Decay theory

The  Decay theory  states that when something new is learned, a neurochemical "memory trace" is formed, but over time this trace tends to disintegrate.

Decay theory suggests that the passage of time always increases forgetting. However there is one circumstance where old memories can be stronger than more recent ones. Older memories are sometimes more resistant to shocks or physical assaults on the brain than recent memories.

The decay theory along with the interference theory are two suggested reasons why people forget. Decay alone, although it may play some role, cannot entirely explain lapses in long-term memory. A trace is formed by sensory neurones -- this trace is a memory. When a person forgets, the trace is lost.

Another theory of forgetting in short-term memory, or STM, is the Displacement Theory which suggests that new memory traces displace or erase old ones.