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Memories are a big part of our identities. Recalling our experiences helps us know who we are. Yet there are some events we can't recall even though they may have helped shape our lives. These are things that occurred in the first three or four years of life.

NPR's Jon Hamilton has this report on the phenomenon known as childhood amnesia.

JON HAMILTON, BYLINE: Francis Csedrik just turned 8 and lives in Washington, D.C. Like most kids, he remembers lots of important events in his life so far. There was the time he got a concussion.

FRANCIS CSEDRIK: I fell, head first, on a marble floor.

HAMILTON: The day he watched the family car get stolen.

FRANCIS CSEDRIK: And my dad had to chase it down the block.

HAMILTON: Then there was the morning he encountered an unexpected visitor.

FRANCIS CSEDRIK: A black bat sleeping right above our door.

JOANNE CSEDRIK: The bat - oh, you remember that.

HAMILTON: That's Francis's mom, Joanne Csedrik. She's been asking her son about things that happened to him when he was 4, or almost 4. Then she asks him about an earlier event that took place when he was just 3, a family trip to the Philippines.

JOANNE CSEDRIK: It was to celebrate someone's birthday.

FRANCIS CSEDRIK: Mm-mm.

JOANNE CSEDRIK: You don't remember? We took a long plane ride, two boat trips.

FRANCIS CSEDRIK: No, I don't remember.

HAMILTON: That's not surprising. Patricia Bauer, of Emory University, says by the time most kids are 7 or 8, they have started losing some of their early memories. And she says experiences will continue to disappear over the next few years.

PATRICIA BAUER: Most adults do not have memories of their lives for the first three to three and a half years.

HAMILTON: Bauer says scientists once thought childhood amnesia occurred because the brains of young children simply couldn't form lasting memories of events. Then in the 1980s, she and other researchers began testing the memories of children as young as 9 months old, using gestures and objects instead of words.

BAUER: And what we found was that even as young as the second year of life, children had very robust memories for these specific past events.

HAMILTON: And that raised a question: If children can remember what happened in their early years...

BAUER: Why is it that as adults we have difficulty remembering that period of our lives?

HAMILTON: Bauer says the evidence suggests that children are somehow losing access to their early memories. She wanted to know when this was happening. So she studied a group of children to see what happened to their memories over time. At age 3, she says, the kids were recorded speaking with a parent about recent events.

BAUER: They tended to be just everyday family activities - visiting an amusement park, a picnic, a visit from a relative.

HAMILTON: Then as the kids got older, Bauer and her colleagues checked to see how much they remembered. She says children as old as 7 could still recall most of the events.

BAUER: In contrast, the children who were 8 and 9 years of age, those children recalled fewer than 40 percent of the events. And so what we observed was actually the onset of childhood amnesia.

HAMILTON: Francis Csedrik, who just turned 8, is right at the age when many childhood memories are fading.

JOANNE CSEDRIK: Do you remember celebrating your grandfather's birthday in the Philippines?

FRANCIS CSEDRIK: Lolo Santo?

JOANNE CSEDRIK: No, not his birthday.

HAMILTON: Joanne Csedrik says her son has clear memories of two more recent trips to the Philippines. But the one when he was 3 is gone.

It's not entirely clear why early memories are so fragile. Bauer says it probably has to do with the structures and circuits in the brain that store events for future recall.

BAUER: The brain systems that are responsible for forming memories at the age of 3, 3 and a half, are still relatively immature. It doesn't mean they're not working at all; they certainly are. But they're not working as efficiently and therefore, not as effectively as they're going to be working in later childhood and certainly in adulthood.

HAMILTON: By the time children are 7 or so, their brains are forming memories that are as robust as those formed by adults.

Of course, some types of early memories are more likely than others to survive childhood amnesia. Carole Peterson, at Memorial University in Newfoundland, Canada, says one example is memories that carry a lot of emotion. She showed this in a study of children who'd been to a hospital emergency room when they were as young as 2.

CAROLE PETERSON: They had broken bones. They were lacerated, had to be stitched up. Things like that. So these were very emotional, very significant events. And what we have found is that even 10 years later, children have enormously good memory of them.

HAMILTON: Francis Csedrik certainly remembers the events that led to his emergency room visit.

FRANCIS CSEDRIK: My friend Asher, he said: I want to carry you down the stairs. I didn't want him to, but he didn't listen. He did it. And I fell, head first, on a marble floor.

JOANNE CSEDRIK: And what did they tell you at the hospital? What did they call it?

FRANCIS CSEDRIK: A concussion.

HAMILTON: That memory is from when Francis was 4. But Peterson says a child in one of her studies remembered an event from when he was just 18 months old. It was the day his mother went to the hospital to give birth to a sibling.

PETERSON: He remembers crying on the floor of the kitchen, and he remembers how upset he was. And he can remember the pattern of his teardrops on the linoleum.

HAMILTON: Findings like that are persuading courts to allow more eyewitness testimony from children. Peterson says even very young children can be good witnesses if they are questioned in a neutral way. She says another powerful determinant of whether an early memory sticks is whether a child fashions it into a good story, with a time and place and a coherent sequence of events.

PETERSON: Those are the kinds of memories that are going to last whereas memories like there was a flower growing up through a crack in the sidewalk, those are the ones that are more likely to be forgotten.

HAMILTON: Because they aren't part of a narrative and have no context. And Peterson says that's where parents can play a big role in what a child remembers. She says with some help, kids learn how to give shape and structure to their memories of an event.

PETERSON: Follow the child's lead. You add some more information, and then encourage them to add more. So essentially, what you're trying to do is co-construct a story.

HAMILTON: A story that won't fade away. That's something Joanne Csedrik and her son Francis have been doing ever since his concussion. He's even written about the events of that day.

FRANCIS CSEDRIK: I just like writing that story because I just don't want to forget it.

JOANNE CSEDRIK: Yeah, because it reminds you to be careful, right? You don't want to have that happen again.

FRANCIS CSEDRIK: Mm-mm. I think that's a day I'll always remember.

HAMILTON: It's not hard to see an evolutionary reason for this. Kids who recall stories about danger or injuries are probably more likely to survive to become adults. Peterson says remembered stories become important for a different reason in adolescence. She says that's when young people begin to create a larger autobiography from their early narratives.

PETERSON: When you start knitting them all together into a life story, you kind of put together a whole bunch of stories in order to explain why you are the kind of person you are.

HAMILTON: Researchers say a person's life story usually includes at least some events that had been lost to childhood amnesia. That's because when our own memories fail, we rely on family members, photo albums, and videos to fill in the blanks.

Jon Hamilton, NPR News.

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