Interference in psychology is a leading explanation for why we forget. It describes what happens when memories compete with each other, making one or both harder to recall. Rather than memories simply fading over time, interference theory says forgetting occurs because other information gets in the way. This can work in two directions: old memories can block new ones, and new memories can disrupt old ones.
How Interference Causes Forgetting
The core idea behind interference is surprisingly intuitive. When you store information in memory, it doesn’t sit in isolation. It exists alongside everything else you’ve learned, and similar memories are especially likely to collide. At the moment you try to recall something, competing information can block the target memory from surfacing. This is called response competition, and it’s been one of the most widely studied explanations for everyday forgetting.
Interference is considered a passive form of forgetting. You don’t choose to forget; it happens because the circumstances of your learning created overlapping memory traces. This distinguishes it from inhibition, which is a more active, goal-directed process where your brain deliberately suppresses certain information. Both contribute to forgetting, but interference is the version most people experience without realizing it.
Proactive Interference: Old Blocks New
Proactive interference happens when something you already know makes it harder to learn something new. The “pro” means forward: the old memory reaches forward in time and disrupts the new one. A classic everyday example is phone numbers. If you memorize a new number, your old number keeps intruding when you try to recall the new one. The same thing happens with foreign currency when you travel abroad. Your brain keeps defaulting to the familiar system even though you’re trying to use a different one.
Students experience this regularly. Studying similar subjects back to back, like two foreign languages or two closely related science courses, tends to produce interference. The vocabulary, concepts, and frameworks from one subject bleed into the other, making both harder to recall accurately.
Retroactive Interference: New Disrupts Old
Retroactive interference works in the opposite direction. New learning reaches backward and damages or disrupts a memory you already had. This is a primary source of forgetting in everyday life.
A landmark study by McGeoch and McDonald demonstrated this clearly. Participants memorized a list of 10 words until they knew them perfectly. Then they were split into six groups, each given a different task: learning synonyms, antonyms, unrelated words, nonsense syllables, three-digit numbers, or simply resting. Afterward, everyone tried to recall the original list. The results showed a direct relationship between similarity and forgetting. The group that learned synonyms (the most similar material) had the hardest time recalling the original words. The group that rested performed best. The more similar the new material was to the old, the more it interfered.
This finding has held up across decades of research. In studies with children and adults, learning new associations that overlap with previously learned ones consistently degrades recall of the originals. Even activities that seem unrelated to the original learning, like listening to a radio recording or solving math problems during a delay, can produce some degree of retroactive interference, though the effect is strongest when the old and new material are similar.
Why Similarity Matters So Much
The single biggest factor that determines how much interference you’ll experience is how similar the competing memories are. This applies to both proactive and retroactive interference. When two pieces of information share features, like the same category, the same format, or similar sounds, your brain has a harder time distinguishing between them at retrieval.
Experimental work on face memory illustrates this well. When people learn to associate names with faces, interference spikes when the faces look similar to each other. Researchers found that making face pairs just 25% more alike noticeably increased the difficulty of keeping the associations straight. The brain resolves this kind of competition partly by exaggerating the differences between similar memories along whatever dimension is most useful for telling them apart, essentially pushing the memories further from each other to reduce confusion.
Interference in Short-Term Memory
Interference doesn’t only affect long-term recall. It’s a major source of errors in short-term memory as well, operating over intervals of just a few seconds. Items that sound alike or look alike are frequently substituted for each other during immediate recall. Similar semantic contexts, where the meanings of words overlap, also build up interference quickly.
One of the most common ways researchers measure this is the recent-probes task. Participants memorize a small set of items, wait a few seconds, then see a test item and have to say whether it was in their set. The key manipulation involves showing items that weren’t in the current set but were in the previous one. People are consistently slower and make more errors when rejecting these “recent no” items. The lingering familiarity from the prior trial creates interference that the brain must actively resolve before giving a correct answer.
Interference vs. Decay
For decades, interference theory competed with decay theory as the dominant explanation for forgetting. Decay theory proposes that memories simply weaken over time, like ink fading on paper, regardless of what else you learn. Interference theory says forgetting is driven not by time itself but by what happens during that time.
Current thinking suggests these aren’t entirely separate processes. One influential framework proposes that decay and interference are functionally related. When your brain needs to update a piece of information frequently, it allows the old value to decay faster specifically to prevent it from interfering with the new value. The decay rate adapts to how often updates occur: when updates are frequent, decay is faster, and when updates slow down, decay slows too. This means decay may actually serve interference resolution rather than being an independent cause of forgetting.
How Your Brain Resolves Interference
When competing memories create confusion, specific brain regions activate to sort out the conflict. Brain imaging studies show that areas in the frontal lobes, particularly on both the left and right sides of the lower and middle frontal regions, become more active during tasks that require distinguishing between competing memories. These regions are involved in the controlled, effortful process of selecting the correct memory while suppressing the wrong one.
Aging affects this process. Older adults recruit additional frontal areas during interference resolution, including regions on the left side of the brain that younger adults don’t need to engage. This extra recruitment appears to be partially driven by errors. When older adults make mistakes on interference tasks, their frontal lobe activation increases substantially, suggesting the brain is working harder to compensate for the difficulty.
Real-World Consequences
Interference has practical implications well beyond the laboratory. In legal settings, retroactive interference plays a role in the reliability of eyewitness testimony. When a witness encounters new information after an event, such as news reports, conversations with other witnesses, or leading questions during interviews, that post-event information can alter or overwrite the original memory. Given the reconstructive nature of memory, stored information can undergo genuine change during the time between an event and when testimony is given.
In education, students studying for multiple exams in related subjects are particularly vulnerable. The overlapping terminology and concepts create exactly the conditions that maximize interference. Spacing out the study of similar subjects, rather than cramming them back to back, helps reduce the overlap.
Strategies That Reduce Interference
Research shows that experience with interference itself can reduce its effects. In experiments where participants encountered interference across multiple rounds, both young and older adults gradually learned to counteract it. They became better at recognizing which items were likely to cause confusion and adjusted their behavior accordingly. Younger adults, for instance, became nearly twice as likely to withhold an incorrect response by the second round of an interference task, jumping from 22% to 46%.
Feedback plays a critical role, especially for older adults. Without being told whether their responses were correct, older adults showed little improvement across rounds. With feedback, they were able to reduce false memories caused by interference just as younger adults did. Both age groups also adjusted their study time when given control over it, spending more time on interference-prone items as they recognized which ones were tricky.
The practical takeaway is that training under conditions of high interference can improve your ability to distinguish between competing memories. Rather than avoiding situations where memories might collide, deliberately practicing recall in those conditions builds the monitoring skills that help you catch errors before they stick.