I. INTRODUCTION: UNDERSTANDING MEDICINES AND MEDICAL INTERVENTIONS.

We live in an increasingly pharmacological and medical world, where children and adults frequently encounter alleged treatments for an enormous range of illnesses. How do we come to understand what heals and why? Here, 15 studies explore how 1,414 children (ages 5-11) and 882 adults construe the efficacies of different kinds of cures. Developmental patterns in folk physics, psychology, and biology lead to predictions about which expectations about cures will remain relatively constant across development and which will change. With respect to stability, we find that even young school children (ages 5-7) distinguish between physical and psychological disorders and the treatments most effective for each. In contrast, young children reason differently about temporal properties associated with cures. They often judge that dramatic departures from prescribed schedules will continue to be effective. Young children are also less likely than older ages to differentiate between the treatment needs of acute versus chronic disorders. Young children see medicines as agent-like entities that migrate only to afflicted regions while having "cure-all" properties, views that help explain their difficulties grasping side effects. They also differ from older children and adults by judging pain and effort as reducing, instead of enhancing, a treatment's power. Finally, across all studies, optimism about treatment efficacy declines with age. Taken together, these studies show major developmental changes in how children envision the ways medicines work in the body. Moreover, these findings link to broader patterns in cognitive development and have implications for how medicines should be explained to children.