There is sometimes difficulty in determining what is actually an occurrence of forgetting and what is simply an inability to retrieve information. However, there are several theories with regard to how forgetting occurs.
Decay Theory-This theory asserts that a memory trace is created every time some new experience occurs. This theory believes that over time these traces fade and disappear. This makes it difficult to access and use the trace which eventually makes the memory disappear.
Interference Theory-this theory believes that memories are made, and memories with very similar characteristics compete against each other. Proactive Interference makes it difficult to remember new memories. Retroactive Interference interferes with your ability to remember old memories.
Failure to Store-This happens when encoding fails, and those things that are meant to be stored in long-term memory are never stored.
Motivated Forgetting-This happens when an individual actively works to forget and eliminate traces of memories that are traumatic. Rape, abuse, incest and other traumas would be subject to motivated forgetting.