Does testing enhance new learning because it insulates against proactive interference?

Taking a test on previously learned material can enhance new learning. One explanation for this forward testing effect is that retrieval inoculates learners from proactive interference (PI). Although this release-from-PI account has received considerable empirical support, most extant evidence is correlational rather than causal. We tested this account by manipulating the level of PI that participants experience as they studied several lists while receiving interpolated tests or not. In Experiments 1 and 2, we found that testing benefited new learning similarly regardless of PI level. These results contradict those from Nunes and Weinstein (Memory, 20(2), 138–154, 2012), who found no forward testing effect when encoding conditions minimized PI. In Experiments 3 and 4, we failed to replicate their results. Together, our data indicate that reduced PI might be a byproduct, rather than a causal factor, of the forward testing effect.