>the amount of books in a household was a strong predictor for IQ. Background matters a whole lot...
That doesn't follow at all. IQ is correlated with parental IQ, due to genetics. Equally plausible, if not more so, than "books cause high IQ" is "high IQ causes books". Smart people a) like to read, and b) have smart children. I don't think it's likely that kids are boosting their IQ scores by cracking open their parent's book collection.
Well, you didn't provide a link to the study, and people mistaking correlation for causation is a frequent thing even in science (replication crisis).
The hypothesis "smart people are likely to buy more books, and smart people are likely to have smart children" sounds quite plausible to me. Does it seem implausible to you?
The best way to convince me that books directly increase IQ is to donate a ton of books to kids who need it, and measure the gains. (As a side effect, you would probably win a Nobel price if that worked.) My experience with charity says that actually many people are willing to donate books to charities; charities refuse them, because they already have enough books and the kids are not reading them anyway.
There are many ways how poverty can hurt intellectual development. This is not one of them.
There are many people who want to get rid of old books for kids, because the kids grew up and don't need them anymore; if you asked them to donate the books to you, you would be doing them a favor -- saving them from a dilemma between the bad feeling of throwing the book away, and the wasted space at home. If any NGO would bring a truck and say "please give us the books for kids you no longer need", the truck would be full in an afternoon. Then... according to your theory, the truck could go a few blocks further and turn hundred poor kids into Einsteins. I would be so happy if that worked! Unfortunately, it does not.
That doesn't follow at all. IQ is correlated with parental IQ, due to genetics. Equally plausible, if not more so, than "books cause high IQ" is "high IQ causes books". Smart people a) like to read, and b) have smart children. I don't think it's likely that kids are boosting their IQ scores by cracking open their parent's book collection.