38 Chapter 3
Persistence
When an individual has a disorder at two different ages, the continuity is
said to behomotypicif the disorders are similar at both ages, andheterotypic
when the type of disorder has changed with age. For example, when
children with conduct disorders are followed up into adult life, some con-
tinue to have disruptive and antisocial problems (homotypic continuity)
while others become depressed as adults (heterotypic continuity). In this
instance, homotypic continuity is more likely in males, and heterotypic
continuity in females.
Many studies have shown that conduct problems are somewhat more
persistent than emotional problems (see Box 3.1). The continuity from
childhood and adolescence into adulthood can be substantial. In the
Dunedin longitudinal study, for example, three-quarters of all 21-year-
olds with psychiatric diagnoses had previously had a mental disorder when
studied between the ages of 11 and 18.
Sex ratio and age of onset
While child and adolescent mental health services tend to see more boys
than girls, epidemiological studies do not show marked gender differences
in the overall rate of psychiatric disorder: males are more likely than
females to have a disorder before puberty, but the reverse is true after
puberty. The sex ratio varies markedly with the type of problem (see
Table 3.1). The usual age of onset also varies markedly from problem to
problem. Some problems characteristically beginning early in childhood,
while other adult-type problems are much commoner in adolescence than
in earlier childhood (see Table 3.2). It is tempting to suppose that these
striking differences in sex ratio and age of onset are important clues to the
underlying aetiology or pathogenesis, but sadly the clues remain largely
undeciphered.
Table 3.1Sex ratio of disorders
Marked male excess Male=female Marked female excess
Autistic spectrum disorder Depression (pre-pubertal) Specific phobias, for example, insects
ADHD Selective mutism Diurnal enuresis
Disruptive behavioural disorders School refusal Deliberate self harm (post-pubertal)
Juvenile delinquency Depression (post-pubertal)
Completed suicide Anorexia nervosa
Tic disorders, for example,
Tourette
Bulimia nervosaNocturnal enuresis in older
children and adolescents
Specific developmental
disorders, for example,
language and reading
disorders