The relationship between family income and fertility is influenced by constraints on passing debts to children. If parents could make their children pay for raising them, income wouldn't affect how many kids they have. But when parents can't do this, income-rich parents tend to have fewer children. This happens when parents see their own consumption as a good replacement for their children's needs. So, if parents can't make their kids pay for raising them, wealthier parents tend to have fewer children.