This is not because they're making this up as they go, of course. At least, they may not be doing it consciously. The state of their mental health is not entirely fractured either, as even the sanest and most “normal” people can have memories that are altered drastically with each retelling of it. Certainly, people with certain mental health issues might have more “false” memories than others, but that doesn't mean that their minds are the only ones capable of such things. The fact is, the human mind has a propensity to create false memories. That much has been known to medical science for decades now, though the reasons for this unusual trait have yet to be fully explored and, if possible, understood.
Human beings tend to have a lot of faith in memories that they can recall in detail, whether the one doing the recollecting is the type to note detail or not. This is a logical mechanism, after all. The more you remember about a certain event, the more likely it is that you were actually there and you didn't dream the whole thing up. However, if fiction writers are any indication, the mind is capable of creating things with as much vivid detail as anything that can be conjured up from memory. There are also times when the foggiest, least detailed memories are the ones that actually occurred, despite the lack of details that a person can draw on as support. The mechanics of this unusual aspect of memory and the mind have recently been put under intensive study by South Korean researchers, under the leadership of Hongkeun Kim of Daegu University.
The study discovered that there were two sections of the brain devoted to memory, with one storing the specific details while another stored the basic gist. Using common memory tricks, the team managed to discover that most people called on the area that stored the gist more often than the area that stored the specifics. Additionally, the more confident the person was in his memories, the more likely that the “detail” area would light up on a scan of the brain while the memory was being recalled.
While this does explain how the brain works with regards to memory, it does not explain the phenomenon of false memories. The researchers assume that it somehow relates to the interaction between the two, along with areas that are more closely linked to the subconscious than memory. At the moment, though, this is still conjecture and further study is needed. The researchers believe that, once understood, the knowledge can help people with memory-impairing problems, such as Alzheimer's. It may also improve our understanding of how certain disorders cause someone to lose recollection, but retain familiarity.
