The Gene Illusion is a 2003 book by clinical psychologist Jay Joseph, in which he challenges the evidence underlying genetic theories in psychiatry and psychology. Focusing primarily on twin and adoption studies, he attempts to debunk the methodologies used to establish genetic contributions to schizophrenia, criminal behaviour, and IQ. In the nature and nurture debate on the causes of mental disorders, Joseph's criticisms of genetic research in psychiatry have found their place among those who argue that the environment is overwhelmingly the cause of these disorders, particularly with psychiatry critic Jonathan Leo, and with Oliver James. Some of the conclusions of The Gene Illusion have been criticized in book reviews by spinal surgeon Daniel Hanson, M.D., and psychologist F.M. Spinath.