Epidemiology 39Table 3.2Age at onset of disorders
Characteristic early onset Mostly teenage onset
Autistic disorders Depression
Hyperactivity disorders Mania
Attachment disorders Generalised anxiety
Selective mutism Psychosis
Oppositional-defiant disorder Suicide and deliberate self-harm
Separation anxiety Anorexia and bulimia nervosa
Specific phobias, for example, insects Panic attacks and agoraphobia
Enuresis Substance abuse
Intellectual disability Juvenile delinquency
Specific developmental disorders, for example, language
and reading disorders
Aetiology
Epidemiological studies have provided evidence for the aetiological impor-
tance of psychosocial, genetic and neurological factors. A particularly in-
fluential study of psychosocial factors involved a direct comparison of chil-
dren from a run-down area of inner London with children from the small
towns and countryside of the Isle of Wight. The same two-phase measures
of psychopathology were applied to representative samples of 10-year-
olds from both areas. By comparison with the Isle of Wight children, the
inner city children had roughly double the rate of conduct, emotional and
reading disorders. These differences seemed to be attributable primarily
to higher rates of the following psychosocial problems in the inner city:
marital breakdown, parental illness and criminality, social disadvantage,
and schools with high turnovers of pupils and teachers.
Epidemiological twin and adoption studies have pointed to substantial
genetic contributions to many psychiatric disorders in childhood and
adolescence. In the case of autism, for example, epidemiological twin
studies have demonstrated a very high heritability for a ‘broad phenotype’
that includes autism and lesser variants. Genetic factors also seem to play a
prominent role in bipolar affective disorders, schizophrenia, tic disorders,
and pervasive hyperactivity, and somewhat lesser roles in the common
behavioural and emotional disorders.
Child and adolescent psychiatric disorders are often associated with
intellectual disability or specific learning disorders. Though these links are
well established, the underlying causal mechanisms are still in doubt. In
some instances, psychiatric problems such as ADHD may interfere with
learning. In other instances, the frustration and stress caused by learning
difficulties may lead to psychiatric problems. In yet other instances, both
learning and behavioural problems may reflect the operation of some
‘third factor’, whether psychosocial, genetic or neurological.
Epidemiological studies of children and adolescents with congenital and
acquired brain disorders have found particularly high rates of associated