This Science Daily article says ADHD may be up to 70% heritable and seeks to find the source of the other 30%. They point to lead in the environment, saying it turns off some genes in the brain.
If this is true, ADHD should have become less frequent when lead interior house paint was replaced with acrylic. I wonder if it did. Unless the lead-ADHD relationship is not dose related. Some masters student should find this out. Free thesis subject.
Of course, the heritability of ADHD should vary according to neighborhood, because heritability is the relative importance of genes to a condition that also has an environmental component, not the absolute importance to the condition, because the environment may change from place to place, so unless it's 100% genetic, heritability can vary. So if lead is one of the environmental culprits, the relative genetic influence on the incidence of ADHD would be high in neighborhoods with little lead in the environment and lower in neighborhoods with more lead.
But the article says it's not by neighborhood at all. I'll bet it used to be, when poor homes still had lead paint in them, but now, they say, there is a universal low level of lead, and every kid has it.
In that case, I'm afraid I'm pessimistic about removing it.