Few would dispute the truth of the statement "People are different" but there is much controversy over "why". This book explains the methods used to understand human variation, and extends them far beyond the primary "nature or nurture" question. After chapters on basic statistics, biometrical genetics, matrix algebra and path analysis, there is a state-of-the-art account of how to fit genetic models using the LISREL package. The authors explain not only the assumptions of the twin methods, but how to test them. The elementary model is expanded to cover sex limitation, sibling interaction, multivariate and longitudinal data, observer ratings and twin-family studies. Throughout, the methods are illustrated by applications to diverse areas such as obesity, major depression, alcohol consumptionm delinquency, allergies and common fears.