Do adults make scale errors too? How function sometimes trumps size

Scale errors-futile attempts to use impossibly sized items as though they were appropriately scaled-have been thought to exist only in young children. Here, we document a similar version of the underlying phenomenon among adults. When asked to select 1 of 2 tools to achieve an instrumental goal, adults in Study 1 frequently selected, via keypress, a tool that was "for" the goal despite the tool being clearly ill sized in the given instance. In doing so, adults ignored an alternative tool that was perfectly sized for the task. Study 2 revealed this outcome did not emerge from a shape bias. Study 3 confirmed findings using a reaching task. Results support proposals that teleofunctional (purpose-based) reasoning is a highly powerful influence on categorization and behavior across development. Toddlers' scale errors may not be a symptom of immature thinking, but reflect a type of reasoning apparent in mature cognition.