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Theory that proposes that memory fades due to the mere passage of time
The Decay theory is a theory that proposes that memory fades due to the mere passage of time. Information is therefore less available for later retrieval as time passes and memory, as well as memory strength, wears away. When an individual learns something new, a neurochemical "memory trace" is created. However, over time this trace slowly disintegrates. Actively rehearsing information is believed to be a major factor counteracting this temporal decline. It is widely believed that neurons die off gradually as we age, yet some older memories can be stronger than most recent memories. Thus, decay theory mostly affects the short term memory system, meaning that older memories (in long term memory) are often more resistant to shocks or physical attacks on the brain. It is also thought that the passage of time alone cannot cause forgetting, and that decay theory must also take into account some processes that occur as more time passes. This simply states that if a person does not access and use the memory representation they have formed the memory trace will fade or decay over time. This theory was based on the early memory work by Hermann Ebbinghaus in the late 19th century. The decay theory proposed by Thorndike was heavily criticized by McGeoch and his interference theory. This led to the abandoning of the decay theory, until the late 1950s when studies by John Brown and the Petersons showed evidence of time based decay by filling the retention period by counting backwards in threes from a given number. This led to what is known as the Brown–Peterson paradigm. The theory was again challenged, this time a paper by Keppel and Underwood who attributed the findings to proactive interference. Studies in the 1970s by Reitman tried reviving the decay theory by accounting for certain confounds criticized by Keppel and Underwood. Roediger quickly found problems with these studies and their methods. Harris made an attempt to make a case for decay theory by using tones instead of word lists and his results are congruent making a case for decay theory. In addition, McKone used implicit memory tasks as opposed to explicit tasks to address the confound problems. They provided evidence for decay theory, however, the results also interacted with interference effects. One of the biggest criticisms of decay theory is that it cannot be explained as a mechanism and that is the direction that the research is headed.

Researchers disagree about whether memories fade as a function of the mere passage of time (as in decay theory) or as a function of interfering succeeding events (as in interference theory). Evidence tends to favor interference related decay over temporal decay,

Both theories are equally argued in working memory. One situation in which this shows considerable debate is within the complex span task of working memory, where a complex task is alternated with the encoding of to be remembered items. This theory gives more credit to the active rehearsal of information, as refreshing items to be remembered focuses attention back on the information to be remembered in order for it to be better processed and stored in memory. Working memory may decay in proportion to information or an event's salience. By broadening the view of this theory, it will become possible to account for the inconsistencies and problems that have been found with decay to date.

Another direction of future research is to tie decay theory to sound neurological evidence. As most current evidence for decay leaves room for alternate explanations, studies indicating a neural basis for the idea of decay will give the theory new solid support. Jonides et al. (2008) found neural evidence for decay in tests demonstrating a general decline in activation in posterior regions over a delay period. Though this decline was not found to be strongly related to performance, this evidence is a starting point in making these connections between decay and neural imaging. A model proposed to support decay with neurological evidence places importance on the firing patterns of neurons over time. The neuronal firing patterns that make up the target representation fall out of synchrony over time unless they are reset. The process of resetting the firing patterns can be looked at as rehearsal, and in absence of rehearsal, forgetting occurs. This proposed model needs to be tested further to gain support, and bring firm neurological evidence to the decay theory.