ADHD was the first disorder found to be the result of a deficiency of a specific neurotransmitter. There are millions of neurons, densely packed into various regions of the brain. Each region is responsible for a particular function. Interpreting vision, hearing, and other sensory inputs that helps you decide how to respond physically, and vocally. Anyway..in order for these regions and do their jobs, they must be linked to another region, like wiring but obviously not wires..but neural circuts, these carry info from one region to another.
Ok sorry, now back to your question. Causes can be from blood relatives, such as a parent or sibling, exposure to environmental toxins such as lead, maternal drug use, alcohol use, and smoking during pregnancy, and premature birth. Hope this helps.