The Evolutionary Benefits of Forgetfulness

Forgetfulness is part of people's everyday lives. A person can walk into a room and forget why they went in there, or perhaps someone they know says hello to them on the street and they can't remember what that person's name is.

But why do people forget? Is it simply a sign of memory impairment, or is there some benefit to it?

One of the earliest discoveries in the field suggests that forgetting may occur simply because the average person's memory is gradually erased. It belongs to the 19th-century German psychologist Hermann Ebbinghaus, whose "forgetting curve" showed that most people forget details of new information fairly quickly, but that the process slows down over time.

More recently, this has been confirmed by neuroscientists. However, forgetting can also serve a functional purpose. The brain is constantly bombarded with information. If people memorized every detail, it would be increasingly difficult for them to retain important information.

Nobel Prize winner Eric Kandel and a host of subsequent studies suggest that memories are formed when connections (synapses) between brain cells (neurons) are strengthened.

Attention to something can strengthen these connections and retain the memory. This same mechanism allows a person to forget all the unimportant details they encounter every day. So although people become increasingly distracted as they age, and memory disorders such as Alzheimer's disease are associated with a decline in attention, everyone still needs to be able to forget unimportant details in order to create memories.

Recall can sometimes change to cope with new information. Suppose a person takes the same route to work every day. He has probably memorized this route firmly, and the connections underlying it are strengthened with each trip.

But suppose that one Monday one of the familiar roads closes and he has to take a new route for the next three weeks. His memory of the trip must be flexible enough to accommodate this new information. To do this, the brain weakens some memory connections and at the same time strengthens new connections to remember the new route.

Obviously, failure to update a memory will lead to significant negative consequences. As an example, post-traumatic stress disorder (PTSD), in which the inability to update or forget a traumatic memory results in a person experiencing constant reminders of it in their environment.

From the perspective of evolution, forgetting old memories in response to new information is certainly beneficial. Human ancestors, hunter-gatherers, may have repeatedly visited safe water sources, only to discover one day a rival settlement or a bear with cubs. Their brains would have had to be able to refresh their memory to label the place as no longer safe. Failure to do so would have been a threat to their survival.

Sometimes forgetting may not be due to memory loss, but to changes in a person's ability to access memories. Studies in rodents have shown how forgotten memories can be recovered (or reactivated) by maintaining synaptic connections.

Rodents were taught to associate something neutral (e.g., the ringing of a bell) with something unpleasant (e.g., a light tap on the leg). After several repetitions, the rodents developed a "fear memory" when they heard the bell ring and reacted as if they were expecting an electric shock. The researchers were able to identify the neural connections that were activated by the juxtaposition of the call and the electric shock in a part of the brain known as the amygdala.

They then wondered whether artificially activating these neurons would make the rodents behave as if they were expecting an electric shock, even if there was no call or shock. To do this, they used the method of optogenetic stimulation, which involves the use of light and genetic engineering, and showed that activating (and subsequently inactivating) such memories is indeed possible.

One way in which this can be relevant to people is through a type of transient forgetting that may not involve memory loss. An example already mentioned is when a person sees someone on the street and cannot remember their name: they may think they know the first letter and will remember the name in a moment, but it doesn't happen. This is known as the "on the tip of the tongue" phenomenon.

When American psychologists Roger Brown and David McNeill first studied this phenomenon in the 1960s, they reported that people's ability to identify aspects of a missing word was greater than chance. This suggests that the information was not completely forgotten.

However, another option is also possible: this phenomenon may serve as a signal to the person that the information is not forgotten, but only inaccessible at the moment.

This may explain why people become more aware as they age, which means their brains have to go through more information to remember something. The "tip of the tongue" phenomenon may be the brain's way of letting them know that the information they need is not forgotten and that persistence can lead to successful memorization.

In general, people can forget information for a variety of reasons. Because they weren't paying attention or because information gets erased over time. A person may forget in order to refresh memories. And sometimes forgotten information is not lost permanently, but simply becomes inaccessible. All of these forms of forgetting help the brain function efficiently and contribute to survival over generations.

This is certainly not to downplay the negative consequences of people becoming very forgetful (e.g., due to Alzheimer's disease or dementia). Nevertheless, forgetfulness has its evolutionary advantages.