In most human conditions, be they physical or psychological, there are characteristically core symptoms; however, in psychology these are often at borderline levels of symptomatology, making diagnosis difficult (especially where comorbidities are present). This search for clarity and definitive diagnosis is often encountered in attention deficit disorder (ADD) and attention deficit hyperactivity disorder (ADHD), due in part to high co-morbidity levels with other neurodevelopmental conditions, such as learning difficulties, social communication disorders, developmental co-ordination disorder and Tourette’s syndrome. This has led to a more pragmatic view of ADD and ADHD as dimensional rather than categorical conditions.