An open question in the developmental causal learning literature concerns how children's beliefs about causal systems impact their inferences. This study investigated how 4- and 5-year-olds' causal beliefs related to their “backwards blocking” abilities, as well as whether associative learning or Bayesian inference better explained their judgements. Children were taught either that two causes together produced a larger effect than that produced by each individually or that they produced the same size effect as that produced by either one. A third group received no training. Results indicated that 4-year-olds engaged in backwards blocking only after additivity training and that their inferences mainly matched an associative model. In contrast, 5-year-olds consistently engaged in backwards blocking and produced responses that largely matched a Bayesian model. These findings suggest that the effect of children's beliefs about causal systems on their inferences undergoes a developmental progression and implicate the role of multiple cognitive mechanisms.