There are two phases in research on forgetting. During the first phase, lasting until the first decade of the 21st century, forgetting was largely regarded as a shortcoming of memory, as a malfunction of actual memory processes, such as memory formation or retrieval. There were some notable exceptions to this view, for example the idea prominently formulated by Freud that some forgetting results from targeted repression, or inhibition, of memory. In the laboratory, this form of forgetting has been studied with intentional forgetting tasks, or directed forgetting, where human participants are instructed to forget some of the material presented to them, while remembering others. Outside this context, however, forgetting was not understood as a true function of the brain, as the result of innate, or constitutive, processes that are part of normal brain function.

This view changed during the second phase, marked by discoveries of mechanisms in the brain dedicated to erase memories. This was observed relatively independent of each other in invertebrate and vertebrate animals. In fruit flies, it was found that certain neurons use dopamine signalling to eliminate memories, so that increasing the activity of these neurons accelerated forgetting, while blocking their activity prevented forgetting. About the same time it was found in rodents that the natural loss of memory over days can be prevented by interrupting the removal of parts of neurons required for neurons to communicate with each other (receptors). Notably, this form of removal requires a signal initiating it, and it was later found that other receptors may provide this signal. Just as in the fruit fly, increasing the activity of these receptors sped up natural forgetting, while blocking their activity preserved memory from obliteration. The conclusion from these findings is that certain forms of forgetting are not the result of failures of other memory mechanisms, but the outcome of dedicated processes, which are now collectively called active forgetting processes, to distinguish them from the view that dominated the first phase of research into forgetting.

One of the first classifications of forgetting stems from the Renaissance scholar Juan Luis Vives (1493-1540), who is regarded as the “Father of Psychology”. He identified three possible causes why memory may fail us. First, a storage failure could lead to a true loss of memory, such that the storage process might not have concluded correctly, or stored memory might have been erased. Second, memory fails us because we cannot retrieve a memory that exists. Third, something might have smeared up a memory, such that it gets partially broken up and corrupted. Currently, these are still the main causes for forgetting, although the terminology has changed as well as the understanding of how these forms of memory loss arise.

The fundamental difference of the contemporary perspective on forgetting is the notion that dedicated forgetting processes exist that are necessary for normal brain function. Forgetting is therefore understood as a constitutive part of the brain, not a deplorable shortcoming or limitation, motivating the search for forgetting processes and efforts to understand their neurobiological mechanisms and contributions to memory.