How many times have you been urgently trying to log into your email or bank account, only to get the message “password not recognized”? You might enter the password again, thinking maybe you just mistyped it, only to get an error reminding you that you changed your password a month ago. Panic sets in as you rack your brain, trying various combinations of old passwords, until you realize that you can’t remember the most recent one you set for the life of you. For me, this usually ends in defeat as I reset my password, only to get the message “your new password cannot be the same as your previous one.” 

Although the stress of being locked out of your accounts may be enough to interfere with your rational thinking and memory, there is another reason that it can be so difficult to remember new passwords: proactive interference. This type of memory contamination, sometimes referred to as proactive inhibition, describes how the things we’ve previously learned tend to make it more difficult to remember newer information, especially if they are similar. Every time you try to memorize a new password, all of the other passwords you’ve previously created (especially if they’re basically the same, just with “123” added at the end) might disrupt your ability to properly recall that memory of your new password. 

Proactive interference not only makes learning new passwords a challenge, but it can make it harder to learn or remember anything new—which is essentially what we do all day, every day. While proactive interference occurs when what we’ve previously learned keeps us from remembering something new, the related concept of retroactive interference explains why it’s harder to remember older information after we’ve learned similar new information.1 However, both phenomena can interfere with activities like learning new languages or job skills, how students function in the classroom, and even learning helpful coping mechanisms when people grapple with mental health challenges. 

Retroactive Interference: A type of memory interference where newer memories interfere with the ability to recall old information, contrasting the effects of proactive interference.1

Memory Consolidation: The process by which short-term memories are stabilized into long-term storage.1

Encoding: The initial process of learning new information, which involves transforming perceptual input into a format that can be stored in memory for later retrieval. Proactive interference can impair encoding, especially when the old and new information are similar.1

Retrieval Failure: When stored information cannot be accessed, often due to interference from older learning.1

Working Memory: The system in the brain that helps you temporarily retain and manipulate a limited amount of information. Overload in working memory can increase susceptibility to interference.1

Long-Term Memory: The system that stores information for extended periods of time. Long-term memories can interfere with the encoding or retrieval of newer information, especially when the old and new material are similar.1

Rehearsal: The mental repetition or processing of information to maintain it in short-term memory or facilitate its transfer to long-term memory.1

Semantic Similarity:  The degree of overlap or resemblance in meaning between two phrases, sentences, or larger chunks of text, even if they are worded differently. It's a measure of how similar two texts are based on their underlying meaning, as opposed to simple lexical matching (comparing exact words).1

Spacing Effect: The idea that when information is learned through repetition, it is more effectively  remembered when spaced over longer, rather than shorter, periods of time. When individuals space out their repetition of information, they can better recall that information in the future.

Cognitive Load Theory: A theory of memory in psychology and instructional design focused on how instructions are processed and learned, considering how information processing capabilities are limited by working memory capacity. A high cognitive load increases the likelihood of interference.

German psychologist Hermann Ebbinghaus was among the first to systematically study memory and forgetting, and the implications of his research apply to both proactive and retroactive interference. His 1885 study using lists of nonsense syllables found that participants’ memories of the lists could intrude on one another, essentially intervening on either the memory encoding or recollection process.2 Later studies on interference confirmed its negative impact on memory. 

The phenomenon of proactive interference was first specifically described in 1894, when early psychological experiments revealed how previously learned material could hinder new learning. At this time, German psychologist Georg Elias Müller documented the difficulties participants had in memorizing word lists when prior lists had already been learned: the information in the older lists was “interfering” with the ability to memorize the information in the newer lists.3

The concept was further developed in the 20th century by the researchers Benton Underwood and  Geoffrey Keppel, who demonstrated how prior learning negatively affected recall of newly learned material. Their studies found that participants who memorized multiple lists performed progressively worse on later lists, suggesting a compounding effect of interference.4

By the 1950s, cognitive psychologist Leo Postman joined Underwood, and they conducted the “Postman word pair study,” which expanded on these ideas, showing that the strength of interference was dependent on the similarity between old and new information. Although this study is usually used as an example of retroactive interference, the link between the two concepts is strong, and the research on the topics is often interrelated.5 

Later, researchers like Delos Wickens discovered that switching to entirely new categories of information could reduce interference, paving the way for the development of techniques to counteract its effects.6 As we learn more about the importance of rehearsal and intentional recall in establishing long-term memories, we can use these tools to improve education and strengthen our memorization capabilities.

G. E. Müller

A German experimental psychologist who inspired much of the later work on interference. Before his research, forgetting was understood as only being due to the brain’s limited volume and duration of information processing, but Müller observed and conceptualized both proactive and retroactive interference. In 1894, he observed that the learning of a second list of items interfered with recall of the first list.3

Geoffrey Keppel & Benton Underwood

American psychologists known for studying how previously learned information disrupts the acquisition of new memories in their 1962 experiment involving nonsense “trigrams” (three-letter groupings). Their work found that proactive interference in short-term memory follows the same laws as proactive interference in the long-term memory of lists of items.4

Leo Postman

An American cognitive psychologist who studied the role of similarity in proactive interference, showing that distinct material reduced interference. His 1960 study, conducted with researcher Benton Underwood, used lists of paired words to test how the similarity of information impacts recall; the results of this study have been used as evidence for the effect of retroactive interference.5 

Delos Wickens

An American experimental research psychologist who developed paradigms to measure proactive interference and demonstrated that switching to different types of information reduced interference.6

Proactive interference influences cognition in a myriad of ways, affecting education, decision-making, professional development, and our day-to-day functioning. When past knowledge disrupts new learning, it can create inefficiencies in incorporating new skills or knowledge and making informed choices.

Interference in Language Learning and Communication

One of the most easily identified and well-documented examples of proactive interference is the challenge it adds to language acquisition. Learning a new language is hard enough as it is, but when individuals learn a second language, their first language often interferes, causing errors in grammar or word choice. People may inadvertently be fixated on a word’s pronunciation in their mother tongue or simply not be able to encode a memory for a second word for the same concept.7 The same can happen with sentence structure; for instance, a Spanish speaker learning English may mistakenly apply Spanish syntax, leading to phrases like "She has 30 years" (the direct translation of the Spanish phrase) instead of "She is 30 years old." Likewise, when switching between multiple languages, speakers may accidentally use words from their native language instead of the intended one.

This interference can persist even in fluent bilinguals, especially when switching rapidly between languages in conversation. The same issues are even more pronounced as people pick up third or even fourth languages, with the previously encoded words interfering with the ability to recall the most recently learned language.7

Though this may cause frustration for speakers of multiple languages, it seems to reflect something uniquely human. It connects to the idea that there is a critical period for language acquisition, a time in early childhood where learning languages is much easier, after which it becomes much more difficult to learn a language to a native-like level. It has been hypothesized that this ability to absorb new languages easily when we are young is due to the high neuroplasticity of our brains in childhood, but it’s still unclear why these languages learned early on stick around (potentially interfering with our ability to learn new ones), even if we’ve stopped using the language in our daily life. 

Recent research has aimed to study whether the critical period for language acquisition is due to human biological constraints or simply emerges as a result of statistical learning, by observing the performance of Large Language Models (LLMs).8 Ultimately, researchers found that because the models don't have a preprogrammed loss of plasticity with age as humans do, they aren’t susceptible to critical period effects when acquiring a second language. The flip side of this is that if you completely switch their training language, they are so plastic that they will rapidly forget their "native" language upon learning the new one—something that doesn't happen in humans beyond the first few years of life. So in humans, this stubborn intrusion from your native language is a feature, not a bug, as it shows measures are in place to prevent it from being completely overwritten.

Workplace Adaptation and Resistance to Change

In professional settings, proactive interference can be just as frustrating. Often, our jobs ask us to continuously learn new workflows as company policies change and technology evolves. For long-standing employees or those who have become accustomed to certain procedures, change can be extra difficult. Besides employees’ natural resistance to change, proactive interference can actually make it harder for people to encode new information, as their memory of the original procedure interferes with their capacity to remember something new.1,9 

For example, a salesperson who has been trained in traditional in-person selling techniques and has been practicing this type of salesmanship their entire career may find it challenging to transition to digital marketing and virtual or over-the-phone sales. While they may have previously relied on a few key phrases that led to success when they were in person, the same techniques (like asking the customer if they want to hold or try out the product) might not work so well over the phone. As salespeople try to switch gears and succeed in this new sales world, proactive interference can hinder their ability to remember the slightly different phrases or techniques they must now turn to. 

Similarly, those in the medical field often must adapt to new procedures or dosage information as more research comes out. However, because information like medication dosages can be incredibly similar or are easily mixed up, proactive interference can limit the ability of doctors and nurses to recall new recommendations because of the intrusion of older ones.10 

There will always be resistance to adopting new policies, technologies, and workflows, especially for employees who are accustomed to certain procedures and have been doing things a certain way for a long time—but change resistance and barriers to encoding new information can lead to inefficiencies and even safety issues in the context of health or safety-related workflows. Organizations can address these issues by offering gradual retraining programs for employees that allow hands-on practice, reducing the impact of proactive reinforcement.9  

Cognitive Rigidity and Habitual Errors

Proactive interference can also lead to cognitive rigidity, where individuals struggle to adapt to new ways of thinking or problem-solving. For most people, once a specific approach becomes ingrained, our brains default to it, making any alternative methods harder to adopt. For example, think of an experienced chess player who has played hundreds or even thousands of games. Over time, she may keep relying on old strategies that worked in past games (her “usual moves”) because not only have they worked before, but they’re also what comes to mind most easily. Even when more effective tactics are available, she may recall the first chess techniques or strategies that she mastered instead of the newer skills she’s learned. 

Similarly, a chef trained in a specific cuisine may instinctively use traditional ingredient pairings, even when learning a completely different style of cooking. Although there’s some value to following tradition or the tried-and-true methods, this rigidity can result in habitual errors, where individuals repeatedly default to outdated methods despite knowing a more effective solution exists.

Proactive interference plays a role in outdated thinking, memory distortions, and difficulty adapting to evolving information. This can take many forms, from issues in recovery from behavioral or substance-use disorders, to barriers in overcoming bias and keeping professional training and standards up to date. 

Cognitive Rigidity in Mental Health Challenges

One of the biggest barriers to recovery from many mental health disorders is patients’ difficulty adopting new, healthier coping mechanisms. For many people suffering from substance use disorders, behavioral addictions like gambling or eating disorders, the behaviors that are currently keeping them trapped in their disorder (like using drugs or binge eating) are the very same familiar coping mechanisms that help soothe them in times of stress.11,12,13 There are few things more stressful on the body and mind than trying to recover from an addiction, and thus many people can find themselves feeling paradoxically incapable of recovery without relying on their self-destructive coping method for a sense of stability. 

Although many people in treatment are presented with suggestions for alternatives to their current unhealthy habits, it can be difficult to learn these alternatives because something akin to proactive interference may intervene.13 In the early stages, patients may actively work on adopting new coping strategies (like deep breathing, exercise, meditation, or mindfulness techniques), but their disordered behavior will likely be so much more practiced, salient, and even preferred, and consequently patients are prone to falling back on these more easily accessible primary coping techniques.10,11, 

Persistence of Outdated Knowledge

Picture a squirrel with a backpack on, ready for his first day of school. Now, forget the image. It’s fun to try to imagine something so specific (and adorable), but it’s not so easy to forget on cue, is it? Once information is learned, it can be difficult to unlearn. Even when newer, more accurate data becomes available, the things we have learned first often persist in our memories. As previously discussed, this can be problematic in scientific and medical fields, where professionals may cling to outdated theories despite being presented with evidence that contradicts them.

Older generations of doctors, for example, may continue recommending dietary guidelines that have since been debunked, simply because they were taught these principles early in their training. So much of our older nutrition information now seems laughable (there was a time when butter and margarine were considered their own food group), and yet dietary guidelines and our understanding of nutrition research can change so quickly that it’s almost inevitable that the appropriate advice will change several times throughout a provider’s career.10 Assuming that the provider even has the bandwidth to stay up to date on these guidelines, it can become harder and harder to remember the most recent information. This bias goes hand in hand with the primacy effect, which is our tendency to remember the first piece of information we encounter over later information. Most often, healthcare providers, just like experts in any other industry, will default to the most easily recalled knowledge, which is often the first thing they learned on the subject. 

Cognitive Bias and Resistance to New Ideas

Proactive interference can contribute to additional cognitive biases, making people resistant to changing long-held beliefs. To some extent, we’re all resistant to change and tend to cling to the beliefs we have about the world around us. Relying on our preexisting perspectives and schemas can be helpful, in that it provides a sense of stability and security as a way to interpret our surroundings, but it can also keep us from having an open mind. This is often known as belief perseverance: when we’re faced with new evidence that challenges existing viewpoints, we may unconsciously rely on our prior knowledge, leading us to dismiss or reinterpret new information. People may subconsciously ignore or, through proactive interference, forget the new contradictory information in order to maintain their preexisting understanding of things.

This resistance is particularly evident in areas like politics, history, social justice, animal welfare, and education reform, where past knowledge competes with evolving perspectives and research. You may even find that it’s easier to recall the instances where you learned something that fit your own belief about a certain emotionally-charged subject than it is to recall a situation where you learned something that challenged your belief. Although this could be because there is genuinely less existing evidence that contradicts your view, or because confirmation bias tells us that you’re more likely to notice and give credence to information that supports your views in the first place, but there is also a chance that proactive interference plays a part in your difficulty to recall new information which contradicts the first material you learned on a subject.

Keppel and Underwood’s 1962 Nonsense Trigram Trial

In 1962, psychology researchers Geoffrey Keppel and Benton Underwood investigated how previously learned information can interfere with the retention of new information in short-term memory.4 Previous research with a similar paradigm had demonstrated the impact of memory decay when information isn’t rehearsed (a practice of repeating the information to ourselves in an attempt to memorize it). However, Keppel and Underwood suspected that proactive interference also played a role in why we tend to forget more recent information, and thus set up an experiment to test their theory. 

In their study, participants were first presented with meaningless three-letter trigrams (e.g., THG, ZXR) at various time intervals (3, 6, 9, 12, 15 seconds, etc.). To prevent the participants from rehearsing the trigrams, they were given a distracting (and perhaps annoying for those who aren’t numerically inclined) task: counting backward in threes from a given number (e.g., starting at 309 and counting 306, 303, 300, and so on). After the time delay, participants were asked to recall the original trigrams.

Similar to the results of previous research documenting memory decay over time, Keppel and Underwood found that participants showed competent recall for the first few trials but struggled in later ones. However, instead of memory simply fading, the researchers noticed that information from previous trials was interfering with participants' ability to recall newer trigrams. In other words, participants typically remembered the trigrams presented first, regardless of the interval length. For example, participants may remember only the first and second trigrams in a list of ten, regardless of whether one minute or five minutes had passed. 

This is related to the primacy effect, which explains why we tend to remember the information presented first better than what is learned later on. However, when the list of trigrams grew to twenty items, for example, even if the same amount of time had passed, they may only be able to recall one trigram. The more trials a participant completed, the worse their recall became, supporting the idea of proactive interference, as it was not the amount of time that passed but the amount of new information that interfered with participants’ memory.4 

Wickens' 1972 Release from Proactive Inhibition

With proactive interference now established as a reliable concept in the field of memory research, researcher Delos Wickens expanded on the idea by investigating whether changing the type of information presented could reduce its effects.6 He theorized that interference occurs because similar types of information overlap in memory, making it harder to distinguish new from old. However, if the category of information was changed, memory performance should improve.

In his study, participants were asked to memorize lists of words from the same category (e.g., different types of fruits: apple, banana, orange). After several trials, the participants’ ability to recall the new words decreased; this showed that proactive interference was building up. However, when the category of items was changed in a subsequent trial (e.g., switching from fruits to professions like teacher, captain, actor), participants’ recall performance suddenly skyrocketed. Wickens called this phenomenon "release from proactive inhibition," as the category switch-up appeared to have “reset” their memory, and kept proactive interference from continuing to inhibit their recall.6

Thus, Wickens' findings highlight the role of semantic similarity in proactive interference—when new information is too similar to old information, interference increases. We can use this understanding of how proactive interference functions to improve our own memorization efforts. If you’re a student or trying to learn multiple new hobbies, it can be beneficial to switch between different subjects or your different skill sets while studying to reduce interference effects.

Retroactive Interference 

As the sister concept to proactive interference, retroactive interference explains the tendency of more recent learning to hinder the recall of previously learned material. This article explains the shared history of retroactive and proactive interference, as well as how their impacts differ. 

Cognitive Load Theory 

Proactive interference suggests a limited ability to process new information when older information interferes, but Cognitive Load Theory (CLT) is a framework that explains how the brain processes and retains information by managing the limitations of working memory. This article explains the concept further and distinguishes between three types of cognitive load—intrinsic (material complexity), extraneous (distractions or poor design), and germane (schema building for long-term retention).

1. Carnero-Sierra, S., Alfonso-Arias, P. (2022). Proactive Interference. In: Vonk, J., Shackelford, T.K. (eds) Encyclopedia of Animal Cognition and Behavior. Springer, Cham. https://doi.org/10.1007/978-3-319-55065-7_750 
2. MacLeod, C. M. (n.d.). Interference theory: History and current status. In M. J. Kahana & A. D. Wagner (Eds.), Handbook of human memory: Foundations and applications. Oxford University Press. 
3. Dewar, M. T., Cowan, N., & Sala, S. D. (2007). Forgetting due to retroactive interference: a fusion of Müller and Pilzecker's (1900) early insights into everyday forgetting and recent research on anterograde amnesia. Cortex; a journal devoted to the study of the nervous system and behavior, 43(5), 616–634. https://doi.org/10.1016/s0010-9452(08)70492-1 
4. Keppel, G., & Underwood, B. J. (1962). Proactive inhibition in short-term retention of single items. Journal of Verbal Learning and Verbal Behavior, 1(3), 153-161. Extraexperimental sources of interference in forgetting.
5. Underwood, B. J., & Postman, L. (1960). Extraexperimental sources of interference in forgetting. Psychological Review, 67(2), 73–95. https://doi.org/10.1037/h0041865 
6. Wickens, D. D. (1972). Characteristics of word encoding. Memory & Cognition, 1(4), 485-490. 
7. Bialystok E, Feng X. Language proficiency and executive control in proactive interference: evidence from monolingual and bilingual children and adults. Brain Lang. 2009 May-Jun;109(2-3):93-100. doi: 10.1016/j.bandl.2008.09.001. Epub 2008 Oct 1. PMID: 18834625; PMCID: PMC2699211. 
8. Constantinescu, I., Pimentel, T., Cotterell, R., & Warstadt, A. (2024). Investigating critical period effects in language acquisition through neural language models (arXiv:2407.19325). https://arxiv.org/abs/2407.19325
9. van Dam, Karen. (2009). Employee adaptability to change at work: A multidimensional, resource-based framework. The Psychology of Organizational Change: Viewing Change from the Employee's Perspective. 123-142. 10.1017/CBO9781139096690.009.  
10. Caddick ZA, Fraundorf SH, Rottman BM, Nokes-Malach TJ. Cognitive perspectives on maintaining physicians' medical expertise: II. Acquiring, maintaining, and updating cognitive skills. Cogn Res Princ Implic. 2023 Jul 25;8(1):47. doi: 10.1186/s41235-023-00497-8. PMID: 37488460; PMCID: PMC10366061. 
11. Setiawan A, Sahar J, Santoso B, Mansyur M, Syamsir SB. Coping Mechanisms Utilized by Individuals With Drug Addiction in Overcoming Challenges During the Recovery Process: A Qualitative Meta-synthesis. J Prev Med Public Health. 2024 May;57(3):197-211. doi: 10.3961/jpmph.24.042. Epub 2024 May 3. PMID: 38726579; PMCID: PMC11164602. 
12. Fitzsimmons EE, Bardone-Cone AM. Differences in coping across stages of recovery from an eating disorder. Int J Eat Disord. 2010 Dec;43(8):689-93. doi: 10.1002/eat.20781. PMID: 19950115; PMCID: PMC2888938. 
13. Forester G, Johnson JS, Reilly EE, Lloyd EC, Johnson E, Schaefer LM. Back to the future: Progressing memory research in eating disorders. Int J Eat Disord. 2023 Nov;56(11):2032-2048. doi: 10.1002/eat.24045. Epub 2023 Aug 18. PMID: 37594119; PMCID: PMC10843822.