What are the forces shaping who we are, how we live, and how we act? Are we shaped primarily by our environment, or by our genes? These very old questions form the basis of the ''nature-nurture'' debate. Increasingly, we are told that research has confirmed the importance of genetic factors influencing psychiatric disorders, personality, intelligence, sexual orientation, criminality, and so on.

Jay Joseph’s timely, challenging book provides a much-needed critical appraisal of the evidence cited in support of genetic theories. His book shows that, far from establishing the importance of genes, family, twin and adoption research has been plagued by researcher bias, unsound methodology, and a reliance on unsupported theoretical assumptions. Furthermore, he demonstrates how this greatly flawed research has been used in support of conservative social and political agendas. This is particularly evident in Chapter 2, which contains the only in-depth critical review of the history of twin research ever published.

Much of the scientific evidence cited in support of genetic theories has been produced by the fields of behavior genetics and psychiatric genetics. It has been delivered to the public in numerous magazine and newspaper articles, as well as by the authors of several popular books. In particular, studies of twins (both reared together and reared apart) have been cited as providing conclusive evidence supporting the importance of genetic influences on psychological trait differences. The reared-apart twin studies performed by researchers at the University of Minnesota have been the subject of much attention, including stories of individual pairs of ''reared-apart'' identical twins who, it is claimed, displayed remarkable similarities upon being reunited. Joseph shows, however, that both systematic reared-apart twin studies, and stories about individual pairs, prove little if anything about the role of genes.