There are many theories on this, for example, Freud put it down to sexual repression. Others say it could be linked to language development or the fact that some areas of the brain linked to memory are not fully mature. This covers episodes of amnesia linked to psychological trauma. It is usually temporary and can be triggered by a traumatic event with which the mind finds it difficult to deal. Usually, the memory slowly or suddenly comes back a few days later, although memory of the trauma may remain incomplete.
Treatment varies according to the type of amnesia and the suspected cause.
Anyone suffering any symptoms of amnesia should seek medical attention. The BBC is not responsible for the content of external internet sites.