Genes and environment in mental illness. Perspectives and ideas for future research

Methodological questions in heredity-environment studies are discussed with special emphasis on schizophrenia and severe affective disorder. Mental disorder is a matter of behavioral and experimental manifestations, caused by complex interactions of genes and environment operating over a lifespan. It is therefore necessary to combine different research strategies in order to advance this field. However, progress in research is hampered by problems in classification and definition of phenotype. Accordingly, family, adoption and twin studies have a lot to contribute before molecular genetic research might be carried out.