Those people are very self-directed. They have a lot of maturity, discipline, patience, ambition, and responsibilty. These are the kind of people we'd like to have procreating more. Unfortunately, those kind of people usually understand just how much of a burden and responsibility children are, so they put it off until they're ready and have fewer children.
The antithesis of this is that those who are immature, uneducated, undisciplined, irresponsible, unthinking, and living only for today are those who are most likely to have a whole litter of children. They have no reason not to. They don't have ambitious goals that restrain them to more responsible actions. Life holds no hope for them and having children at least gives them a feeling that they're loved. Being irresponsible and undisciplined is the easiest life in the world.