The main problem is in the name. People fixate on the attention deficit aspect of the impairment because it's right there in the name, however an attention deficit is simply one of a number of symptoms that are caused by ADHD.
ADHD is a developmental disorder that results in a generalized impairment of ALL executive functions. This includes the ability to purposefully direct attention, to modulate emotions, and to perceive the passage of time. It also impairs ones ability to hold an inner monologue, or 'voice'.
There are also other psychological illnesses that may present as "co-morbidity" with ADHD because of the detrimental effects of ADHD on ones quality of life, such as depression.
It is know that the disorder has an unusually high correlation to heredity, and no environmental causes (save one virus which may causes a similar executive function impairment). It is absolutely clear that it is an inherited disorder. It is not caused by modern society and constant interruption.