I added the book to my "to read" list, but my quick reply is that there may be different incentives for having the first child and having the fifth child.

Also the book describes situation in USA, while I am usually thinking about Roma communities in Slovakia. So we should not generalize across cultures, which was my fault in the first place. There are some differences, e.g. in poor communities of my country the male-to-female ratio is close to 1:1, and the marriages tend to be stable, as far as I know. On the other hand, their ability to think long-term is sometimes pretty low. (For a specific example, imagine poor people who in every spring throw away their winter clothes, because the winter is over, so they won't need them anymore. Or spend all their money on food on the first day, make a huge party, and then starve towards the end of the month; predictably month after month, year after year.) As far as I know some women in this community are aware of the fact that unprotected sex will lead to more starving children, and would like to prevent it, they are just not very good at planning and handling money; especially because their culture does not support the concept of private property, so even if they set away some money for contraception, any family member, or actually any member of the village, is free to take that money and spend it on alcohol. (There were some political proposals to provide free contraception, but they were opposed for religious reasons.) -- This is just a situation in one specific culture, where cute robotic babies would not help, and probably even free contraception would not help if there would be any trivial inconvenience, such as having to remember to use it every time.

To be clear I don't think that dumb people (unless seriously retarded) don't understand the concept that sex causes children. It's more like that beliefs don't propagate automatically, and the thought chain: "any sex has a chance to result in pregnancy... so even this specific instance of sex could result in a specific pregnancy... which means that there will be one more baby... and the baby will need to eat... which means that after a period of breastfeeding we will have to buy food... and it will cost money... and we do not have enough money... so there may be not enough food... so the child may starve... and I don't want my child to starve... so I should use contraception now or avoid sex for now" is too long to be thought clearly and in a near mode during the moment of passion.

Not everybody makes the link:

But the most fascinating and strange part about the islanders are their beliefs on the subject of pregnancy, also described in Malinowski’s classic article “Baloma: The Spirits of the Dead in the Trobriand Islands”. When people die, you see, their spirit takes a canoe to the island of Tuma, which works very much like the normal island except everybody is a spirit of the dead. When the spirit gets old and wrinkled it shrugs off its skin and turns back into an embryo, which a spirit then takes back to the island and inserts into a woman. This, you see, is how women get pregnant.

That’s right. The islanders do not believe that sex causes pregnancy. They don’t believe in physiological fatherhood. Malinowski was incredibly skeptical about this, so he tried all sorts of ways to see if this was simply a story they told, while they actually the real deal. But no, they assured him that it was really true, that all the white people who insisted otherwise were being silly, that the spirits caused pregnancy, not sex.

They argued the case quite logically. After all, they noted, one fellow went on an expedition for a year or two and when he came back, he had a new son. He obviously wasn’t having sex with her while he was away, so where did the kid come from? (Cough.) And, they note, there are some really hideous people on the island who nobody would dare have sex with, yet they manage to become pregnant. (Malinowski spies some kids looking sheepish when this subject is raised.)

They also argue the other way: people on the island are having sex all the time from a very early age and yet they very rarely get pregnant. (Naturally, the islanders don’t practice any form of contraception; the very idea doesn’t make sense when sex doesn’t cause pregnancy.) The white man’s argument just doesn’t make sense. Indeed, recent visitors report, the islanders still believe that sex doesn’t cause pregnancy, despite the best efforts of health workers.