The diagnostic dilemma in psychiatry.- The nature - nurture controversy.- Methods of clinical genetics in psychiatry.- General remarks on twin origination.- Biology of twin origination.- Frequency of twin births.- The twins method in research.- Characteristics of the twin constellation.- The methodological basis of twin research.- Classical twin studies in schizophrenia research.- Severity of illness and concordance rates.- Concordance/discordance and familial pattern.- Concordance/discordance and handedness.- Concordance rate of dizygotic twins and other sibling morbidity rate.- Monozygotic twins reared apart.- Frequency of schizophrenia in twins.- Studies of monozygotic twin pairs discordant for schizophrenia.- Further questions.- Methodology of a systematic twin study.- Twin survey methodology and zygosity diagnosis.- Methodology of psychiatric diagnostics.- Determining the criteria for concordance/discordance.- Selection of further test variables.- Results.- Results of the twin survey and zygosity diagnosis.- Psychiatric diagnoses.- Demographic data of the subjects.- Concordance rates.- Case studies.- Summary of the family history.- Birth history in the intra-pair comparison.- Role assignment in the twin constellation.- Handedness of test persons.- Discussion.- Role assignment (dominant/subordinate) and handedness .....- Comparability of diagnoses.- Demographic data in polydiagnostic comparison.- Varying genetic dispositions.- The absence of schizophrenic subgroups among twins.- The influence of pre- and perinatal complications.- The role of prenatal developmental disorders.- Acknowledgements.