I think Sonia will have great answer for this. A lot of how we develop and who we become is an interaction between our DNA (our genes) and our environment. The brain is important in mental illnesses and disabilities, but they’re often really complex problems that are due to many factors.
Oh that is a really good question. The short answer is we are not 100% sure yet. That is why research into brain development is so important at the moment. We know that mental illnesses occur in both adults and children but we do not know if mental illnesses are something that is present when we are born and takes a few years to show themselves or if they are solely to do with the environment we live in and the traumas we face. Schizophrenia a disorder that affects the brain. We are not sure what causes it but we believe that it is a combination of genetics, brain chemistry, and the environment. So you can see how complicated these illnesses are and why we need to do so much research into them as we do not know enough about them!