Background. Several studies during recent years have reported
an increased occurrence of schizophrenia in selected immigrant groups.
However, difficulties in establishing the population denominator for immigrant
rates of mental disorder, selective referral for treatment, and other
problems may have influenced such results. The current study aims at testing
empirically the influence of some of these methodological problems.
Methods. Using nationwide case register data from Denmark,
diagnosis specific first-admission rates were compared between persons
born in Denmark and other countries respectively.
A case–control design was used to compare schizophrenia risk
between different immigrant groups.
Results. Incidence rates of mental disorders among non-Danish
residents calculated from admission data showed increased rates of schizophrenia
(RR 1·7) and non-affective functional psychoses (RR 1·9).
A case–control analysis utilizing non-psychotic admissions as control
for schizophrenic admissions yielded essentially the same result, thus
excluding selective referral as the
sole explanation of the increased schizophrenia rate. However, this was
almost exclusively due
to increased rates in individuals born in countries neighbouring on Denmark.
Conclusions. While selective risk factors may be operating
various groups of immigrants, caution
should be warranted in the interpretation of immigrant studies as large
portions of transient visitors
may obscure actual rates of mental disorders.