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.