I'm on board with sourland - just as egg laying is hormonal, so is broodiness. Of course some breeds (and some chickens) are not very broody. I have a 7-year old Delaware who has never been broody even once. Of course, she has a little "mini broody" session for 40 minutes after laying, then hops to her feet and walks out of the nest box like nothing happened. I think she may be a freak. Where there's eggs, there's a least potential for broodiness.