Memory of what happened

     A aspect of memory believed to depend on the prefrontal cortex is what’s sometimes called autobiographical, or episodic, memory-the kind of memory that lets you consciously recall where you were and what you were doing while you were, say, on your last vacation. If a four-year-old  sees you put a marble in a box, or hears you tell him you’ve put a marble in a box, he’ll remember not just that there’s a marble in the box but also how he knows – whether he saw it or was told. A three-year-old, on the other hand, may remember there’s a marble in the box but not the circumstances of learning it, just as you probably don’t remember how or when you learned that Paris is the capital of France. This kind of memory, called source memory, doesn’t develop until about age four.

     That’s why a three-year-old isn’t lying if he says he saw Rudolf on the roof. He won’t remember whether he actually saw something, or heard it in a story, or even just imagined it. All he’ll remember is the fact of having gotten the idea in his head. It’s no coincidence that it’s also not until about age four that a child can explicitly relate the past to the present to understand, say, that the reason the cat ran away is that he left the door open.