure 1). These results extend previous work linking motivation to cognitive performance (e.g., [7,8]) and highlight the interactive nature of motivational and cognitive processes by demonstrating that implicit theories about willpower can affect people’s ability to recruit cognitive resources to sustain learning over time. Whereas previous work assessing the impact of these implicit theories focused on decrements in performance following self-control demands [4], this experiment suggests that people’s beliefs about the nature of willpower can also limit or facilitate the acquisition of a cognitive skill. The simple nature of the manipulation employed in this study suggests that beliefs about willpower can be easily modified by subtle input, at least in the short-term. This point is especially important given that much recent academic and popular literature claims that human willpower is inherently limited (e.g., [9,10]). The current findings suggest that disseminating such notions may create self-fulfilling prophecies; leading people to believe that willpower is limited might contribute to decrements in willpower and undermine persistence and learning. Further, the demonstra- tion that implicit theories about willpower can affect performance over a sustained duration stands in contrast to recent work suggesting that these theories only improve performance under relatively mild conditions [11]. The current results instead demonstrate that implicit theories can improve performance even for a very difficult and lengthy task. The present findings suggest many important directions for future research. For instance, we have shown that theories of willpower influence learning during a task, but we have not determined whether or how long acquired information or skills are retained. Another important open question involves directionality. In particular, we have only demonstrated a relative benefit of priming a non-limited theory of willpower relative to a limited Figure 1. Learning over time on 3-back task. Improvement in percent accuracy on the 3-back task relative to baseline for the limited and non- limited willpower groups over the full 20-minute time course (each block is averaged over 67 trials). doi:10.1371/journal.pone.0038680.g001 PLoS ONE | www.plosone.org 2 June 2012 | Volume 7 | Issue 6 | e38680