theory. We cannot state whether a non-limited theory improves sustained learning, whether a limited theory undermines sustained learning, or both. Finally, we have taken the approach of priming one theory of willpower, but behaviorally relevant individual differences do exist in beliefs about willpower [4]. It will be important to determine how promoting a belief interacts with preexisting theories of willpower to produce effects on behavior. The present results are especially meaningful in light of evidence that links training-based improvements in performance on working memory tasks (e.g., a modified n-back task) to increased fluid intelligence [12]. Whereas the working memory training implemented in this previous work was conducted over a series of sessions, the manipulation employed in the present study produced meaningful improvements in performance within a much shorter time frame. A more robust implementation of the intervention employed in the current study may produce effects that compound over time. As people improve on the task, the resulting success may feed back and reinforce the implicit theory that initially gave rise to these advantages. The potential of such a recursive process References 1. Ericsson KA (1996) The Road To Excellence: the Acquisition of Expert Performance in the Arts and Sciences, Sports, and Games. Mahwah: Lawrence Erlbaum Associates. 384 p. 2. Blackwell LS, Trzesniewski KH, Dweck CS (2007) Implicit theories of intelligence predict achievement across an adolescent transition: A longitudinal study and an intervention. Child Dev 78: 246–263. 3. Molden DC, Dweck CS (2006) Finding ‘‘meaning’’ in psychology: A lay theories approach to self-regulation, social perception, and social development. Am Psychol 61: 192–203. 4. Job V, Dweck CS, Walton GM (2010) Ego depletion–Is it all in your head?: Implicit theories about willpower affect self-regulation. Psychol Sci 21: 1686– 1693. 5. Rydell RJ, Shiffrin RM, Boucher KL, Van Loo K, & Rydell MT (2010) Stereotype threat prevents perceptual learning. P Natl Acad Sci USA 107: 14042–14047. Theories of Willpower Affect Sustained Learning Text S1 (DOCX) Text S2 (DOCX) to expand working memory and fluid intelligence over extended training remains an important topic for future work. Supporting Information Figure S1 Limited and non-limited questionnaire re- sponses. The distribution of responses for the limited and non- limited willpower questionnaires. Both groups indicated overall agreement with the questionnaires. (DOCX) Limited and non-limited questionnaire items. Growth curve analysis methods. Author Contributions Conceived and designed the experiments: EMM GMW CSD VJ SMM. Performed the experiments: EMM. Analyzed the data: EMM GMW CSD VJ KHT SMM. Wrote the paper: EMM GMW CSD VJ KHT SMM. 6. Li F, Duncan TE, Duncan SC, Hops H (2001) Piecewise growth mixture modeling of adolescent alcohol use data. Struct Equ Modeling 8: 175–204. 7. Colquitt JA, LePine JA, Noe RA (2000) Toward an integrative theory of training motivation: a meta-analytic path analysis of 20 years of research. J Appl Psychol 85: 678–707. 8. Eccles JS, Wigfield A (2002) Motivational beliefs, values, and goals. Annu Rev Psychol 53: 109–132. 9. Baumeister RF, Bratslavsky E, Muraven M, Tice DM (1998) Ego depletion: Is the active self a limited resource? J Pers Soc Psychol 74: 1252–1265. 10. Baumeister RF, Tierney J (2011) Willpower: Rediscovering the Greatest Human Strength. New York: Penguin Press. 304 p. 11. Vohs KD, Baumeister RF, Schmeichel BJ. (in press). Motivation, personal beliefs, and limited resources all contribute to self-control. J Exp Soc Psychol. 12. Jaeggi SM, Buschkuehl M, Jonides J, Perrig,WJ (2008) Improving fluid intelligence with training on working memory. P Natl Acad Sci USA 105: 6829. PLoS ONE | www.plosone.org 3 June 2012 | Volume 7 | Issue 6 | e38680