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Individuals frequently choose between accomplishing goals using unaided cognitive abilities or offloading cognitive demands onto external tools and resources. For example, in order to remember an upcoming appointment one might rely on unaided memory or create a reminder by setting a smartphone alert. Setting a reminder incurs both a cost (the time/effort to set it up) and a benefit (increased likelihood of remembering). Here we investigate whether individuals weigh such costs/benefits optimally or show systematic biases. In two experiments, participants performed a memory task where they could choose between a) earning a maximum reward for each remembered item, using unaided memory, or b) earning a lesser amount per item, using external reminders to increase the number remembered. Participants were significantly biased towards using external reminders, even when they had a financial incentive to choose optimally. Individual differences in this bias were stable over time, and predicted by participants’ erroneous metacognitive underconfidence in their memory abilities. Bias was eliminated, however, when participants received metacognitive advice about which strategy was likely to maximize performance. Therefore, there was no evidence for an intrinsic bias against cognitive effort. We conclude that individuals have stable biases towards using external versus internal cognitive resources, which result at least in part from inaccurate metacognitive evaluations. Finding interventions to mitigate these biases can improve individuals’ adaptive use of cognitive tools.