Heredity and Environment in the Functional Psychoses involves an epidemiological clinical study of twins carried out at the University Psychiatric Institute in Oslo. The author reviews the history and nosological aspects of psychiatry. In his study of twin genetics, the author seeks to answer the questions "What is inherited?" and "How is it inherited?" In his review of 342 pairs of twins in the Central Register of Psychosis, his study had the following aims: to obtain true concordance figures for all types of functional psychoses; to study problems pertaining to nosology; and to study a larger sample of discordant pairs in order to clarify crucial environmental factors. His findings show that concordance figures for schizophrenia are 25-38 per cent in monozygotic twins and 4-10 per cent ion dizygotic twins. He points out that the difference in such concordance rates between the two types of twins is statistically significant. The data supports the (weak) assumption of a genetic factor in the etiology of schizophrenia. Behavioral scientists, psychiatrists, child psychologists, and researchers involved in genetics will find this book helpful and informative.