This is pure speculation, but is it possible that we're less strict nowadays in child-rearing behavior (none of that children are meant to be seen, not heard nonsense.) So perhaps the children don't develop the same degree of control over their behavior because their parents aren't forcing them to behave themselves to the same degree?
I remember when I was a kid, we had to learn to be quiet when my dad was home. Making too much noise would inevitably lead to escalating admonitions from my father to be quiet.
People voted this down, but it's plausible. The brain is like a muscle in some ways, you work it out and it gets stronger. So I think it's entirely possible that if you work out your pre-frontal cortex by having to control your behavior all the time because your parents are strict - you develop stronger self control. Sample size of 1, but it seems true enough in my case.
I remember when I was a kid, we had to learn to be quiet when my dad was home. Making too much noise would inevitably lead to escalating admonitions from my father to be quiet.