Gabor Mate claims that based on his observations as a GP + coordinator of the Palliative Care Unit, ADD/ADHD is caused by maternal neglect/lack of love. He doesn't really offer any solutions on how to deal with it... time travel being ruled out, I guess:).

He's a strong communicator + a compelling speaker, which is probably why his theories enjoy the popularity they do.

Great reading for getting yourself thinking and exploring other perspectives, but I'd recommend going to more reliable/scientific sources when looking for meaningful solutions.