Children get a lot of attention for being powerful “learning machines” in the popular press, but the truth is they remember much less than adults. Even still, many of children’s individual memories are rich and complex and similar in quality to adults’, suggesting what improves in development is not just the quality of our memories, but the frequency with which we form them. Here, we ask why children form memories less often than adults; instead of focusing on memory mechanisms, we focus on an entirely separate aspect of cognition: sustained attention. In adults, sustained attention fluctuates to shape memory in each moment, but we know little about how attention fluctuates in childhood to shape memory formation. To address this gap, 7–10-year-old children and adults (n=120) completed a sustained attention task in which they classified trial-unique images as living or nonliving. We then tested memory for each image, and related attentional fluctuations during classification to subsequent memory. We found that attention fluctuated between states more frequently in children than adults, and that across children, attentional lapse rates correlated with lower memory performance. Within participants, attentional fluctuations shaped the fate of individual memories, such that lapses predicted memory failures. While these fluctuations shaped memory for expected events in both children and adults, they only shaped memory for unexpected events in children, highlighting their particularly detrimental and pervasive influence in development. Our findings raise the possibility that broad developmental differences in cognitive performance reflect the ability to sustain attention.

Attentional lapses have been found to impair everything from basic perception to learning and memory. Yet, despite the well documented costs of lapses on cognition, recent work suggests that lapses might unexpectedly confer some benefits. One potential benefit is that lapses broaden our learning to integrate seemingly irrelevant content that could later prove useful–a benefit that prior research focusing only on goal relevant memory would miss. Here, we measure how fluctuations in sustained attention influence the learning of seemingly goal-irrelevant content that competes for attention with target content. Participants completed a correlated flanker task in which they categorized central targets (letters or numbers) while ignoring peripheral flanking symbols that shared hidden probabilistic relationships with the targets. We found that across participants, higher rates of attentional lapses correlated with greater learning of the target-flanker relationships. Moreover, within participants, learning was more evident during attentional lapses. These findings address long-standing theoretical debates and reveal a benefit of attentional lapses: they expand the scope of learning and decisions beyond the strictly relevant.