Interference theory
We forget because other memories compete with and crowd out the target.
What it means
Interference theory attributes forgetting not to the erosion of traces but to competition among memories: proactive interference occurs when older learning disrupts newer learning, and retroactive interference when newer learning disrupts retrieval of older material. Similarity between the competing memories increases interference, which is why confusable items are especially error-prone. The account explains many forgetting phenomena that pure decay cannot, including the release from proactive interference when category shifts. It complements cue-dependent views, since interference is partly a problem of overloaded retrieval cues. It is one of the dominant explanations of forgetting in long-term memory.
Examples
After moving, you keep dialing your old address from memory — proactive interference from the well-learned former one.
A week of holiday Spanish leaves your school French coming out mangled — the newer words crowd out retrieval of the older ones.
You cannot recall where you parked today because a year of parking in the same garage has left dozens of near-identical memories competing for the same cue.
First described in McGeoch (1932); Underwood (1957).