Epidemiology 41explanation is that this excess of autism is due to prenatal infections
with viruses that immigrant mothers had not previously been exposed
to in their countries of origin.
Even from these few examples, it is evident that many possible explana-
tions need to be considered when cross-cultural differences are found.
Time trends
A growing body of empirical evidence supports the pessimists’ view that
things are getting worse as the years go by. Since diagnostic criteria and
research tools have changed over time, it is obviously hard to establish
whether some particular problem has really become more common, or
whether clinicians are setting their threshold lower nowadays, or getting
better at recognising particular types of problem. Taking account of these
methodological issues, it still seems likely that young people over the past
50 years have become increasingly prone to behavioural problems, crime,
substance abuse, depression and suicide – problems which are known to be
sensitive to psychosocial circumstances. For eating disorders, the evidence
for a real rise in prevalence is suggestive but not conclusive. For autism,
it is more likely but not proven that the substantial increase in accepted
prevalence in the past 30 years (from less than 1 in 1,000 to around 1%)
is due to better recognition and ascertainment rather than a large rise in
the number of individuals with the disorder.
Subject review
Costello EJet al.(2005) 10-Year Research Update Review: The Epidemiol-
ogy of Child and Adolescent Psychiatric Disorders: I. Methods and Public
Health Burden.Journal of the American Academy of Child and Adolescent
Psychiatry 44 , 972–986.
Costello EJet al.(2006) 10-Year Research Update Review: The Epidemi-
ology of Child and Adolescent Psychiatric Disorders: II. Developmental
Epidemiology.Journal of the American Academy of Child and Adolescent
Psychiatry 45 , 8–25.
Further reading
Collishaw Set al.(2004) Time trends in adolescent mental health.Journal
of Child Psychology and Psychiatry 45 , 1350–1362. (Evidence that mental
health has worsened over the last quarter of the twentieth century, and
that this was not simply a methodological artefact.)
Fleitlich-Bilyk B, Goodman R. (2004) The prevalence of child psychiatric
disorders in south east Brazil.Journal of the American Academy of Child and
Adolescent Psychiatry 43 , 727–734.
Goodman Aet al. (2010) Why do British Indian children have an apparent
mental health advantage?Journal of Child Psychology and Psychiatry 51 ,
1171–1183.