Variability in the Y chromosome and variability of human behavior

Two kinds of variability in the Y chromosome have been assessed in relation to variability of human behavior. The first is the presence of a supernumerary Y chromosome, which is found in two karyotypic variants, 47,XYY and 48,XXYY. The second is inherited variability in the length of the Y chromosome, which results from variability in the length of the quinacrine-binding fluorescent segment. In this article, I review the evidence that these two kinds of variability in the Y chromosome are related to variability of human behavior, and I indicate the methodological and statistical difficulties that are encountered in assessments of such a relationship. My final purpose is to underscore the need for mapping of the Y chromosome and to elucidate the function of the constitutive heterochromatin that constitutes its fluorescent segment so that progress can be made in the study of the Y chromosome and the possible effect of its variability on behavior.