Classification schemes of psychiatric disorders in childhood and adolescence have to take into account three particular features. Firstly, the individual is continually changing and growing. Sound knowledge is therefore required of the normal range of development and its limits. For example, some fears may be normal in a 5-year-old but abnormal in an 8-year-old. Once identified, it is helpful to decide if abnormalities are due to delay in or deviance from the usual pattern of development. The implications of each differ, and should be classified differently. Secondly, the majority of childhood mental health problems arise from an excess of behaviours exhibited by many young people, such as aggression or dieting. They are seldom due to qualitatively distinct phenomena of the kind more often seen in adult conditions, such as hearing voices or hanging oneself. Consequently choosing a cut-off point to make a categorical entity from a dimensional construct is more often used in child psychiatry. This is inevitably an arbitrary process (albeit informed by empirical criteria), which may lead to loss of information, and may be held to be labelling the child unnecessarily. That dimensions can be interchanged with categories does not necessarily mean they are unhelpful—after all, day and night are useful terms yet the boundary between them is continuous and arbitrary. Psychiatrists in particular may be criticized for ‘medicalizing’ a child’s difficulties by talking about disorders or diagnoses, whereas other professionals and parents may prefer to see them as understandable variations in child development, and prefer to call them ‘emotional and behavioural difficulties’. However, diagnoses are a quick way to convey a lot of information that dimensions may not. Thus to only say a child is at an extreme of an antisocial behaviour dimension does not necessarily convey the association with specific reading retardation and ADHD, which could be (and often are) consequently missed.

Thirdly, children’s difficulties nearly always arise in the context of relationships within the family. More often than in adulthood, some or all, of the problem may appear to be the result of the functioning of the family, rather than in the individual child who may merely be reacting to the situation. For example, a child who is disobedient and shouts in class may simply be behaving the way his parents do at home. A classification system will be stronger if it can take family functioning into account—it will have a greater chance of capturing clinically important causal and therapeutic considerations.

A valid and useful classification system will need to take into account these features and be based on a thorough understanding of normal development and how it can go wrong, rather than merely include static descriptions of presumed pathological states. The term of developmental psychopathology was coined in the early 1980s to denote the scientific study of how abnormalities can be understood in terms of processes underpinning human development.^(1,2) There are now journals and books incorporating the term into their titles.^(3,4) Many disciplines are relevant, from embryology and genetics to social learning theory and criminology. Developmental psychopathology, besides studying the impact of pathogenic influences on pathways through life, such as the way the
monoamine oxidase-A (MAOA) genotype interacts with an abusive upbringing to cause conduct disorder,⁽⁵⁾ or how specific reading retardation leads to low self-esteem, also investigates protective mechanisms, such as the ameliorating effect of high IQ on the propensity to juvenile offending, or the beneficial effect of a trusting and caring relationship with an adult on the impact of childhood abuse. This chapter aims to show how findings from developmental psychopathology have informed current classification systems, and what challenges remain.