Another creative method has made use of the natural experiment of family migration to demonstrate gene-environment interaction: The high rate of schizophrenia among African-Caribbean individuals who emigrated to the United Kingdom is presumed to result from gene-environment interaction. Parents and siblings of first-generation African-Caribbean probands have risks of schizophrenia similar to those for white individuals in the area. However, the siblings of second-generation African-Caribbean probands have markedly elevated rates of schizophrenia, suggesting that the increase in schizophrenia rates is due to an interaction between genetic predispositions and stressful environmental factors encountered by this population (Malaspina et al. 1999, Moldin & Gottesman 1997). Although family studies provide a powerful design for demonstrating gene-environment interaction, there are limitations to their utility. High-risk studies are very expensive to conduct because they require the examination of individuals over a long period of time. Additionally, a large number of high-risk individuals must be studied in order to obtain a sufficient number of individuals who eventually become affected, due to the low base rate of most mental disorders. Because of these limitations, few examples of high-risk studies exist.
