Forgetting: Interference Theory

Forgetting is the inability to recall or recognise information that has previously been stored in memory. There are several explanations for why we forget. One of the most important is interference theory, which explains forgetting from long-term memory.

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What is Interference?

Interference theory proposes that forgetting occurs because memories disrupt or interfere with each other. This is most likely to happen when two (or more) memories are similar to each other. The more similar the memories, the more likely interference is to occur.

Think of it this way: if you have had many similar experiences, it becomes difficult to distinguish between them. For example, if you park your car in a similar car park every day, you may struggle to remember where you parked today because your memory for today's location is interfered with by memories of previous parking locations.

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Types of Interference

There are two types of interference:

flowchart LR
    A["Old memory<br/>e.g. old phone number"] -->|Proactive<br/>old disrupts new| B["New memory<br/>e.g. new phone number"]
    B -->|Retroactive<br/>new disrupts old| A
    C[Similarity of memories] --> D{More similar?}
    D -->|Yes| E[Greater interference]
    D -->|No| F[Less interference]

1. Proactive Interference (PI)

Proactive interference occurs when old (previously learned) information interferes with the ability to recall new information.

• The word "proactive" means acting forward — the old memory acts forward in time to disrupt the new memory
• Example: You have learned your old phone number so well that you keep accidentally reciting it instead of your new phone number

2. Retroactive Interference (RI)

Retroactive interference occurs when new information interferes with the ability to recall old (previously learned) information.

• The word "retroactive" means acting backward — the new memory acts backward in time to disrupt the old memory
• Example: After learning French vocabulary at school, you find it harder to remember the Spanish vocabulary you learned on holiday last year

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Summary of Interference Types

Type	Direction	Old vs New	Example
Proactive	Old disrupts new	Old → interferes with → New	Old phone number prevents recall of new one
Retroactive	New disrupts old	New → interferes with → Old	New teacher's name prevents recall of old teacher's name

Exam Tip: To remember the difference, think of the prefix: Pro = forward (old affects new), Retro = backward (new affects old). A very common exam mistake is to get these the wrong way round.

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Research Evidence

McGeoch and McDonald (1931)

McGeoch and McDonald investigated retroactive interference by asking participants to learn a list of words until they could recall them with 100% accuracy. They were then given a second list to learn. The second list varied in how similar it was to the first:

Second List Content	Recall of First List
Synonyms (similar meaning)	12% — worst recall
Antonyms (opposite meaning)	18%
Unrelated words	21%
Nonsense syllables	24%
Three-digit numbers	37%
No second list (control)	45% — best recall

Key finding: The more similar the second list was to the first, the worse the recall of the first list. This demonstrates that similarity between materials increases interference.

Underwood (1957)

Underwood investigated proactive interference. He reviewed studies where participants had to learn and recall word lists. He found that:

• Participants who had previously learned no other lists recalled about 70% of the words after 24 hours
• Participants who had previously learned 15 or more lists recalled only about 20% after 24 hours

This shows that the more previously learned material there is, the more it interferes with the recall of new material — demonstrating proactive interference.

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Evaluation of Interference Theory

Strengths

• Supported by considerable research evidence (e.g. McGeoch and McDonald, Underwood)
• The findings have been replicated many times, demonstrating reliability
• It has real-life applications — for example, it explains why students who study similar subjects (e.g. French and Spanish) may confuse vocabulary
• It provides a clear explanation of why forgetting increases when memories are similar

Weaknesses

• Most research uses artificial materials (e.g. word lists) in laboratory settings, which may lack ecological validity — people do not usually learn lists of words in everyday life
• Interference effects may be less significant in real life than in the lab, because real-life memories tend to be more distinctive and meaningful than word lists
• The theory does not explain all forgetting — some forgetting may be due to other factors such as retrieval failure, decay, or motivated forgetting
• It is difficult to determine whether interference causes permanent forgetting or just temporary difficulty in retrieval

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Interference and Everyday Life

Despite the criticisms, interference does appear to play a role in everyday forgetting:

• Mixing up similar passwords or PIN numbers
• Confusing the names of similar-looking people
• Struggling to remember where you parked when you always park in the same area
• Learning a new language and mixing up words from languages with similar vocabulary

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Key Points

• Interference theory explains forgetting from LTM as a result of similar memories disrupting each other.
• Proactive interference: old information disrupts new learning.
• Retroactive interference: new information disrupts old learning.
• Similarity between materials increases interference (McGeoch and McDonald, 1931).
• The more previously learned material, the greater the proactive interference (Underwood, 1957).
• Criticisms include low ecological validity of research and the theory not explaining all types of forgetting.

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Study case: McGeoch and McDonald (1931) — Similarity and Retroactive Interference

Aim: To test whether similarity between materials increases retroactive interference — i.e. whether new information that is similar to previously learned material disrupts recall of the original information more than dissimilar material does.

Procedure: Participants were asked to learn a list of 10 adjectives until they could recall it with 100% accuracy. After reaching criterion, participants were then given a second list to learn. There were six independent conditions, each using a different kind of second list: