Of course everyone remembers precisely where they were when they heard about 9/11 - what they were doing, who told them, how they felt - but imagine you had been the president of the United States. That singular moment would have burned into your memory like acid etching steel. It would never fade.

"I was in Florida," President George W. Bush recalled on Dec. 4, 2001, a mere 11 weeks after the attacks. "And my chief of staff, Andy Card - actually I was in a classroom talking about a reading program that works. And I was sitting outside the classroom waiting to go in, and I saw an airplane hit the tower - the TV was obviously on, and I used to fly myself, and I said, 'That's one terrible pilot.' And I said 'It must have been a horrible accident.' But I was whisked off there - I didn't have much time to think about it, and I was sitting in the classroom, and Andy Card, my chief who was sitting over here walked in and said, 'A second plane has hit the tower. America's under attack.'"

On Dec. 20, 2001, Bush told a somewhat different story to Dan Balz and Bob Woodward of the Washington Post. "Bush remembers senior adviser Karl Rove bringing him the news, saying it appeared to be an accident involving a small, twin-engine plane. ... 'This is pilot error,' the president recalled saying. ... At 9: 05 am, United Airlines Flight 175, also a Boeing 767, smashed into the south tower of the trade center. Bush was seated on a stool in the classroom when Card whispered the news: 'A second plane hit the second tower. America is under attack.'"

On Jan. 5, 2002, someone in an audience asked Bush about how he reacted to the news. "I was sitting there, and my Chief of Staff - well, first of all, when we walked into the classroom, I had seen this plane fly into the first building. There was a TV set on. And you know, I thought that was pilot error and I was amazed that anybody could make such a terrible mistake. And something was wrong with the plane, or - anyway, I'm sitting there, listening to the briefing, and Andy Card came in and said 'America is under attack'."

Read those statements carefully and you'll spot significant inconsistencies. And recall that on the morning of the attack there were no images of the first plane striking the World Trade Center.

Now, put those two facts together. Conclusion? For many, it was obvious: The president was lying. No other explanation fit. Everyone remembers exactly how they heard the news, after all. Even a decade later, it is unforgettable. And this is the president recalling 9/11 mere months later! Oh, yes. He lied.

But what was he hiding? Cue the conspiracy theory.

"Fortunately, students of memory can offer a more benign hypothesis," wrote psychologist Daniel L. Greenberg in a 2004 examination of Bush's recollections. "We need only consider the frailty of human memory."

Memories fade. Everyone knows that. Memories can also get jumbled and distorted and we sometimes remember things that didn't happen. Everyone knows that, too. But we also believe that memories of sudden, emotional shocks - like where you were when you heard about 9/11- are different than others. They're more vivid and resonant. They don't get mixed up. They don't fade. They are fixed forever.

That's not even close to true.

Modern memory research really began in the 1970s. In 1986, when the space shuttle Challenger exploded, killing everyone on board, psychologists realized they had an opportunity to test flashbulb memory. They quickly rounded up students and had them write down the circumstances under which they had heard the news. Later, they went back to the same students and asked them to again describe how they heard about the tragedy. The researchers found the second recounting routinely deviated from the first.

The Challenger study became famous and others like it were conducted, so when 9/11 happened many psychologists immediately rounded up people and asked them to put their memories on paper. The result is irrefutable proof that flashbulb memories are not remotely as reliable as we think they are.

The biggest research effort is a consortium that interviewed 3,000 people in seven American cities. Several follow-ups have been conducted, including one just last month.

A year after the event, the overall consistency level was 63 per cent. Three years out, it was 57 per cent. People's recollections of how they felt were even less reliable. A year out, they were only 40-per-cent reliable.

Of course this doesn't mean their memories were entirely false. As New York University psychologist Elizabeth Phelps explained to Scientific American, "When we talk about 'accuracy' here, we mean accuracy for details like, 'How did find you find out about 9/11,' or 'Who were you with?' It's not the case that you don't have a fairly vivid image in your head of the planes crashing into the building. No one's forgetting 9/11 occurred."

Taking a different angle, Jennifer Talarico and David Rubin, of Duke University, asked students for their personal recollections of 9/11 the day after the attack and then tested them again either one, six, or 32 weeks later. At the same time, they asked them to recount ordinary memories of personal activities.

Not surprisingly, they found that the accuracy of the mundane memories declined rapidly. But the same was true of the flashbulb memories of 9/11. The only difference was how people felt about their memories: They consistently said their flashbulb memories were more vivid and more accurate than their ordinary recollections. "Flashbulb memories are not special in their accuracy," the researchers concluded, "but only in their perceived accuracy."

Some psychologists object to that conclusion, at least to some degree. But everyone who works in the field agrees our confidence in the accuracy of flashbulb memories is grossly inflated. And it's very hard to shake. In fact, when researchers show people the hard evidence that proves their memories have shifted, they often flatout reject it. "I remember it just the way it happened," they say. "I don't care what I wrote."

Granted, it doesn't really matter whether it was your neighbour, and not your wife, who told you a plane had crashed into the World Trade Center. But sometimes our faith in flashbulb memories can have real consequences. Like the witness whose testimony is dismissed because his recollections changed slightly. Or the president whose ordinary memory missteps fed suspicions of conspiracy.

Memories change. Remember that.