Why can't you remember being a baby?

Nobody has one settled answer, but the clues point to a brain still under construction and to memories that exist yet are hard to reach. Childhood amnesia, also called infantile amnesia, is the inability of adults to retrieve episodic memories, memories of situations or events, from before the age of three to four years. It is a puzzle rather than a blank, because babies clearly do remember.

Infants can remember the actions in a sequence, the objects used to produce them, and the order in which the actions unfold. These look like the precursors of autobiographical memory. The forgetting is not unique to us either: infantile amnesia has been observed in animals, so it cannot come down to something only people have, like reading and writing or an understanding of self.

One explanation lies in the brain's machinery. The hippocampus and the prefrontal cortex, two key structures for memory, do not develop into mature structures until around the age of three or four. The hippocampus also shows high levels of neurogenesis, the making of new neurons, in early childhood, tapering off into adulthood. It has been proposed that this neurogenesis degrades existing memories, and in mouse models, increasing neurogenesis increased forgetting.

Were those early memories erased, or just locked away? It is unknown. But in one experiment, researchers stimulated the neurons that had encoded a forgotten fear memory in infant mice, and the fear response returned for up to three months. That suggests a failure to access the memory rather than a failure to encode it. The memories of babyhood may be less deleted than lost on a shelf nobody can reach.