Van Der Waals agrees with Freud that both heredity and environment are important in the psychic development of the individual. He emphasizes that investigations of each are in fact mutually dependent, rather than mutually exclusive, as seems so often to be assumed. The most fruitful field of investigation for genetic psychology in humans is the study of identical (monozygotic) twins; most of the paper consists of a clear and critical review of the literature on this subject. It is concluded that 'the supposition of an innate, basic character on a hereditary basis is strongly supported by the results of the investigations of twins'. However, 'there is still much to learn from a prolonged investigation of twins and families, especially when conducted by scientists who are conversant with analytical developmental psychology'. We still do not know the relationship between endowment ('basic character') and mental illness or maldevelopment.