Recent developments in DNA-based techniques may revolutionize the study of human behavioral genetics. However, unless these methods are used with great care, many of the same mistakes which have plagued non-molecular genetic analyses of behavior will reoccur. Errors in the application of genetic approaches and in the interpretation of results have been a common feature of published studies in this field. We review studies in human behavioral genetics, focusing on those using identical twins and DNA-based linkage techniques in order to draw attention to recurrent problems in molecular and non-molecular studies. We suggest possible guidelines for future research in the area of the biological basis of human behavior.