The idea is simple. You have starting traits, but over time depending on your actions they can change. An easy example would be cannibalism. Lets say your colony hits hard times and you have to start eating people for a while. Each time you do so there would be a small chance that your pawn would develop a taste for his fellow man and gain the trait cannibal. Another easy example would be psychopath. lets say after 432 raids and witnessing 985 strangers deaths your pawn becomes a hollow shell of a person and develops the trait psychopath.
I think this would be a fun way to experience character change, which going with the theme of the game is key to story telling.
+1
and of course they can lose some traits when they don't use it or start fear fire (after they was burning)
This is a feature I've been asking myself why it wasn't already included.
This is complicated.
Although the logic that people do change with time is sound, in a game I would prefer it to be something the player can control in some remote but precise way, so it doesn't appear randomly/irreversible and can be voluntarily done by players for storytelling purpose (though, too many here would consider cannibalism to just be a +1 attribute)
for example :
psychopath : Kill a number of incapacitated pawn
Trigger Happy : spend the time to destroy some object with gun pointlessly
or Neurotic and Psychic related trait to be related to (eventual) machinery in Rimworld.