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Think back to when you were very young. What's the earliest memory you can recall? For years, science has concluded that children begin to form lasting memories from around age 3 or 4 — and not before.

Now a new study published in the scholarly journal Psychological Science estimates that 40% of the 6,641 people surveyed in the study have completely fictional first memories of their lives. That doesn't mean that those 40% of people woke up one morning and decided to simply make up a good story for the survey. Here's what scientists think may be responsible.

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We might be lying to ourselves without even realizing it.

Participants in the study were given very specific instructions regarding what they were to report. The memories, the study authors said, must be things that the people responding had directly experienced — not memories triggered because they'd seen a photograph, or because a relative had told them a story about it once and said, "don't you remember?"

Taking these results and the language used to describe these instances, scientists believe that fragments— a half-remembered crib, for example, or a favorite blanket from when you were a baby — combined with contextual knowledge gathered later have helped us to inadvertently construct "memories" of things that happened before we were fully capable of storing real memories inside our heads.

So these people weren't lying on purpose about what they remembered — they truly believed it, and may still continue to believe it.

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Here's why we lie to ourselves — and how those lies become memories.

Paper co-author Professor Martin Conway of the University of London said in a statement,

"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 becomes 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. This partly due to the fact that the systems that allow us to remember things are very complex, and it's not until we're five or six that we form adult-like memories due to the way that the brain develops and due to our maturing understanding of the world."

We as humans actually forget things pretty quickly.

There are numerous reasons why the approximate age at which children form lasting memories is the case — not the least of which is that most of our memories are things we speak about using some kind of verbal language. A specific smell, or sound, or even the sight of your grandmother's music box can trigger a sudden flood of memories associated with that object. But in the time before you develop verbal skills, it's much harder to associate things that way.

If that's not enough, humans are also very good at forgetting — and young children are extremely good at forgetting but slow down in this skill as they get older. By the time they become first young adults and then reach full adulthood, they're much better equipped to hang onto memories and not forget them as quickly as they did when they were younger.

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False memories can be relatively harmless — or they can cause serious damage.

The scientific study of memory in humans — at all ages — is an ever-growing field. Humankind likes to have a complete story — but there's so much going on around us at any given time, it's difficult or impossible to take everything in.

Kimberly Wade of the University of Warwick described it like this to the BBC:

"Our perceptual systems aren't built to notice absolutely everything in our environment. We take in information through all our senses but there are gaps. So when we remember an event, what our memory ultimately does is fills in those gaps by thinking about what we know about the world."

On the more serious end of the spectrum, sometimes false memories can have far-reaching and even life-destroying consequences.

For example, through advances in forensic science and the ability to analyze DNA acquired and stored years and even decades earlier, the Innocence Project is now able to overturn cases where an eyewitness mis-remembered what — or who — they'd seen commit a heinous crime.