The Kennedy assassination. The Challenger explosion. The Sept. 11 attacks on the World Trade Center.
Nearly anyone who was alive at the time of these tragedies would be able to provide a vivid recollection of the event. As for more commonplace emotional memories — a wedding, the loss of a loved one — some people seem more inclined toward this type of recall than others.
Now, according to new research, there's a genetic reason why some have the gift or curse of retaining emotional memories.
The research, published Sunday in the journal Nature Neuroscience, found that those with a certain common genetic variation tend to more readily remember emotionally charged events, for better or for worse.
There are specific chemicals in the brain that have the effect of covering up old emotional memories — like a photo that fades with time. But for those with this particular genetic variation, these "images" are preserved, resulting in enhanced emotional memory.
About a third of Caucasians and 12 percent of African-Americans possess the variant.
"This mechanism plays an important role in remembering dangerous situations, as well as happy ones," said lead study author Dr. Dominique de Quervain, a researcher in the psychiatric division at the University of Zurich.
"For example, remembering where along a street an accident happened is important, so it doesn't happen again."
The researchers showed 435 young Swiss adults 30 pictures — 10 that were neutral, 10 that had a positive emotional connotation and 10 with a negative emotional meaning. The team asked the subjects to rate their reaction to each photograph and, after 10 minutes, to describe each picture in a few written words.
The participants who carried the gene variant had a much better recall of both the negative and positive pictures, but there was no difference between the groups when it came to remembering the neutral pictures.
"So what we found was that this genetic variant related to enhanced emotional memory had nothing to do with normal memory," said de Quervain.
To understand the gene variant in other populations, the researchers tested Rwandan civil war refugees and found similar results; those who carried the gene variant had an easier time recalling both the positive and the negative pictures.
Brain researchers said that there is a reason why humans have evolved such a mechanism to remember certain events, but not others.
"We can't remember everything," said Dr. Jeff Victoroff, associate professor of clinical neurology and psychiatry at the University of Southern California Keck School of Medicine.
"As the human brain evolved, we needed a good system that would ensure that we remember those things that are truly important for survival and reproduction," he said. "Virtually everything else is optional."
Victoroff agreed with the study's authors that emotional memory in particular serves two purposes: "The immediate benefit of getting us revved up for fight or flight, and the long-term benefit of helping us remember dangerous circumstances so that we can judge whether the gains of putting ourselves through the same situation again are likely to be worth the risk."
Humans who were adept at utilizing these tricks were the ones who survived and became our ancestors.
Those who had brains that didn't separate the really important memories from the forgettable ones were more likely to die off, according to Victoroff.