The findings have highlighted the fallibility of memory - and explain why two people can have varied recollections of the same conversations and events.
University of Hull researchers asked 1,600 students whether they had experienced a false memory.
A fifth said they had experienced 'fictional' memories, mostly dating back to when they were four to eight years old, reported the journal Psychological Science.
"Autobiographical memory provides us with a sense of identity and it is usually accurate enough to help us negotiate our lives," the Daily Mail quoted researcher Giuliana Mazzoni as saying.
"But as our study shows, not all that we remember about our past is true. Our research also shows that this phenomenon of non-believed memories is much more frequent than people had imagined.
"Crucially, if these memories are not challenged by some form of evidence, they would still be considered part of the individual's autobiographical experience," said Mazzoni.
The work has also raised concerns about the way therapists 'recover' lost memories of child abuse in adults.