Interference is an explanation for forgetting in long-term memory. It states that memories interfere with and disrupt one another, so forgetting happens because other memories get in the way (Baddeley & Logie, 1999).
This idea suggests that information in long-term memory may become confused or combined with other information during encoding thus distorting or disrupting memories.
There are two ways in which interference can cause forgetting:
1. Proactive interference (pro=forward) occurs when you cannot learn a new task because of an old task that had been learnt. When what we already know interferes with what we are currently learning – where old memories disrupt new memories.
2. Retroactive interference (retro=backward) occurs when you forget a previously learnt task due to the learning of a new task. In other words, later learning interferes with earlier learning – where new memories disrupt old memories.
Proactive and retroactive Interference is thought to be more likely to occur where the memories are similar, for example: confusing old and new telephone numbers.
Chandler (1989) stated that students who study similar subjects at the same time often experience interference.
Previous learning can sometimes interfere with new learning (e.g. difficulties we have with foreign currency when travelling abroad).
Also, new learning can sometimes cause confusion with previous learning. (Starting French may affect our memory of previously learned Spanish vocabulary).
Key Takeaways
- PI: old learning disrupts recall of something learned more recently, so the interference runs forward in time.
- RI: new learning disrupts recall of something learned earlier, so the interference runs backward in time.
- Similarity: interference is strongest when the competing memories resemble each other, such as an old and a new phone number.
- Key Study: Underwood and Postman (1960) showed that learning a second word list disrupted recall of an earlier list, demonstrating retroactive interference.
- Limitations: most evidence comes from lab word-lists with low ecological validity, and the theory says little about the underlying mechanism.
- Modern Evidence: brain imaging shows the brain actively suppresses competing memories during retrieval, reframing interference as adaptive rather than a simple failure (Wimber et al., 2015).
Key study: Underwood and Postman (1960)
Aim: To investigate how retroactive interference affects learning. In other words, to investigate whether information you have recently received interferes with the ability to recall something you learned earlier.
Method: A lab experiment was used.
Participants were split into two groups, and both groups had to remember a list of paired words – e.g. cat – tree, jelly – moss, book – tractor.
The experimental group also had to learn another list of words where the second paired word if different – e.g. cat – glass, jelly- time, book – revolver. The control group was not given the second list. All participants were asked to recall the words on the first list.
Results: The recall of the control group was more accurate than that of the experimental group.
Conclusion: This suggests that learning items in the second list interfered with participants’ ability to recall the list. This is an example of retroactive interference.
Classic Experimental Evidence
Two further classic experiments explain why similarity matters and how much everyday “forgetting” is really old learning getting in the way. Both studies are treated as foundational, load-bearing evidence for interference theory rather than passing mentions.
Similarity and Retroactive Interference: McGeoch and McDonald (1931)
Aim: to test whether retroactive interference increases with how similar the newly learned material is to the material learned earlier.
Method: Participants learned a word list to perfect recall. They then spent ten minutes resting, learning numbers, nonsense syllables, or words related to the list (adjectives, antonyms, synonyms).
Results: recall fell as the interpolated task grew more similar to the original list, from about four items after rest to about one item after synonyms.
Conclusion: retroactive interference is graded by similarity. Because the time interval was identical across conditions, time alone could not explain the difference, which is strong evidence for interference over simple decay.
Evaluation: the design isolates similarity while holding the time interval constant, which is exactly what a pure decay account cannot explain. Its limitation is that it uses word lists learned in the laboratory, so it demonstrates the mechanism convincingly but says less about ordinary, real-world learning.
Underwood (1957): How Much “Decay” Was Really Interference?
Aim: to explain why the amount forgotten after 24 hours varied so widely between published experiments, and to test whether interference from previously learned lists was responsible.
Method: Underwood pooled results across many earlier nonsense-syllable studies, plotting 24-hour recall of the newest list against how many earlier lists each participant had already learned.
Results: one list alone left 70-80% recall a day later. Twenty prior lists cut that to about 20%. The more competing lists, the worse recall got.
Conclusion: much of the “forgetting” attributed to decay was really proactive interference. Time was not the cause. What mattered was how much competing material came before.
Evaluation: the re-analysis reframed lab forgetting as interference, explaining why labs reported different rates. It relies on nonsense syllables, so is artificial, but recurs consistently across datasets.
Evaluation
Although proactive and retroactive interference are reliable and robust effects, there are a number of problems with interference theory as an explanation of forgetting.
First, interference theory tells us little about the cognitive processes involved in forgetting. Is the old memory destroyed, or just blocked?
Secondly, most research into interference has been carried out in a laboratory using lists of words. This is a situation unlikely to occur often in everyday life, meaning low ecological validity.
As a result, it may not be possible to generalize from the findings. Real-world memory looks different.
Baddeley (1990) states that the tasks given to subjects are too close to each other and, in real life; these kinds of events are more spaced out.
Real-World Evidence: Baddeley and Hitch (1977)
Nevertheless, one study took interference outside the laboratory altogether.
Aim: to test, outside the lab, whether forgetting depends on how many events happen in between (interference) or on how much time has simply passed (decay).
Method: rugby union players recalled the names of every team they had played that season. Because injury and selection meant players missed different numbers of games, the matches actually played since a given fixture varied widely between players. This held even when time elapsed was equal.
Results: recall depended on how many games a player had played since the fixture. Time itself did not matter. A player who had played many games remembered a match worse than one who had played fewer games, even over the same time.
Conclusion: forgetting tracked intervening events rather than the passage of time. Events, not minutes, drove the forgetting. This favours interference over decay, using real memories outside the lab.
However, there is no doubt that interference plays a role in forgetting, but how much forgetting can be attributed to interference remains unclear (Anderson, 2000).
Contemporary Research
Recent neuroscience research has moved past this debate toward explaining the mechanism itself.
Retrieving one memory makes related, competing memories harder to recall later, a pattern called retrieval-induced forgetting. A meta-analysis pooling many earlier studies confirmed the effect is genuinely reliable (Murayama et al., 2014).
Wimber et al. (2015) then supplied a mechanism. Using brain scans that could read the activity of individual memories, they showed that repeatedly retrieving one memory actively suppressed its competitors’ brain patterns. The amount of suppression predicted how much those competitors were later forgotten.
The picture is now mechanistic, not just descriptive.
A causal animal study extends this picture to forgetting’s biology. Epp et al. (2016) manipulated hippocampal neurogenesis, the brain’s ongoing production of new neurons, in mice, then tested how well they learned information that conflicted with an older memory.
Increasing neurogenesis weakened the old memory and made room for the new, conflicting one. The reverse also held. Decreasing neurogenesis protected the old memory but blocked the new one.
The pattern is adaptive. It reframes forgetting as a biologically regulated process that clears out old, competing memories to make way for new learning.
A broader theoretical review ties these findings together (Yonelinas et al., 2019). Memories are bound to a time and place. New experiences drift, and old contexts become confusable with newer ones.
This is a modern echo of the interference idea the whole article has been building toward.
References
Anderson, J. R. (2000). Learning and memory: An integrated approach. New York: John Wiley & Sons.
Baddeley, A. D., & Hitch, G. (1977). Recency re-examined. In S. Dornic (Ed.), Attention and Performance VI (pp. 647-667). Lawrence Erlbaum Associates.
Baddeley, A. D., & Logie, R. H. (1999). Working memory: The multiple-component model. In A. Miyake & P. Shah (Eds.), Models of working memory (pp. 28-61). Cambridge, UK: Cambridge University Press.
Chandler, C. C. (1989). Specific retroactive interference in modified recognition tests: Evidence for an unknown cause of interference. Journal of Experimental Psychology: Learning, Memory, and Cognition, 15(2), 256-265. https://doi.org/10.1037/0278-7393.15.2.256
Epp, J. R., Silva Mera, R., Köhler, S., Josselyn, S. A., & Frankland, P. W. (2016). Neurogenesis-mediated forgetting minimizes proactive interference. Nature Communications, 7, 10838. https://doi.org/10.1038/ncomms10838
McGeoch, J. A., & McDonald, W. T. (1931). Meaningful relation and retroactive inhibition. American Journal of Psychology, 43(4), 579-588. https://doi.org/10.2307/1415159
Murayama, K., Miyatsu, T., Buchli, D., & Storm, B. C. (2014). Forgetting as a consequence of retrieval: A meta-analytic review of retrieval-induced forgetting. Psychological Bulletin, 140(5), 1383-1409. https://doi.org/10.1037/a0037505
Underwood, B. J. (1957). Interference and forgetting. Psychological Review, 64(1), 49-60. https://doi.org/10.1037/h0044616
Underwood, B. J., & Postman, L. (1960). Extraexperimental sources of interference in forgetting. Psychological Review, 67(2), 73-95. https://doi.org/10.1037/h0041865
Wimber, M., Alink, A., Charest, I., Kriegeskorte, N., & Anderson, M. C. (2015). Retrieval induces adaptive forgetting of competing memories via cortical pattern suppression. Nature Neuroscience, 18(4), 582-589. https://doi.org/10.1038/nn.3973
Yonelinas, A. P., Ranganath, C., Ekstrom, A. D., & Wiltgen, B. J. (2019). A contextual binding theory of episodic memory: Systems consolidation reconsidered. Nature Reviews Neuroscience, 20(6), 364-375. https://doi.org/10.1038/s41583-019-0150-4
