I had to go back to work and now my 18 month old todler has been under my sister's care for a couple of months now(from 8-4p.m). She has 2 ADHD sons and my son has been spending quite some time with them. AS of 3 weeks now, his behavior has drastically changed. He has become violent and disrespectful. He tends to hit his 4 yr old sister alot. Can ADHD be a learned behavior that my son has picked up from my nephews?