How well do you trust your own memory? How sure are you that the childhood events that defined your sense of self and shaped you happened just as you recall? Science says that most of us make up, or at least ‘alter’, almost as many memories as we save correctly.

This phenomenon is not exclusive to individual memory either. Collective memory hallucinations, while rare, are far from unknown. One of the most famous examples is the so called ‘Mandela Effect’, coined by paranormal researcher Fiona Broome, who said that since 2010, ‘perhaps thousands’ of people have written and admitted in public that they remember Nelson Mandela dying in prison in the 1980s, and are convinced that the man who became president was an imposter.