Hivemind Memory

One of the crucial elements of worldbuilding is creating-- or researching-- the historical events which lead your world to the situation portrayed in your narrative. Not just what those events are, but how much accurate detail the characters know about them, how much mythology has been built up around those events, and how those events are viewed by the specific characters and by the general population.
The collective memory is a powerful thing. I come from cultures which have a strong oral tradition, and an amazing ability to pass on correct historical information. However, the further back you go, the more likely facts are to get muddled or elaborated upon or lost or turned into allegory.
This also means that life expectancy will have a major effect on the collective memory. If your population has a number of people over 80 who clearly remember what life was like before the aliens landed 50 years ago, your society will have a different narrative and outlook than one in the same world whose entire population is under 30, so no one remembers anything different than the status quo except by hearsay.