How does the game choose which sim is head of household?

@GalacticGal I almost always do that, but I've been doing challenges lately that start with a full house, and I can only control one sim. So I've been a bit frustrated with the create 8 sims, have no clue who the household will be named after, or where my one controllable sim will end up on the household portraits in game. Bleh. Just messy.