When Karen learns that the problems between her parents began about six or seven years ago, she comes to the conclusion that since this was around the time Amy was born, the rift between her parents must have been because her father chose Amy as his favorite, and her mother retaliated by making Jeff hers...that if Amy hadn't been born, the marriage might have survived. Adult readers who read between the lines might come up with a different interpretation...the problems did begin with Amy, but not in the way Karen rather naively and simplistically thinks. Jeff would have been in school for two years around that time, and Karen just getting ready to start kindergarten...and that would have been around the time Ellie got pregnant with Amy. It may have been that Amy wasn't exactly planned, and that with both children in school, Ellie (who is said to have married very young, and was eager to get a job and more independence after the divorce) may have been looking forward to getting a job or maybe going back to school...then along came an unplanned child and upset her plans, and the resentment may have eaten away at the marriage until it was unsalvageable.
Jeff's moodiness and bad temper seem to go beyond the usual teenage mood swings: he yells at and curses out his SIX-YEAR-OLD SISTER at the beginning of the book for spilling her milk and "ruining dinner for everyone" when their mother loses her temper over it (due to her tension over her fraying marriage), and generally lashes out at everyone and everything. But when you realize that things have not been going well between his parents for about seven years, and that their way of handling conflicts is generally to lash out at each other in rather nasty outbursts of temper, you also realize that this is the kind of behavior that's been modeled for Jeff since HE was about seven. (Also crosses over with Fridge Horror.)
