THE FIRST REPORT OF AN XYY HUMAN MALE APPEARED IN 1961, AND SINCE THAT TIME SCIENTISTS HAVE ATTEMPTED TO ESTABLISH A RELATIONSHIP BETWEEN THE CHROMOSOME ABNORMALITY AND ANTISOCIAL BEHAVIOR. STUDIES BY TELFER, BROWN, JACOBS, CARSTAIRS, AND OTHERS HAVE DEALT WITH CHARACTERISTICS OF XYY'S. TALLNESS, LONG ARMS, FACIAL ACNE, MILD MENTAL RETARDATION, AGGRESSIVE BEHAVIOR AND ANTISOCIAL BEHAVIOR AT AN EARLY AGE ARE CHARACTERISTICS OFTEN REFERRED TO IN XYY RESEARCH. LATER, RESEARCHERS BEGAN INVESTIGATIVE WORK ON THE FREQUENCY OF THE CHROMOSOME IN THE MALE POPULATION. A NUMBER OF SCREENING DEVICES HAVE BEEN USED TO SELECT INSTITUTIONALIZED ADULT SUBJECTS--E.G., HEIGHT; NONINSTITUTIONALIZED POPULATIONS UNSCREENED ON THE BASIS OF HEIGHT HAVE ALSO BEEN SURVEYED IN A NUMBER OF COUNTRIES. EVIDENCE RESULTING FROM ALL OF THIS RESEARCH IS INCONCLUSIVE, AND ALMOST EVERY ASPECT OF THE MORPHOLOGY OF THE XXY AND XYY INCIDENCE REQUIRES ADDITIONAL SYSTEMATIC INVESTIGATION. THE USE OF XYY CHROMOSOMES IN INSANITY DEFENSES HAS ALREADY HAD SOME INFLUENCE ON THE OUTCOME OF CRIMINAL TRIALS IN THE U.S., AUSTRALIA, FRANCE, AND WEST GERMANY. EVEN SO COURTS ARE USUALLY SKEPTICAL OF THE LINK BETWEEN THE CHROMOSOME AND CRIMINAL BEHAVIOR, AND THE XYY DEFENSE HAS NOT BEEN SUCCESSFUL IN OBTAINING ACQUITTALS AND LESSER SENTENCES. IN FACT, THE XYY DEFENSE COULD CONCEIVABLY HAVE THE REVERSE EFFECT, RESULTING IN INDEFINITE SENTENCES FOR PERSONS WITH XYY'S, ELIMINATION OF PAROLE ELIGIBILITY, OR ULTIMATELY THE IMPRISONMENT OF PERSONS HAVING GREATER HEIGHT AND LARGER BUILDS. IN THE PRESENT STATE OF KNOWLEDGE, NO ONE SHOULD BE SUBJECTED TO ANY ADDITIONAL SANCTION OR SUFFER ANY OTHER DISABILITY FOR HAVING AN XYY ABNORMALITY. REFERENCES AND TABULAR DATA ARE INCLUDED. (DAG)