SENSATIONAL PUBLICITY CONCERNING MURDERS COMMITTED BY MEN WITH THE XYY SEX CHROMOSOME AND THE INTRIGUING DISCOVERY OF A DISPROPORTIONATE NUMBER OF XYY CARRIERS IN PENAL INSTITUTIONS HAVE INCORRECTLY LED MANY PERSONS TO BELIEVE THAT THE XXY TRAIT PREDISPOSES A CARRIER TO VIOLENCE. STUDIES HAVE INDICATED THAT NEARLY 4 PERCENT OF A PRISON POPULATION MAY HAVE THE XYY TRAIT, IN CONTRAST TO LESS THAN 0.1 PERCENT IN THE GENERAL POPULATION. COMPARISONS OF POPULATIONS, HOWEVER, ARE MORE MEANINGFUL IF THE XYY MALES ARE COMPARED WITH MALES EXHIBITING KLINEFELTER'S (XXY) SYNDROME. FROM A GROUP OF 200 TALLER PRISON INMATES, 4 HAD THE XYY TRAIT AND 3 HAD THE XXY. AMONG 103 MEN IN A HOSPITAL FOR THE CRIMINALLY INSANE, 4 KLINEFELTER'S AND 3 XYY CARRIERS WERE FOUND, AND 6 KLINEFELTER'S AND NO XYY WERE FOUND AMONG 347 MENTAL PATIENTS. WHILE ONLY TWO OF THE ABNORMAL PATIENTS WERE PREVIOUSLY SUSPECTED OF HAVING GENETIC DISORDERS, THE XYY SUBJECTS WERE ABOVE AVERAGE HEIGHT. THE XYY AND XXY GROUPS DEMONSTRATED FEW DIFFERENCES IN CRIMINAL RECORD, FAMILY BACKGROUND, AND PSYCHIATRIC EVALUATION. THE XYY GROUP DID EXHIBIT A SIGNIFICANTLY LOWER AGE OF FIRST CONVICTION THAN THE KLINEFELTER'S GROUP. THE EFFECTS, HOWEVER, OF AN EXTRA SEX CHROMOSOME MIGHT BE EXPECTED TO VARY CONSIDERABLY IN DIFFERENT INDIVIDUALS, WITH CONSIDERABLE OVERLAP IN THE PHYSICAL AND SOCIAL CHARACTERISTICS OF DIFFERENT ABNORMAL GROUPS. IT IS POSSIBLE THAT SOME XYY CRIMINALITY MAY RESULT FROM EARLY STIGMATIZATION OF A BOY WITH THE EXTRA Y CHROMOSOME DUE TO HIS GREAT HEIGHT. INCREASING NUMBERS OF REPORTS INDICATE THE PRESENCE OF 'NORMAL' XYY'S IN THE GENERAL POPULATION. REFERENCES ARE PROVIDED. (TWK)