Why a failure to remember everything is a feature, not a bug, of memory systems.
First, the bad news: Most of us do not remember everything. But there’s a silver lining: Forgetting is a feature, not a bug, of our memory systems.
So how do our brains sort out what information we remember and what we don't?
Time, repetition, and emotion all contribute to whether we remember information or an event, and the basic rule is that events and information with significance are remembered. Remembering recent events is likely to be more relevant for functioning than most things from long ago: You want to remember where you parked your car this morning, but remembering where you parked last Wednesday can be counterproductive for finding your car today.
Events and information that are repeated over and over are likely to be more important than one-off events—I’m sure that you can all find your way from your middle school or high school back to the house you lived in at the time, even though for many of us, that was a long time ago. And yes, this is why studying (repetition) works. We also easily remember events with high emotional significance—it’s why we remember “where we were” when we heard about major, devastating public events; or the specific table in the restaurant where you got engaged—even if you have visited the restaurant enough to have sat in every seat at some time. The emotion related to the experience strengthens the formation of that memory and causes an indelible and easily retrieved memory that lasts long into the future.
Why is this selectivity in memory important? Why not just remember everything? There are a few different possibilities for why we might not remember everything. One is simply a question of capacity: Do we have enough synapses in the brain to store all the information from every day? We don't know.
Even if we do have the storage capacity, storing all memories is inefficient. If the purpose of memory is to guide behavior and allow survival, then a lot of what we experience every day will be largely irrelevant to our ability to function in the future. But some events will be critical for survival. Recency, repetition, and emotion all indicate how important this information is likely to be in our current environment.
For example, if you know where to find delicious food (maybe a bakery that makes particularly good chocolate croissants in your town), you are more likely to remember that place than any of the other bakeries in town. If you are mugged one day, then you are likely to remember both the mugging itself and details about the place than other places you visited as often. You are also likely to have a fear response to that place, to avoid it, and remember the mugging and feel fear in places resembling that spot. A mugging, and maybe even an exceptional croissant, are intense, complex experiences with many different factors, including previous experience, influencing why this specific event is to be remembered.
In the laboratory, stories and images that trigger emotion are remembered in more detail and for longer than similar stories or images without emotional connotations (Cahill & McGaugh, 1995). This is also true for non-human animals—places in which a stressful event is presented are remembered faster and for longer than an equivalent environment without a stressor. So emotion contributes to not only the content of the memory but also whether something is remembered at all.
If it’s adaptive to be selective in what we remember, then are people who have “better memories” somehow at a disadvantage? Individuals with “highly superior autobiographical memory” (HSAM) (Parker et al., 2006; LePort et al., 2012), remember almost everything about events from their lives—yet they lead normal lives, exhibit normal IQ, and are indistinguishable from others across a range of other cognitive functions.
Why? This may be because HSAM individuals do not show the same kinds of extreme memory for all kinds of memory. In addition to autobiographical memory (conscious memories about your own experiences), many other kinds of memories are critical for modifying our behavior, including memories for facts (semantic memory) and unconscious memories. including habits (driving to your old house while distracted) or associative conditioning (why we avoid eating mushrooms if we got sick after eating them as a kid, even if we don’t consciously remember the incident).
It may also be that because HSAM individuals remember not only what happened, but also when events happened, they are less likely to have interference between the memory of where the car was parked yesterday versus where it is today. But we really don’t know. I, for one, can imagine problems arising from remembering every detail—every breakfast, every argument, every political event I read about—every day of my life.
What if the brain gets it wrong? Of course, if the brain can do something, it can also go wrong, and this is no exception: How the brain codes for “important” information can be hacked or disrupted by our experiences. For example, drugs are widely thought to hijack the brain’s system for the kinds of pleasure and reward that is more usually derived from food or sex; in this way, they also cause stronger memories for the people, places, and things associated with the drug, which in turn contributes to drug-taking and relapse to drug-taking in addiction. And strong emotions during trauma contribute to strong, long-lasting, and intrusive memories that are one component of post-traumatic stress disorder.
And now the good news: For most of us, failing to store memory, or forgetting over time (a topic for another day) are not because we have “bad memory”; rather this selectivity of what we remember allows is a filter for the massive quantities of information we encounter every day. By selecting significant information based on emotional arousal, among other things, means that we sometimes “forget” information we wish we could recall, but more importantly, it means we can efficiently use the information we retain to function—and survive—in our current environment.
References
Cahill, L., & McGaugh, J. L. (1995). A novel demonstration of enhanced memory associated with emotional arousal. Consciousness and Cognition, 4:410–421.
LePort AK, Mattfeld AT, Dickinson-Anson H, Fallon JH, Stark CEL, Kruggel F, Cahill L, McGaugh JL (2012) Behavioral and neuroanatomical investigation of Highly Superior Autobiographical Memory (HSAM). Neurobiology of Learning & Memory, 98, 78‐92.
Parker ES, Cahill L, McGaugh JL. (2006) A case of unusual autobiographical remembering. Neurocase, 12(1):35–49.