>>510542822 (OP)Surprised this is still born out in the data but the answer is really eugenics.
Before FDR there wasn’t much of a social safety net and the main draw of the country was free land out west so it created a natural culling where people who couldn’t make it fighting off inidians, raising crops and managing resource stores for winter on their own died off and their genes died with them.
Since the last wave of large scale white immigration came in the 1910s the majority of American whites alive today are the product of this natural selection.