Almost one in eight young people are in youth services (e.g. mental health services). Abnormal development refers to development not being normal and often includes excessive parts of behaviour or behaviour that is not shown enough. Cognitive distortions are common. There are several definitions:Absence of disordersThis is limited as this does not allow for treatment of a problem when it does not classify as a disorder. Statistical fact (i.e. what the majority of the people do is ‘normal’).This includes a deviation from the mean and is based on standard deviations. One limit is that it is arbitrary where the cut-off of the norm is. Desired or ideal stateThis views normality as a desired or ideal state. It relies on considerations of what is normal (e.g. Erikson’s theory of psychosocial development). Successful adaptationThis views normality as a successful adaptation to the environment, meaning that normality depends on the context and not necessarily on the behaviour. Classification is important as it gives directions to what is normal and what is abnormal. It contains a meaningful grouping of symptoms and syndromes. It is important to have knowledge of what is normal to determine when there is a problem and when there is a disorder. There are two levels of classification:Mode of functioningThis includes determining whether functioning is abnormal, deviant and/or in need of treatment.Mode of distinguishingThis includes distinguishing among different dimensions or types of psychological functioningThere are two main functions of classification systems:CommunicationIt allows for better communication among professionals as it defines the rules by which psychological constructs are defined. Documentation of need for servicesIt allows for the documentation of need for services (e.g. whether a child is eligible for special education services).There are several advantages of using a classification system such as the DSM-V:...