How many parents know that on average children raised in a big city grow up smaller and have more health problems than those that grow up in the countryside? That's been known for some time but the truth is suppressed due to country children more likely to turn into Republican voters, even if they later move to a big city. While the Democrat bell curve is wider, the average Republican is both taller and smarter than the average Democrat. You'll never hear that truth from the urban media.
Maybe that's because their parents are smaller.
Think about it. Two brothers, one big and burley and the other smaller and frail. Which one is going to take over the farm and which one is likely to move to the city to get a job?
All these data are after the fact. The only experimental data - the kind of data you need to answer the question of causality - is from genetic studies. They mainly from agriculture and animal breeding. Genetics of humans is a taboo topic.