The present investigation examines the development of children's diagnostic reasoning abilities when such inferences involve belief revision about uncertain potential causes. Four- to 7-year-olds observed an event occur that was due to one of four potential causes. Some of those potential causes were revealed to be efficacious; others were revealed to be inefficacious, but there was always one potential cause presented with unknown efficacy. While all children could make appropriate predictive inferences about this situation, 4- and 5-year-olds were less capable of making correct diagnostic inferences about the cause of the event under these circumstances than older children. We discuss possible mechanisms for this development, as well as speculate on the relation between these findings and literature in children's scientific reasoning. [ABSTRACT FROM AUTHOR], Copyright of PLoS ONE is the property of Public Library of Science and its content may not be copied or emailed to multiple si)