We examined the mechanisms involved in the development of the easily learned, easily remembered (ELER) heuristic in three groups of young children (4-5 years, 6-7 years, and 8-9 years). A trial-to-acquisition procedure was used to evaluate how much these children's judgment of learning depended on the ELER heuristic. Moreover, a new experimental paradigm, composed of six phases a pretest, four training phases, and a posttest was employed to implicitly influence the validity of the ELER association that underlies this metacognitive rule. Results revealed that the ELER heuristic develops early (4-5 years), but its use is reduced after implicit training. Furthermore, executive monitoring was found to account for the smaller changes observed in older children (8-9 years) after training. From a developmental perspective, these findings present a coherent picture of children's learning of metacognitive heuristics, wherein early automatic and implicit learning is later followed by effortful control. (C) 2014 Elsevier Inc. All rights reserved.