Two fifths of people have a fictional first memory based on fragments of early experiences, psychologists have found.
Scientists questioned participants in a survey that identified more than 2,000 individuals claiming to have memories from the age of two or younger.
Current research suggests that memories cannot be formed before about three-and-a-half years.
Yet 893 of those taking part in the survey said they had memories extending to before their first birthday.
The researchers studied the content, language, nature and descriptive detail of the early memory descriptions.
They found that the memories were fictional patchworks based on fragments of early remembered experiences combined with facts derived from photos and family conversations.
Professor Martin Conway, one of the scientists from City, University of London, said: "In our study we asked people to recall the very first memory that they actually remembered, asking them to be sure that it wasn't related to a family story or photograph.
"When we looked through the responses from participants we found that a lot of these first 'memories' were frequently related to infancy, and a typical example would be a memory based around a pram.
"For this person, this type of memory could have resulted from someone saying something like 'mother had a large green pram'. The person then imagines what it would have looked like. Over time these fragments then become a memory and often the person will start to add things in such as a string of toys along the top.
"Crucially, the person remembering them doesn't know this is fictional. In fact when people are told that their memories are false they often don't believe it."
The research, published in the journal Psychological Science, suggested that 40% of people had fictional first memories.
First author Dr Shazia Akhtar, from the University of Bradford, said: "We suggest that what a rememberer has in mind when recalling fictional improbably early memories is an episodic-memory-like mental representation consisting of remembered fragments of early experience and some facts or knowledge about their own infancy/childhood.
"Additionally, further details may be non-consciously inferred or added, eg that one was wearing a nappy when standing in the cot."