36 Chapter 3
Epidemiological surveys vary not just by geographical location but also
by focus:
A survey may cover a wide age range (for example, the British studies
included in Box 3.1) or be focused on a narrower age band, for example
10 and 11-year-olds in the first Isle of Wight survey; very young children
in the Preschool to School Study (see Box 22.1); and 14 and 15-year-
olds in the Isle of Wight study of adolescence (see Box 23.1).
The focus may be on a broad range of psychiatric disorders or on just
one or two specific disorders, for example, depression, anxiety, hyper-
activity, obsessive-compulsive disorder, tic disorders, autistic disorders,
or eating disorders.
The primary focus may be on specific risk factors such as head injury,
marital discord or divorce, low-level lead, disasters, or school influences.
Epidemiological studies of twins and adoptees have increasingly been
used to investigate the relative aetiological importance of genetic and
environmental factors.
The main conclusions that have emerged from epidemiological studies
are discussed below.
Overall prevalence
Recent surveys have generally reported that psychiatric disorders are
present in roughly 10–25% of children and adolescents, though some
older surveys estimated that up to 50% of children were affected, probably
reflecting the inadequacy of DSM-III and DSM-III-R diagnostic criteria.
Until DSM-IV, the criteria for a disorder were met when individuals had
a particular set of symptoms even if those symptoms had no significant
impact on the child’s life (in terms of distress or social impairment). As a
result, many of the individuals who met DSM-III and DSM-III-R symptom
checklists were not in need of treatment and did not correspond to what
clinicians recognised as ‘cases’. Thus, many of the individuals identified
as psychiatrically disordered in epidemiological studies using unmodified
DSM-III or DSM-III-R criteria were probably not ‘real’ cases in any mean-
ingful sense. With DSM-IV and ICD-10 now using impact as well as symp-
tom criteria, prevalence estimates are more conservative (see Box 3.1).
At the same time, there is increasing recognition that some children and
adolescents have serious mental health problems that warrant treatment
but that do not fit neatly into the current diagnostic systems. These are
typically individuals who do not have the right number or pattern of psy-
chiatric symptoms to meet operationalised diagnostic criteria, but whose
symptoms nevertheless cause substantial distress or social impairment. For
every three individuals in an epidemiological survey who meet opera-
tionalised diagnostic criteria, there is roughly one other individual whose
problems are equally relevant to mental health services, but who does
not meet current operationalised criteria. Epidemiological surveys need
an element of clinical judgement to detect those individuals who would
otherwise slip through the cracks between operationalised diagnoses.