In this series of articles, leading American geneticists report on their studies. In recent years it has been generally recognized that even in those types of mental illness in whose origin hereditary factors are known to play an important part, environmental factors are important. This is an area where co-operation between geneticists and psycho-analysts seems to be very promising. So far this has not materialized, but there are several articles in this book which might stimulate such joint research. It is to be hoped that when this important Association returns to this subject again,