I think these IQ things come down to two factors:
- Development and health: It has been shown that poor nutrition during childhood leads to stunted development, which also affects intelligence. Toxins like lead, drug use, illnesses, etc. all affect IQ in a bad way and are functions of poverty and development.
- Culture of writing and book learning: IQ tests test your ability to read and solve problems with a pencil and paper. If you're used to that, you will do better. If you can barely write and just work on the field all day you will do worse than someone who sits at a desk and does similar tasks all day.
So if you want to improve the IQ of a population, health and nutrition are things to focus on. Education is always good, but it also depends on the development level of the culture. If no bookworms are needed, only farmers, the kind of IQ you measure on such a test is not very meaningful anyway.
I really dislike the racists who want to twist this into some biological constant, because the facts are very clear that the environment affects these results very much. If you choose to ignore that to confirm your racist bias, you're stupid yourself.