If our brains are so active and developing during infancy, why don’t we remember anything from that time?Fabian van den Berg:
Ah, infantile amnesia as it’s better known. Weird, isn’t it? It’s a pretty universal phenomenon where people tend to have no memories before the age of four-ish and very few memories of the ages five to seven. What you say in the question is true, our brains are indeed very actively developing in that time, but they are still developing after five years as well.
The specifics aren’t known just yet. It’s tricky because memory itself is very complicated and there are swaths of unknowns that make it difficult to say for certain why we forget these early memories. This will be mostly about consensus and what can be supported with experiments.
(Image based on data from Rubin & Schulkind, 1997  )
I’ll skip the whole introduction to memory bit and state that we focus on the episodic/autobiographical memories only—events that happened to us in a certain place at a certain time. And we have two forgetting phases, the early one until about four years old, and a later one from about five to seven years old, where we have very few memories.
The first notion to go is that this is “just normal forgetting,” where it’s just difficult to remember something from that long ago. This has been tested and it was found that forgetting happens quite predictably, and that the early years show less memories than they should if it was just regular old forgetting.
This leaves us with infantile amnesia, where there are probably two large camps of explanations: One says that children simply lack the ability to remember and that we don’t have these memories because the ability to make them doesn’t develop until later. This is the late emergence of autobiographical memory category.
The second big camp is the disappearance of early memory category, which says that the memories are still there, but cannot be accessed. This is also where the language aspect plays a part, where language changes the way memories are encoded, making the more visual memories incompatible with the adult system.
Both of them are sort of right and sort of wrong; the reality likely lies somewhere in between. Children do have memories, we know they do, so it’s not like they cannot form new memories. It’s also not likely that the memories are still there, just inaccessible.
Children do remember differently. When adults recall, there is a who, what, where, when, why, and how. Kids can remember all of these too, but not as well as adults can. Some memories might only contain a who and when (M1), some might have a how,
where, and when (M3), but very few, if any, memories have all the elements. These elements are also not as tightly connected and elaborated.
Kids need to learn this; they need to learn what is important [and] how to build a narrative. Try talking to a child about their day: It will be very scripted [and] filled with meaningless details. They tell you about waking up, eating breakfast, going to school, coming home from school, etc. Almost instinctively an adult will start guiding the story, asking things like, “Who was there?" or "What did we do?”
It also helps quite a bit to be aware of your own self, something that doesn’t develop until about 18 months (give or take a few). Making an autobiographical memory is a bit easier if you can center it around yourself.
(Image from Bauer (2015) based on the Complementary Process Account  )
This method of forming memories makes for weak memories, random spots of memories that are barely linked and sort of incomplete (lacking all the elements). Language acquisition can’t account for all that. Ever met a three-year old? They can talk your ears off! So they definitely have language. Children make weak memories, but that doesn’t completely tell you why those memories disappear, but I’ll get there.
The brain is still growing, very plastic, and things are going on that would amaze you. Large structures in the brain are still specifying and changing, the memory systems are part of that change. There’s a lot of biology involved and I’ll spare you all the science-y sounding brain structures. The best way to see a memory is as a skeleton of elements, stored in a sort of web.
When you remember something, one of the elements is activated (which can be by seeing something, smelling something, or any kind of stimulus), which travels through the web activating all the other elements. Once they are all activated, the memory can be built, the blanks are filled in, and we “remember."
This is all well and good in adults, but as you can imagine this requires an intact web. The weak childhood memories barely hung together as they were, and time is not generous to them. Biological changes can break the weak memories apart, leaving only small isolated elements that can no longer form a memory. New neurons are formed in the hippocampus, squeezing in between existing memories, breaking the pattern. New strategies, new knowledge, new skills—they all interfere with what and how we remember things. And all of that is happening very fast in the first years of our lives.
We forget because inefficient memories are created by inefficient cognitive systems, trying to be stored by inefficient structures. Early memories are weak, but strong enough to survive some time. This is why children can still remember. Ask a four-year-old about something important that happened last year and chances are they will have a memory of it. Eventually the memories will decay over the long term, much faster than normal forgetting, resulting in infantile amnesia when the brain matures.
It’s not that children cannot make memories, and it’s not that the memories are inaccessible. It’s a little bit of both, where the brain grows and changes the way it stores and retrieves memories, and where old memories decay faster due to biological changes.
All that plasticity, all that development, is part of why you forget. Which makes you wonder what might happen if we reactivate neurogenesis and allow the brain to be that plastic in adults, huh? Might heal brain damage, with permanent amnesia as a side-effect ... who knows!
 Rubin, D. C., & Schulkind, M. D. (1997). Distribution of important and word-cued autobiographical memories in 20-, 35-, and 70-year-old adults. Psychol Aging.
 Bauer, P. J. (2015). A complementary processes account of the development of childhood amnesia and a personal past. Psychological review, 122(2), 204.
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