A recent study shows that to persist, memories need to be encoded into language. The result: kids aren't able to remember things that predate their ability to put it into language.
[update: at 11:43PM one night, two months after his daughter is born, Matt Haughey puts it in terms of our brain's programming language:
"Imagine we have no media and everything's in RAM, as it happens.
10 PRINT "CRY LOUDLY"
20 GOTO 10
A few months into life and we move on to encoding everything in MS-DOS..."]
The results make so much sense the first time you hear them that you wonder why this took so long to get out...
Still, I always assumed that the heavy drinking I did in university had something to do with the loss of my childhood memories... :)
And yet, I don't remember anything prior to 4 years old, and I believe I was speaking before then. Am I just particularly dense. And no, I haven't RTFA yet. Just saying that it might not be the only reason.
I realized recently that some of my earliest "memories" are actually memories of me being told about things that happened to me a few years later.
I suspect video footage and photographs of the first four years strongly shape later memories, too. So, be sure you don't overdo the crying, cranky moments.
I read an article about a mom taking her four year old back to a hotel where he had been before he could talk. He pointed out things to her he remembered from the first visit and pointed out things they would encounter around the corner before he saw them, like chairs or flowers. The idea here was we do remember, it just gets pushed way back in our memories as we pile up life experience.