Reasons for exclusion Studies that coded the marshmallow test in ways other than time to delay: (e.g. waited full time yes/no; Callaway et al., 2007; Carlson, 2005; Da Silva et al., 2014; Paulsen & Johnson, 1980; van Reet, 2014; Likert-scale-type coding: Krietler & Zigler, 1990; Merz et al., 2015; Putnam et al., 2002; Weber, 2015; self-estimates of elapsed time: Miller & Karinol, 1976; numbers instead of wait time: Atance & Jaclson, 2009). Studies that tested whether children wanted one reward now or two later (choice delay) were also not included (e.g. Bandura & Mischel, 1965; Herzberger & Dweck, 1978; Hongwanishkul et al., 2005; Imutaet al., 2014; Labuschagne et al., 2017; Mischel, 1958, 1961; Mischel & Grusec, 1967; Mischel & Metzner, 1972; Nisan et al., 1967; Prencipe & Zelazo, 2005; Schwarz et al., 1983; Seeman & Schwarz, 1974; Staub, 1972; Thompson et al., 1997; Tynan et al., 2014; Yu et al., 2016). Children who were over the age of ten were not considered for this meta-analysis (e.g. Avci, 2013; Ayduk et al., 2000; Bruce et al., 2011; Corvi, 2010; Corvi et al., 2012; Duckworth et al., 2013; Funder & Block, 1989; Herndon, 2011; Kirby & Maraković, 1996; Meade, 2012; Rachlin et al., 1991; Rodriguez et al., 1989; Trommsdorff & Schmidt-Rinke, 1980; Waclawik, 2014; Watson & Milfont, 2015; Wulfert et al., 2002; Zytkoskee et al., 1971). The gift delay task is very similar to the marshmallow task. In the gift delay, a gift is wrapped and children are told not to touch the gift until they get back. We excluded studies that solely used this task as it is substantively different from the marshmallow task. As the gift delay task has only one reward present (the gift), we a priori chose not to include it in this investigation. Studies that were eliminated sue to this inclusion include (but not limited to: Crivello et al., 2016; Cuskelly et al., 2006; Joyce et al., 2016; Mulder et al., 2014; Olson et al., 1999; Vaughn et al., 1984, 1986). An alternate version of the delay of gratification task involves the alternate reward not being two of the preferred treat, but instead an increasing accumulation of treats, one at a time, periodically until the child breaks. Again, to allow for accurate estimation across ages, we exclude these studies as times are consistently lower than the marshmallow task. Studies excluded for these reasons include: (Hrabic, 2015; Lomranz et al., 1983; Richie & Toner, 1984; Toner, 1981; Toner & Smith, 1977; Toner et al., 1977, 1979, 1980). Studies that altered the marshmallow task substantially were also excluded if no control group was available with standard instructions (e.g. Corfield et al., 1976; Kanfer et al., 1981; Kidd et al., 2013; Kochanska et al., 2000; Leonard et al., 2014; Maccoby et al., 1965; Michaelson & Munakata, 2016; Mischel & Moore, 1980; Mischel & Underwood, 1974; Yates et al., 1981). One study was excluded for dropping all children who waited the full time (Joseph, 2015). One study was axcluded for running the marshmallow test in an fmri scanner (Luersson et al., 2015). **References** Atance, C., Jackson, L. (2009) The Development and Coherence of Future-oriented Behaviors During the Preschool Years. Journal of Experimental Child Psychology. 102 379-391. Avci, S. (2013). Relations between self regulation, future time perspective and the delay of gratification in university students. Education, 133(4), 525-537. Ayduk, O., Mendoza-Denton, R., Mischel, W., Downey, G., Peake, P. K., & Rodriguez, M. (2000). Regulating the interpersonal self: strategic self-regulation for coping with rejection sensitivity. Journal of personality and social psychology, 79, 5, 776-792. Bandura, A., & Mischel, W. (1965). Modifications of self-imposed delay of reward through exposure to live and symbolic models. Journal of personality and social psychology, 2(5), 698. Bruce, A., Black, W., Bruce, J., Daldalian, M., Martin, L., Davis, A. (2011) Ability to Delay Gratification and BMI in Preadolescence. Obesity, 19, 5, 1101-1102. Callaway, N., Lutes, B., & Schlatter, C. (2007). Authoritarian Parenting and its Effects on the Impulsivity of Children. Undergraduate Research Journal of the Human Sciences, 6(1), 2-3. Carlson, S. M. (2005). Developmentally sensitive measures of executive function in preschool children. Developmental neuropsychology, 28(2), 595-616. Corfield, V., Al-Issa, I., & Johnson, B. (1976). Effects of verbal cues on the delay of self- gratification. The Journal of Psychology, 94(2), 167-171. Corvi, A. P. (2010). Subjective Time Perception Predicts Delay of Gratification (Doctoral dissertation, Case Western Reserve University). Corvi, A. P., Juergensen, J., Weaver, J. S., & Demaree, H. A. (2012). Subjective time perception and behavioral activation system strength predict delay of gratification ability. Motivation and Emotion, 36(4), 483-490. Crivello, C., et al. (2016) The effects of bilingual growth on toddlers’ executive function. Journal of Experimental Child Psychology, 141, 121-132. Cuskelly, M., Jobling, A., Gilmore, L., & Glenn, S. (2006). Parental strategies for assisting children to wait. Down Syndrome Research and Practice, 11(2), 55-63. Da Silva, S., Moreira, B., & Da Costa Jr, N. (2014). 2D: 4D digit ratio predicts delay of gratification in preschoolers. PloS one, 9(12), e114394. Duckworth, A. L., Tsukayama, E., & Kirby, T. A. (2013). Is it really self-control? Examining the predictive power of the delay of gratification task. Personality and Social Psychology Bulletin, 39(7), 843-855. Funder, D. C., & Block, J. (1989). The role of ego-control, ego-resiliency, and IQ in delay of gratification in adolescence. Journal of personality and social psychology, 57(6), 1041. Herndon, J. S. (2011). The effects of delay of gratification on the academic achievement, substance abuse, & violent behavior of middle-school students in alternative learning settings (Doctoral dissertation, University of Central Florida Orlando, Florida). Herzberger, S. D., & Dweck, C. S. (1978). Attraction and delay of gratification. Journal of Personality, 46(2), 215-227. Hongwanishkul, D., Happaney, K. R., Lee, W. S., & Zelazo, P. D. (2005). Assessment of hot and cool executive function in young children: Age-related changes and individual differences. Developmental neuropsychology, 28(2), 617-644. Hrabic, M. (2015). Social Models Influence Children's Delay of Gratification Strategy Use and Delay Performance. Imuta, K., Hayne, H., & Scarf, D. (2014). I want it all and I want it now: delay of gratification in preschool children. Developmental psychobiology, 56(7), 1541-1552. Joseph, N. A. (2015). Delayed Gratification Behavior Among Elementary School Children: An Intervention Model. Journal of Research Initiatives, 1(3), 11. Joyce, A. W., Kraybill, J. H., Chen, N., Cuevas, K., Deater-Deckard, K., & Bell, M. A. (2016). A Longitudinal Investigation of Conflict and Delay Inhibitory Control in Toddlers and Preschoolers. Early Education and Development, 27(6), 788-804. Kanfer, F., Stifter, E., Morris, S. (1981) Self-Control and Altruism: Delay of Gratification for Another. Society for Research in Child Development. 52(2) 674-682 Kidd, C., Palmeri, H., & Aslin, R. N. (2013). Rational snacking: Young children’s decision- making on the marshmallow task is moderated by beliefs about environmental reliability. Cognition, 126(1), 109-114. Kirby, K. N., & Maraković, N. N. (1996). Delay-discounting probabilistic rewards: Rates decrease as amounts increase. Psychonomic bulletin & review, 3(1), 100-104. Kochanska, G., Murray, K., Harlan, E. (2000) Effortful Control in Early Childhood: Continuity and Change, Antecedents, and Implications for Social Development. Developmental Psychology, 36, 2, 230-232. Kreitler, S., & Zigler, E. (1990). Motivational determinants of children's probability learning. The Journal of Genetic Psychology, 151(3), 301-316. Labuschagne, L. G., Cox, T. J., Brown, K., & Scarf, D. (2017). Too cool? Symbolic but not iconic stimuli impair 4-year-old children’s performance on the delay-of-gratification choice paradigm. Behavioural Processes, 135, 36-39. Leonard, J. A., Berkowitz, T., & Shusterman, A. (2014). The effect of friendly touch on delay- of-gratification in preschool children. The Quarterly Journal of Experimental Psychology, 67(11), 2123-2133. Lomranz, J., Shmotkin, D., & Katznelson, D. B. (1983). Coherence as a measure of future time perspective in children and its relationship to delay of gratification and social class. International Journal of Psychology, 18(1-4), 407-413. Luerssen, A., Gyurak, A., Ayduk, O., Wendelken, C., & Bunge, S. A. (2015). Delay of gratification in childhood linked to cortical interactions with the nucleus accumbens. Social cognitive and affective neuroscience, nsv068. Maccoby, E. E., Dowley, E. M., Hagen, J. W., & Degerman, R. (1965). Activity level and intellectual functioning in normal preschool children. Child Development, 761-770. Meade, T. L. (2012). I want it now: Do new media affect ability to delay gratification? (Doctoral dissertation, The University of Alabama TUSCALOOSA). Melikian, L. (1959). Preference for delayed reinforcement: An experimental study among Palestinian Arab refugee children. Journal of Social Psychology. 50, 81-86. Merz, E. C., Landry, S. H., Zucker, T. A., Barnes, M. A., Assel, M., Taylor, H. B., ... & Spinrad, T. L. (2015). Parenting Predictors of Delay Inhibition in Socioeconomically Disadvantaged Preschoolers. Infant and Child Development. Michaelson, L., Munakata, Y. (2016) Trust matters: Seeing how an adult treats another person influences preschoolers’ willingness to delay gratification. Developmental Science. 19(6) 1011–1019 Miller, D. T., & Karniol, R. (1976). The role of rewards in externally and self-imposed delay of gratification. Journal of Personality and Social Psychology, 33(5), 594-600. Mischel, W. (1958). Preference for delayed reinforcement: An experimental study of a cultural observation. The Journal of Abnormal and Social Psychology, 56(1), 57-61. Mischel, W. (1961). Father-absence and delay of gratification. The Journal of Abnormal and Social Psychology, 63(1), 116. Mischel, W., & Grusec, J. (1967). Waiting for rewards and punishments: Effects of time and probability on choice. Journal of personality and social psychology, 5(1), 24-31. Mischel, W., & Metzner, R. (1962). Preference for delayed reward as a function of age, intelligence, and length of delay interval. The Journal of Abnormal and Social Psychology, 64(6), 425. Mischel, W., & Moore, B. (1980). The role of ideation in voluntary delay for symbolically- presented rewards. Cognitive Therapy and Research, 4,211-221, Mischel, W., & Underwood, B. (1974). Instrumental ideation in delay of gratification. Child development, 1083-1088. Mulder, H., Hoofs, H., Verhagen, J., van der Veen, I., & Leseman, P. P. (2014). Psychometric properties and convergent and predictive validity of an executive function test battery for two-year-olds. Frontiers in psychology, 5. Nisan, M. (1976). Delay of gratification in children: Personal versus group choices. Child Development, 195-200. Olson, S. L., Schilling, E. M., Bates, J. E. (1999) Measurement of Impulsivity: Construct Coherence, Longitudinal Stability, and Relationship with Externalizing Problems in Middle Childhood and Adolescence. Journal of Abnormal Child Psychology, 27(2), 151- 165. Paulsen, K., & Johnson, M. (1980). Impulsivity: A multidimensional concept with developmental aspects. Journal of Abnormal Child Psychology, 8(2), 269-277. Prencipe, A., & Zelazo, P. D. (2005). Development of affective decision making for self and other: Evidence for the integration of first-and third-person perspectives. Psychological Science, 16(7), 501-505. Putnam, S. P., Spritz, B. L., & Stifter, C. A. (2002). Mother‐Child Coregulation During Delay of Gratification at 30 Months. Infancy, 3(2), 209-225. Rachlin, H., Raineri, A., & Cross, D. (1991). Subjective probability and delay. Journal of the experimental analysis of behavior, 55(2), 233-244. Ritchie, F. K., & Toner, I. J. (1984). Direct labeling, tester expectancy and delay maintenance behavior in Scottish preschool children. International Journal of Behavioral Development, 7(3), 333-341. Rodriguez, M. L., Mischel, W., & Shoda, Y. (1989). Cognitive person variables in the delay of gratification of older children at risk. Journal of personality and social psychology, 57(2), 358. Schwarz, J. C., Schrager, J. B., & Lyons, A. E. (1983). Delay of gratification by preschoolers: Evidence for the validity of the choice paradigm. Child Development, 620-625. Seeman, G., & Schwarz, J. C. (1974). Affective state and preference for immediate versus delayed reward. Journal of Research in personality, 7(4), 384-394. Staub, E. (1972). Effects of persuasion and modeling on delay of gratification. Developmental Psychology, 6(1), 166. Thompson, C., Barresi, J., & Moore, C. (1997). The development of future-oriented prudence and altruism in preschoolers. Cognitive Development, 12(2), 199-212. Toner, I. J. (1981). Role involvement and delay maintenance behavior in preschool children. The Journal of Genetic Psychology, 138(2), 245-251. Toner, I. J., & Smith, R. A. (1977). Age and overt verbalization in delay-maintenance behavior in children. Journal of Experimental Child Psychology, 24(1), 123-128. Toner, I. J., Holstein, R. B., Hetherington, E. M. (1977). Reflection-Impulsivity and Self-control in Preschool Children. Child Development, 48(1), 239-245. Toner, I. J., Lewis, B. C., & Gribble, C. M. (1979). Evaluative verbalization and delay maintenance behavior in children. Journal of Experimental Child Psychology, 28(2), 205- 210. Toner, I. J., Moore, L. P., & Emmons, B. A. (1980). The effect of being labeled on subsequent self-control in children. Child Development, 618-621. Trommsdorff, G., & Schmidt-Rinke, M. (1980). Individual and situational characteristics as determinants of delay of gratification. Archiv für Psychologie, 133, 263-275. Tynan, S. A. (2014). Inhibitory Control and Classroom Behavior in Kindergarten Children. Van Reet, J. (2013) The relationship between temperament and pretense in young preschoolers. NIH Public Access. 33(4) 383-401. Vaughn, B. E., Kopp, C. B., & Krakow, J. B. (1984). The emergence and consolidation of self- control from eighteen to thirty months of age: Normative trends and individual differences. Child development, 990-1004. Vaughn, B. E., Kopp, C. B., Krakow, J. B., Johnson, K., & Schwartz, S. S. (1986). Process analyses of the behavior of very young children in delay tasks. Developmental Psychology, 22(6), 752. Waclawik, K. (2014). Effects of Choice Framing and Affect on Delay of Gratification. The Huron University College Journal of Learning and Motivation, 52(1), 13. Watson, S., & Milfont, T. (2016) A short-term longitudinal examination of the associations between self-control, delay of gratification and temporal considerations. Personality and Individual Differences, 106, 57–60. Weber, J. D. (2015). Executive Function Predictors of Children's Talk. East Tennessee State University Wulfert, E., Block, J. A., Santa Ana, E., Rodriguez, M. L., & Colsman, M. (2002). Delay of gratification: Impulsive choices and problem behaviors in early and late adolescence. Journal of personality, 70(4), 533-552. Yates, J., Revelle, G. (1979) Processes Operative During Delay of Gratification. Motivation and Emotion, 3, 2, 103-115. Yates, G., Lippett, M., Yates, S. (1981) The Effects of Age, Positive Affect Induction, and Instructions on Children's Delay of Gratification. Journal of Experimental Child Psychology. 32 169-180. Yu, J., Kam, C. M., & Lee, T. M. (2016). Better working memory and motor inhibition in children who delayed gratification. Frontiers in psychology, ~7. Zytkoskee, A., Strickland, B. R., & Watson, J. (1971). Delay of gratification and internal versus external control among adolescents of low socioeconomic status. Developmental Psychology, 4(1p1), 93.