The cost of "what might have been": false memories for counterfactual actions

Deciding what to do should typically involve considering what could have been done, but in fact was not done. We tested whether such unchosen alternative action paths leave traces in memory. Across two behavioural experiments, participants solved a modified Tower of London (ToL) task in which each problem afforded two alternative and equivalent optimal solution paths. They then undertook a recognition test in which they viewed configurations of the ToL drawn either from the chosen path, the unchosen path, or from new, unviewed configurations. Participants were consistently more likely to judge (falsely) that they had previously seen configurations from the unchosen but plausible path, compared to novel configurations. Signal detection analyses further showed reduced discriminability and a more liberal decision criterion for alternative-path configurations compared to control novel configurations. Unchosen action alternatives may be difficult to distinguish from actions that are actually executed, but may have a feeling of familiarity that leads to an "old" response. This effect replicated when novel items were matched to alternative items in visual similarity, ruling out explanations based on perceptual confusability. The results are consistent with the idea that action planning generates representations of unchosen alternatives that can persist and bias later memory, even if not executed. Planning therefore leaves behind more than a route toward action goals: it also leaves mnemonic traces of paths not taken.