Interference theory is the dominant explanation for forgetting from long-term memory. Rather than memories simply fading with time (decay), interference theory proposes that forgetting occurs because other memories block or impair retrieval of the target memory. The more similar the competing memories, the greater the interference. Two forms are distinguished: proactive interference (older memories interfere with newer ones) and retroactive interference (newer memories interfere with older ones).
Proactive Interference
Proactive interference (PI) occurs when previously learned information impairs memory for subsequently learned information. In the classic A-B, A-C paradigm, learning a second list of word pairs (A-C) is harder when a first list with the same cue words but different responses (A-B) was previously learned. The old associations (A-B) compete with the new ones (A-C) at retrieval. PI accumulates across learning episodes: the more prior lists learned, the worse recall of the most recent list — a finding demonstrated by Underwood (1957), who showed that much of the "forgetting" in verbal learning experiments was actually PI from previous experimental sessions.
Retroactive Interference
Retroactive interference (RI) occurs when newly learned information impairs memory for previously learned information. Learning new material between study and test worsens recall of the original material, especially when the new material is similar. Classic demonstrations involved learning two word lists: recall of the first list was worse when a similar second list intervened than when participants rested during the interval.
Retroactive interference: Learn A-B → Learn A-C → Test A-B (impaired by A-C)
Control: Learn A-B → Rest → Test A-B (best performance)
The interference vs. decay debate has a long history. Pure decay theories predict that forgetting depends only on time. Interference theories predict that forgetting depends on what happens during the retention interval. Evidence generally favors interference: forgetting is greater when the retention interval is filled with similar activities than when it is filled with dissimilar activities or sleep (which minimizes new learning and hence retroactive interference). However, some time-dependent forgetting may occur even in the absence of identifiable interference, and modern theories often incorporate both mechanisms.
Similarity and Interference
The degree of interference is critically dependent on the similarity between competing memories. Highly similar memories (two lists of words from the same category) produce more interference than dissimilar memories (a word list and a motor task). This similarity gradient is consistent with interference arising from competition among overlapping memory representations at retrieval.
Modern Perspectives
Contemporary research has refined interference theory by identifying specific mechanisms. Competition at retrieval (multiple associations to the same cue produce competition) is complemented by associative unlearning (new learning may weaken old associations, not just compete with them) and inhibitory mechanisms (retrieval of some items may actively suppress related competing items, as in retrieval-induced forgetting). These mechanisms operate together to determine which memories can be successfully accessed at any given moment.