It is believed by some that we have ancestral memory within our brain going back several generations.  Usually those memories stay hidden in the subconscious though they may have some affect on us anyway.  In the "Dune" novels, the Bene Geserit are able to release these memories and get advise from long dead relatives.  Can something like that really happen?
    If a person has a traumatic experience that causes his personality to withdraw, another personality may come forward and take over.  This personality that takes over, permanently, or periodically is the personality of an ancestor.  Is this what really happens when a person has Multiple Personality Disorder?  Psychiatrists think the person is making up alternate personalities, and maybe that is true in some cases,  but it may also be personalities from our ancestral memory that are coming forward.  It sounds like a far-fetched idea, but is it really any stranger than making up entirely new personalities, sometime several of them?  I don't think so.