Look into the marginal cost of raising a child. Children are a bigger expense for upper-middle class people who expect to send their children to a private school, pay for them to go to an expensive college, want each kid to have their own room, etc. For poorer families the main expense of a child is food. The younger ones can wear hand me downs, it is possible to shove two or three children in one room, etc. The poorer the family is, the less having another kid costs until it is actually a negative cost because each additional child actually brings in income by working.