How we know
The foundational study is Nader, Schafe, and LeDoux 2000, published in Nature. They showed that recalling an already-consolidated long-term memory returns it to an unstable, labile state that has to be actively re-stored to survive. When they blocked protein synthesis in the amygdala during that window, the reactivated memory was erased. Retrieval is not a read-only operation. It reopens the file for editing.
That memory is reconstructive in the first place was established long before. Frederic Bartlett showed in 1932 that people recalling a story systematically reshaped its details to fit their own expectations, rebuilding it from fragments rather than replaying a stored recording. Elizabeth Loftus and John Palmer 1974 sharpened the point: changing a single word in a question, "smashed" instead of "hit", raised witnesses' speed estimates and led them to misremember broken glass that was never there. Post-event information rewrites the memory itself.
Combine the two and you get the loop that defines your favourite memories. Every recall rebuilds the scene from fragments plus your current mood, then saves that version back. The more often you revisit a moment, the more copy-of-a-copy drift it accumulates. You do not remember the day. You remember the last time you remembered it.
What it means
The unsettling part is confidence. Flashbulb memories, the vivid snapshots of where you were during a shocking event, feel permanent. Talarico and Rubin 2003 tracked memories of September 11 and found that vividness and certainty stayed high while the actual consistency of the recollection decayed at the same rate as ordinary memories. Hirst and colleagues confirmed the pattern over ten years. Feeling sure about a memory tells you nothing about whether it is accurate.
This is not a licence to distrust everything. The honest framing is that the gist is usually stable while the details drift. You did go to that birthday; the broad shape survives. What erodes is the fine detail: the exact words, who stood where, the colour of a shirt, how it felt. The memories you treasure most are the ones you have rewritten most. Love makes a memory less accurate, not more, and you can never go back and check, because there is no original copy left.