That's not the only available explanations. You can find many alternatives. Specially now that we have discovered the influence of epigenetics. For instance, a

That's not the only available explanations. You can find many alternatives. Specially now that we have discovered the influence of epigenetics. For instance, average height of a population is not a fixed value but changes with the progress of a country. Presumably due to the diet improvement in the mother's generation. Another consideration: parasites, I would be surprised if they don't influence the development of child intelligence.