Recent studies hypothesize causal connections between the XYY genotype in male subjects and criminal behavior: the current state-of-the-art in this area both in Italy and in other countries indicates a reluctance to accept the findings of genetics research on the correlation of the XYY syndrome and deviance. Acceptance of this theory would involve wide-ranging ramifications in criminal law, penology, and criminology. Criminal responsibility would be especially implicated and the problem of designing a reliable methodology to discriminate between relevant and irrelevant chromosomal abnormalities in criminogenic and criminological terms would arise. The extremely complex phenomenon of criminal behavior should be addressed on a multidisciplinary basis, especially refraining from monocausal etiological theories. In addition to the necessity of proving the reality of the XYY syndrome, criminologists must realize the need for extending their research on the criminogenic effects of the extra-Y factor to noninstitutionalized (e.g., outside of prisons and psychiatric hospitals) subjects, using standardized methods to assure scientifically reliable findings. At present, a wait-and-see attitude in this area is advisable. Extensive footnotes contail bibliographic references. Italian, French, English, Spanish, and German abstracts are appended. --in Italian.