When we think of our memories, it’s natural to imagine a kind of personal library, a bit like Sherlock Holmes’s memory palace, where we have stored the most precious events of our lives. Along the shelves, you can pull out that fifth birthday when you dressed up as Superman, or that family picnic when you found a worm in your sandwich.

Good and bad, these events define who we are; it’s the reason that amnesia is so scary. We certainly wouldn’t want anyone else interfering with those intimate recollections, or we would risk losing a vital part of our selves.

Except it turns out that your friends, family and colleagues are already ransacking your memory palace. They are rearranging the books on the shelves; they are tearing out pages and scattering them on the floor, or they are scribbling over our most precious volumes. “Our memories are constantly being reshaped by social interactions,” says William Hirst at the New School for Social Research in New York. “People can implant memories, people can induce you to forget or they can reinforce other memories.”

Every time you have a conversation, you are inviting someone to ghost-write bits of your autobiography

These are not rare events. Every time you have a conversation, you are inviting someone to ghost-write bits of your autobiography. It sounds troubling and it may cause you to rethink everything you thought you knew about your past – yet you may be relieved to discover that there are also some unexpected benefits.

It takes only a second’s thought to realise that memory is rarely a solitary activity. During a day at work, for instance, we will deliberately recall events to tell our partner in the evening; we may even rehearse and refine the story on the train journey home. We will also recall and reminisce for the sake of nostalgia alone – even if the events are long in the past and familiar to everyone concerned. “I don’t know of any other species that does that,” says Hirst. “You can think of bees conveying where the pollen is, but it is very limited – there’s no intention behind it and they are only conveying new information.”

When Hirst first began this research more than a decade ago, he was among just a handful of people examining the ways those interactions change our memories. But times have changed, and it is now becoming clear that social networks can mould and sculpt our minds in profound ways.

1. Collaborative inhibition

Let’s first consider a phenomenon known as “collaborative inhibition”. Imagine that you and your friends John and Jane attended a football match, where you see a fight break out between the two sides. Afterwards, the three of you get together to discuss the event. You may expect that you will each trigger each other’s recall, helping each person to get a better understanding of the event. Although the group as a whole may record more than any single person, each individual will find that their own memory has been slightly impaired by the discussion.

John is essentially inhibiting Jane’s ability to remember with full potential

It’s all down to the subtle dynamics of the conversation. If John is particularly talkative, for instance, everyone will be paying attention to his point of view, leading their memories down one avenue while distracting them from their own path. Jane might have been more likely to think about different players from a different team, for instance, or she might have noticed an unusual disturbance in the crowd – but John’s reminders have caused her to lose that train of thought. “John is essentially inhibiting Jane’s ability to remember with full potential,” says Hirst. For this reason, you would gather more details of the event if each person had sat down quietly and recorded all that they had known, before sharing notes afterwards.

2. Shared forgetting

Importantly, the effects of our conversations can linger into our long-term memory. Hirst’s own research has focussed on a phenomenon known as “socially-shared retrieval-induced forgetting”. Through subtleties in the way he talks about an event, John can encourage Jane to forget something over time.

The “silence” has altered the memory trace, so that Jane will find it harder to retrieve details in the future

It hinges on the fact that whenever we reactivate a memory, it becomes fragile and malleable. Suppose John is talking about a wedding he attended with Jane. He might mention his speech – reinforcing the memory – but he might neglect to mention a fight on the dancefloor. Through association, this memory may still have been activated at the cellular level (rendering it vulnerable) but Jane may have suppressed that thought to concentrate on the rest of John’s anecdote. As a result, that “silence” has altered the brain’s memory trace, so that Jane will find it harder to retrieve details in the future.

So if you want someone to forget something, the trick is to pick a subject that will activate the memory, while then distracting the person from the crucial detail you would prefer to ignore. Over time, it may then fade.

Hirst has investigated the phenomenon extensively and it appears to be remarkably persistent. In one experiment, for instance, Hirst and his colleague Alin Coman asked pairs of participants to discuss 9/11. The participants did not know each other previously, but they still found that the conversation could still subtly nudge people to forget certain details.

If John forgot to mention the time of day, for instance, Jane would also be less likely to bring up the fact from her own story at a later point. Again, this probably works through activation through association, and suppression. John has triggered the memory in Jane’s mind, and by inhibiting that detail, she later forgets it.

Hirst says that people are often surprised by his work. Surely people would realise that certain details are being neglected, and fight against it? In reality, it rarely happens. “I think it requires a great deal of effort – you have to be really motivated to go beyond what people are talking about,” says Hirst.

3. Infectious thoughts

To understand a third way your friends may be manipulating your memories, consider the eye-witness testimonies following the murder of Swedish foreign minister Anna Lindh in 2003, who was stabbed to death in a department store. Many of the witnesses agreed that the perpetrator had worn a baggy green army jacket; yet the CCTV footage later showed that he had been wearing a grey sweater.

How could they have got it so wrong? It turned out that the police had allowed the eyewitnesses to sit in the same room as they waited to be interviewed. As they discussed the event, a false memory had spread from witness to witness.

Following the pioneering research of Elizabeth Loftus, we already know that it is alarmingly easy to plant false information in someone’s memory. One time, for instance, she hired some subjects who had all visited Disneyland as a child. Beforehand, some of them saw a fake advert for Disneyland featuring Bugs Bunny. Around 30% of these subjects subsequently “remembered” having met the cartoon rabbit at the resort – despite the fact he was a Warner Bros character and would never appear alongside Mickey and friends.

Starting in the early 2000s, Michelle Meade at Montana State University has shown that false memories are contagious and can easily spread from one person to another. She would ask pairs of participants to view a household scene; they were then allowed to discuss what they had seen before they took a test. If one of the pair happened to drop in a few false details, they would stick in the other’s mind, so that they would swear they had seen it themselves.

Again, the effect is remarkably persistent. Even explicit warnings – explaining the flaws in their partners’ recall – failed to reduce the errors entirely. “The flipside to that is that sometimes the explicit warnings also reduce correct recall – they think that person is unreliable I should cut off everything they say,” explains Meade.

Meade is currently investigating the phenomenon in education, to see if one students’ mistakes may contaminate another’s understanding, but it’s easy to see how it could also have important implications for the courtroom, as the case of Anna Lindh’s murder shows.

4. Planting doubts

Besides seeding a false memory that we believe to be true, our acquaintances can also sow a grain of doubt about the memories we thought we could trust. Robert Nash, a psychologist at Aston University in the UK, knows this only too well. Looking back at his sister’s graduation, he could clearly recall that the British newsreader Trevor McDonald had attended the event. “I was absolutely convinced,” he says. But when, before his own graduation, he mentioned the event to his parents, he found them laughing in disbelief. A bit of research online only left him with more doubts. “And the more I thought about it, the more I knew that it wasn’t plausible.”

Despite his suspicions, the memory hasn’t disintegrated under the scrutiny, though. “I can still picture it.” This is an example of a “non-believed memory” and it shows the fourth way our social interactions influence our recollections – by questioning them and forcing us to confront our own failings.

Nash thinks non-believed memories are probably very common, pointing to a recent study showing that at least a quarter of participants could describe at least one questionable recollection. “But that survey asked if they could think of one on the spot – my guess is that everyone has a non-believed memory at one point.” And in many cases, it’s another person who first seeded the disbelief.

To understand the characteristics of these experiences, Nash and his colleagues recently explored surveys from hundreds of participants, finding three distinct flavours of non-believed memories. The “classic non-believed memories” might be similar to the kind Nash described: you have a vivid recollection, but you now strongly suspect that it is false; with others, there’s a grain of doubt – you have the sense it’s not true but you might still defend it. The third kind are weak non-believed memories. They are vaguer; you might feel confident that you remember something but you aren’t clear about the details, and you now doubt its very occurrence.

Nash has also investigated the ways we test the truth of our memories. Previous research had shown that our feelings of authenticity may depend on the assumption that our memories are accurate – so you might expect that people would put in a lot of effort to verify the facts. So along with his collaborators, he asked participants to imagine that someone had challenged a cherished memory, and asked them to describe how they would test whether it was true or not. They also had to rate how much effort it would take. In almost every situation – whether the memories were important or trivial, from the distant past or more recent – he found that participants would opt to use quicker but less reliable options. These might include asking a friend or family member who may be unreliable themselves instead of more difficult, but more accurate, attempts to get to the truth, such as checking the hard evidence of medical records or looking through old diaries.

The ‘principle of least effort’ was true even if they asked the participants to imagine that they would need to verify the memory for the police

This ‘principle of least effort’ was true even if they asked the participants to imagine that they would need to verify the memory for the police – a situation with serious consequences. “They still chose the ‘cheap’ strategy over the reliable strategy,” he says. We may think we value the truth, but “people don’t question their memories enough to think it’s worth putting in the effort”. (Truman Capote was strongly in this camp; when writing his non-fiction novel In Cold Blood, he claimed never to have used a tape recorder for his interviews, instead relying purely on his own recall.)

Even after all his research, Nash still finds it hard to shake that belief. “I’m acutely aware of the fact that my memories aren’t reliable and I have just as many false memories as everyone else – but it’s still hard to budge the idea that I can trust my memory. We [psychologists] are not more immune than anyone else.” He does, however, try to remain open-minded if someone does question his memory. “I remind myself to entertain the idea that I might be wrong – that it’s all we can ever do.”

The extended mind

No mind is an island, after all – and despite the errors that other people may bring, our memories benefit from their input. This ties into the concept of ‘the extended mind’ – the increasing recognition that our environment plays a crucial role in our thoughts. “We tend to think of the mind as something that’s beneath the surface of skin but really so much of our actions are scaffolded by external artefacts and practices,” says Hirst.

Consider a recent study by Nicole Iannone at Purdue University in the USA, which examined the relationships between friends. She was interested in their “transactive memories” – a shared system of storing and recalling information. You may often lean on your friend for recipes for instance, while he may ask you for advice on holiday destinations, or you may turn to them to help you recall events from your past.

The longest, strongest and most trusting friendships seemed to be built around these shared, interconnected memory systems

To study that system, Iannone asked subjects to rate statements such as “my best friend and I can remind each other of things we know” and questioned them about the quality of the friendship. Sure enough, she found that the longest, strongest and most trusting friendships seemed to be built around these shared, interconnected memory systems. Iannone suspects that we may choose to build our memory around our friendship; if you know your friend is around for restaurant recommendations, you may opt never to read good food guides yourself. “Is it possible that you don't develop knowledge in an area your best friend has a lot of knowledge in?”

Even the aspects of social influence that may at first seem like a disadvantage – such as the retrieval-induced forgetting and the contagious false memories – may provide some unexpected benefits, by sculpting our recollections so that we all remember the same details. “For me, one of the things that promotes sociality is common understanding of the past,” says Hirst. “All memories shape our identity, and collective memories may shape our collective identities.” We are not the sole authors of our autobiography – and we may all be stronger for that fact.

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David Robson is BBC Future’s feature writer. He is @d_a_robson on Twitter.

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