With a new approach and fortified by a wealth of experimental findings this book reverberates the hereditary-environment discussions of earlier decades. Little, if any, new data were obtained specifically for this research. Instead the author utilized available results from numerous longitudinal studies in a variety of disciplines.
Testing the Overlap Hypothesis on these studies is the primary contribution of the book. The Overlap Hypothesis used here was formulated by J. E. Anderson who suggested that correlations between scores at two temporal points in longitudinal studies approximate the square root of the ratio obtained by dividing the mean of the first set of scores by the mean of the second. Hypothetical assumptions underlying this formulation are too involved to be included in a review; thus, it may suffice to say that this formula provides a method for comparing actual correlations with those expected on the basis of