There is plenty of photographic evidence that you were once a tiny, onesie-sporting newborn with a penchant for putting everything except food in your mouth. Yet as hard as you try, you can't nail down the faintest memory of those years.
Scientists recently came across a physical mechanism to explain our baby amnesia: When new cells sprout in young brains, they crowd out the circuits where memories are formed.
So the reason you remember your best friend's wedding day but can't seem to recall the time you decorated your hair with mashed potatoes is because making new memories destroys the older snapshots.
When researchers experimented with mice, they found that when they slowed down their ability to make new brain cells, they helped them craft more solid memories. Conversely, when they sped up their brain-cell-generation, the critters had a harder time remembering. In other words, more new brain cells meant fuzzier memories, and less new brain cells meant clearer ones.
It sounds counter-intuitive, but here's why it makes sense: We can't remember everything because our ability to store memories is limited. "Some kind of forgetting is important for memory. There's finite capacity," Paul Frankland, a neurobiologist at the Hospital for Sick Children in Toronto who led the study, told Vox. "You want to get rid of all the junk, and you want to remember the important features and important events."
Frankland's findings are published in the journal Science.
Here's how they made the recent discovery: After instilling memories in rodents by creating an association between a place and a mild electric shock, they played with their brain cell regeneration rates, to see how forming new cells would affect their ability to remember.
Why it matters: Our strange inability to remember being a child has puzzled scientists, psychologists and philosophers for decades. Freud, one of the first to write about the phenomenon, thought we repressed our early memories because they were so heavily charged with psychosexual content that we wouldn't be able to handle them — even as adults. Others have said we can't remember our early life because we couldn't speak at that age (words help us encode some types of memories), or because we didn't yet have a sense of self.
But after discovering that other animals are also missing memories of their infant lives, scientists realized there had to be another mechanism that was causing us to forget.
The recent research is a step towards a fuller understanding of how and why we remember what we do. Given that our capacity for remembering is finite, perhaps some memories are more important than others. Would you trade your memory of the look on your partner's face when you propose for a snapshot of spitting up on someone's shoulder? I would not.