Mental Disorders

What causes mental disorders? How do the doctors know when something is not right with someones brain? I want to  know all of these things. Even what in the brain causes people to have problems, or what makes it happen. Are you born with it or does it process as you grow up? Growing up with a younger brother, 2 cousins, and an aunt with all different mental disorders make me curious about them. I want to know the different types of disorders and what happens in the brain to make someone have a disorder. Being around my cousins and my brother made me realize that not all kids act the same. One of my cousins was very mean and always hyper. he never listened and he was out of control. As he got older my aunt took him to the doctor and he was diagnosed with sensory problems. What is sensory  problems? 

Mental Disorders Chart (1)