Your question gets at the heart of what is known as the 'nature-nurture' issue. The most fundamental distinction is whether what Mother nature gives us when we are born shapes our development or whether the nurture we receive from the environment leads to our state of development. As with most theories, it is a little bit of both.

Certainly, the disorders you mention have something to do with physical brain states, which is why they are receptive to certain drugs. However, to a large extent, they are affected by the environment. For example, Attention Deficit Disorder (ADD) is not surprising to me in the least given that many youths have to deal with a preponderance of information that begs their attention: TV, Instant Messaging like AIM, video games, email, cellphones, and parents once in a while.

Besides these environmental factors, some scientists have claimed that certain toxins in the environment might cause developmental disorders. Most shampoos, hand creams, and cosmetics have a chemical called methylisothiazolinone (MIT for short) that kills bacteria and helps keep the shampoos more 'fresh'. I remember reading a study where a neurologist, a doctor that studies brains, did a study exposing rat embryo brain tissue to MIT. He found that the brain cells, also called neurons, grew only half as big. It is a big step to take this finding and generalize it to human beings, but it is an interesting piece of evidence nonetheless.

This is a question that is still under research.

To some extent, this is genetic. To a different extent, it's environmental. Also, some conditions, hyperactivity in particular, can actually be good under certain circumstances. Most scientists, for example, are obsessive (about science). It's what makes them successful - they're willing to work on it.