I deleted my comment talking about it because it usually goes badly with the HN audience and their preconceived notions.

But I have had the hypothesis (as a layman) that growing around smokers affects the brain of a child, they self-medicate with tobacco, and the cycle continues, and it has for many generations until today when smoking is frowned upon and self-medication is no longer readily available. I don’t think smoking is the cause of all of ADHD, but we’ll eventually learn that dopamine dysfunction has a lot of different causes.