I would also wager having a "better" child of the sort that would have good outcomes also helps hold families together better. If you have a kid that just is downright "broken" somehow in a way that places unusual demands from the parents, it often harms the marriage. That is, perhaps it is also the case the kind of children that have better outcomes tend to place less unusual demands on their parents, allowing the family to continue to bond. A particular violent, psychologically psychopathic child that might be measured as having bad outcomes might also drive the parents apart as family life becomes unbearable.
