OH! Sorry for misunderstanding @loubyloulou, the trait for getting interest is the reward for completing the Fortune->Fabulously Wealthy aspiration. The name of the trait is "Shrewd" and it gives interest of 5% of your household funds added to the bank roll every week.
It's pretty awesome to have once you have some bank added up, which I usually do by generation 2 of a game (until I spend it all like a fool...obviously). My sims, before I spent all their money on the move, were getting about 23,000 a week from it.
Once I have progressed a bit in the game, it's pretty easy to complete the aspiration. The heir sim in my current game completed the aspiration instantly when i switched him to it after he had completed his first adult aspiration. He already met every qualification for every tier prior to me even taking it.