The diagnostic dilemma in psychiatry.- The nature - nurture controversy.- Methods of clinical genetics in psychiatry.- General remarks on twin origination.- Biology of twin origination.- Monozygotic and dizygotic twins.- The effect of differences in intrauterine environment.- Frequency of twin births.- Frequency of monozygotic twins.- Frequency of dizygotic twins.- Decline of dizygotic twin births in civilized countries.- The twins method in research.- Characteristics of the twin constellation.- The methodological basis of twin research.- Classical twin research.- Methodological variants.- Definition of terminology.- 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.- Prenatal developmental disorders.- Peri- and postnatal findings.- Neuroradiological findings.- Biochemical and other findings.- Pre-psychotic personality differences.- Children of discordant monozygotic twins.- Leonhard's findings on twins.- Further questions.- Methodology of a systematic twin study.- Twin survey methodology and zygosity diagnosis.- Methodology of psychiatric diagnostics.- Determining the criteria for concordance/discordance.- Criteria for concordance/discordance in DSM-III-R.- Criteria for concordance/discordance in ICD 10.- Criteria for concordance/discordance in the Leonhard classification.- Selection of further test variables.- Results.- Results of the twin survey and zygosity diagnosis.- Psychiatric diagnoses.- Demographic data of the subjects.- Age, age at onset and duration of illness.- Social situation (school, occupation, marital status).- Concordance rates.- Pair-wise calculation of the concordance rates.- Proband-wise calculation of the concordance rates.- Case studies.- Monozygotic concordant pairs.- Monozygotic discordant pairs.- Dizygotic concordant pairs.- Dizygotic discordant pairs.- 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.