Who is the "they" here? AD/HD has been studed for decades, well before there were any medical interventions for it. The research is in peer reviewed journals, independent of financing from drug companies. You can read the research yourself, and if you follow the "Methods" section in these studies, you can replicate it on your own. It is completely transparent and there for you if you care to look.

I, personally, have spent thousands of hours researching this issue. Many of those hours were as an unpaid graduate student. Now, as a professor, I supervise ongoing research in AD/HD. Neither I or my students receive any financial support from a drug company. Our work is born out of a desire to help and understand.... not make money for a drug company.