4. The false underlying premise

Finally, even if researchers found a consistent difference between children who act a certain way ("ADHD") and children who don't, and even if they could somehow prove that the difference caused the behaviours, there is no reason to believe there is any "disorder". There may be physiological differences between people who are right-handed and left-handed, or people who prefer the colour red over the colour blue. But it doesn't make either group "sick". 

We know that people have individual physical differences, but it is dangerous ground to say that those differences are a "disorder", just because they are in the minority, or because they cause problems with fitting into society's rigid structures (like public school). So all these millions of dollars are being spent to try to prove something which will, if proven, be meaningless. Maybe children with higher activity levels, or children with shorter attention spans, or children who don't like doing their homework, tend to have some physical difference. That will still not prove that "ADHD" exists as a valid "disorder".