Forgetting

Proactive interference (PI) occurs when older memories interfere with the recall of newer material — earlier learning acts forward in time to disrupt later learning. Retroactive interference (RI) occurs when newer learning disrupts recall of older material — more recent learning acts backward in time. A useful mnemonic: PI points forward (old disrupts new); RI reflects backward (new disrupts old). Both are strongest when the competing materials are similar.

Storage failure means that information was not durably encoded, or that the memory trace has decayed — the information is genuinely no longer present in memory. Retrieval failure means that information is stored in memory but is currently inaccessible — the right cues are absent. Tulving and Pearlstone (1966) demonstrated retrieval failure: items that participants "failed to recall" in free recall were successfully retrieved when category name cues were provided. The tip-of-the-tongue phenomenon — knowing that you know something but being temporarily unable to retrieve it — is a common everyday experience of retrieval failure.

Context-dependent forgetting occurs when the physical or environmental context at retrieval differs from the context during encoding. Environmental cues present during learning become part of the memory trace; their reinstatement at retrieval facilitates access. Godden and Baddeley (1975) demonstrated this with deep-sea divers who recalled word lists significantly better when the learning and recall environments matched (both underwater or both on land) than when they mismatched. The effect is explained by Tulving's encoding specificity principle.

Ebbinghaus (1885) found that forgetting follows a negatively accelerating curve: memory loss is most rapid in the first hour or two after learning, then progressively slows. He used the "saving method" — measuring how much less time was needed to relearn a list compared to the original learning — as a sensitive measure of retained memory. Approximately 50% of the material was forgotten within the first hour, with diminishing further loss over subsequent days and weeks. The curve has been replicated across many materials and conditions and forms the basis of spaced-practice learning strategies.