It's not unusual for siblings to quibble over ownership of something – a cherished toy, a coveted seat in the car, the last ice cream sandwich. As they were growing up in Quebec, Canada, Mercedes Sheen and her identical twin sister used to quarrel over who owned memories.

"I think the best example is my first kiss – what I perceived to be my first kiss," says Mercedes Sheen, now a professor of psychology at Heriot-Watt University in Dubai.

She remembers all the details: Summer camp in New Brunswick, the day spent canoeing, the walk back up the hill.

"And this guy called Jeff Levitt, who was the camp catch, the most gorgeous guy at camp, he pulled me aside and pulled me into a bush and kissed me on the lips, very briefly," she says.

It made a big impression. Then months later, when she brought up the story with Michaela, her twin insisted that it had been her whom Jeff had smooched, not Mercedes.

"We both felt that it was 100% us, when the event could have only happened to one of us," she says.

The sisters would run into this situation fairly often: They would remember the same experience but disagree on which twin it had happened to. Sheen says sometimes the disputes felt personal.

"Our memories tie us to our personal past, our 'this-is-us,'" she says. "My thesis was called 'Stealing the Past' because it really feels like someone's taking your history from you. ... One could say that I took my arguments with my twin to a great extent by doing a PhD on it."

Disputing ownership of a memory

As she pursued studies in psychology, Mercedes Sheen was still thinking about those questions of twins and memory.

"It's kind of bizarre to think that you have such a strong memory, and have all those visual properties, the sounds and smells, and then to think that it didn't happen," she says. "It kind of makes you think, 'Well, wow, so, what is real?'"

Sheen was considering various dissertation topics at Canterbury University in Christchurch, New Zealand, when she had lunch with a visiting professor – David Rubin, a Duke University neuroscientist and a noted expert on memory.

The fact that Sheen is an identical twin came up in conversation, and by the end of the lunch the two had plans to launch a research project together: They would test whether disputed ownership of memories might be a common trait in twins.

Working with the local multiple births society in Christchurch, Sheen recruited dozens of volunteers – a task she said was relatively easy to do at the time. "There was a big increase in women doing IVF in the early eighties, and they ended up with an influx of twins. So I had these beautiful participants that were there," she says.

Next, she asked them in various ways: Do you have any memories where you disagree on whose it is?

"I used 20 or 30 cue words," Sheen says, "words that would cue everyday experiences, like birthday, McDonald's, road trip, injury, things like that. And just by asking them both to come up with a memory in response to those cue words, they just happened spontaneously."

Once they found a disputed memory the researchers would ask a series of detailed questions: What do you remember seeing? What do you remember hearing? Do you see the memory from your own point of view or an observer's?

The researchers found that in most cases of disputed memories, based on the supporting details and their level of confidence, both twins were equally credible – even though the event could have only happened to one of them. In fact, each twin tended to remember a disputed memory with more certainty than they did with memories they agreed on.

It all felt very familiar to Mercedes Sheen – as did the study subjects' resulting squabbles.

"They all had the same types of arguments that I had with my twin – like, 'You always do this! You steal my memories,'" she says.

Follow-up experiments would show that in addition to identical twins, fraternal twins also experience this effect to a lesser extent, followed by non-twin same-sex siblings.

A reconstruction, not a recording

Sheen says another finding was that disputed memories tend to be self-aggrandizing. As in her tale of the contested first kiss, they tend to show the person remembering in a positive light or as the main character.

"Our memories are selective, you know," says Sheen. "Our memories are not a file that we pick out from our brain. They are reconstructions."

Experts say creating those reconstructions is a complicated, even messy neurological process.

"My dad used to say to me, if you've got a machine with many moving parts, like a car or whatever, there's just so many more ways it can go wrong. And memory is one of those machines with many moving parts," says Charles Fernyhough, professor of psychology at Durham University in the United Kingdom.

In his book Pieces of Light, Fernyhough explains that a memory is made up of different kinds of information: the facts of what actually happened, the subjective, sensory information connected to it, "semantic knowledge" of how the world generally works. When the brain remembers, Fernyhough says, it must recombine all that data.

"It takes those different kinds of information spread across all those different bits of the brain and it puts them together right here, right now, when you're being asked to remember. It reconstructs a version of the past according to the demands of the present," he says.

Each time the brain reconstructs a memory, it creates opportunities for that memory to be further shaped by factors besides what actually happened – emphasis can shift, other memories can leak in, and, according to Sheen's research, the protagonist's identity can change.

Why are disputed memories common in twins?

Sheen points to a handful of factors that might explain this tendency for twins to dispute ownership of a memory, most of which are unconscious and not necessarily selfish. One is something she calls "empathy inflation."

"If I see my twin in pain, I feel that pain as well as much as she does. So I'm more likely to take that on as a personal experience and then later subsequently remember that as something I experienced myself," she says.

There's also contamination of the memory by outside information, such as other people's version of what happened. Sheen says parents misremember all the time which child belongs in a given story, and the parents of identical twins likely make those errors even more frequently.

"Parents often make mistakes," she says, "and so they go, 'I remember when you did that.' And you kind of go, 'Oh yeah, that happened to me,' when it really didn't."

And then there are a series of what Sheen calls "source monitoring errors," where a person mixes up a memory's origins.

"I did a study once on the confusion between real and dreamt experiences. Because you have so much imagery involved with dreams, you can actually remember it as [if it were] a real event," Sheen says.

Sheen's work builds on decades of research from people like Elizabeth Loftus, the University of California, Santa Cruz psychologist who has demonstrated how unreliable eyewitness testimony can be in a courtroom setting.

Studies show that the vast majority of wrongful convictions for rape and murder were based on eyewitness testimony.

Sheen says an eyewitness account's sensory details may sound convincing, but they're still often wrong. Her own research finds that twins tended to impute even more details to disputed memories than to ordinary ones.

"Twins often say, 'this is my memory, I remember his red hair. I remember ice cream melting in my hair. I remember the humiliation. I remember the sound it made or the smell of fire burning.' All these qualities that are used in eyewitness testimony are also used between twins when they want to argue about their memories," she says.

As for her own twin, Mercedes says her sister greeted her research with eye rolls.

"When I when I said I was doing this, she was like, 'what?'" says Mercedes. "She thought that I was very stubborn and that I took took an argument too far. We do laugh about it still."

Check out more of NPR's series on the Science of Siblings.

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Today's episode was produced by Berly McCoy. It was edited by Rebecca Ramirez. Gabriel Spitzer checked the facts. Patrick Murray and Stu Rushfield were the audio engineers.

Tags: memories  Series: The Science of Siblings