My notes list decay theory and interference theory as two explanations of forgetting, then move on without saying which one the evidence supports.
Are they competing explanations, and is there a way to tell them apart experimentally?
Accepted Answer
They are separable in principle, and the classic design that separates them is sleep.
If forgetting were pure decay, time alone would do it, and an hour asleep should cost as much as an hour awake. It does not. Material learned before sleep is retained better than material followed by an equal period of ordinary activity.
That points at interference doing much of the work, because sleep removes the new input without removing the time.
The current picture is that both operate, plus retrieval failure, which is a third thing: the trace is intact but the cue is missing. Tip of the tongue is the everyday version.
One thing to be careful of when reading about the sleep result.
Sleep does more than remove interference. There is good evidence for active consolidation during sleep, especially slow wave sleep for declarative memory. So better retention after sleep is not clean evidence against decay; it has a second explanation built in.
That is the recurring difficulty in this area. The two theories are not easy to separate cleanly because almost every manipulation of time also changes what else happens during it.
Interference also splits in two, and the names are worth keeping straight. Proactive is old material blocking new, retroactive is new material blocking old. Learning a second language interferes in both directions.
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