OF THE 342 PATIENTS AT THE FACILITY, 249 HAD BEEN COMMITTED BY THE COURTS AND 93 HAD BEEN TRANSFERRED FROM OTHER INSTITUTIONS. NEARLY ALL HAD CRIMINAL RECORDS. A CHROMOSOME CHECK OF 315 PATIENTS FOUND 9 WITH THE XYY SEX CHROMOSOME ABNORMALITY. THE PERSONALITIES, CRIMINAL RECORDS, AND FAMILY BACKGROUNDS OF THESE MEN WERE ASSESSED AND COMPARED WITH THOSE OF A CONTROL GROUP OF 18 CHROMOSOMALLY NORMAL (XY) PATIENTS. ALL OF THE XYY PATIENTS SUFFERED FROM A SEVERE PERSONALITY DISORDER CHARACTERIZED BY EXTREME INSTABILITY AND IRRESPONSIBILITY. SEVEN OF THE NINE WERE MENTALLY SUBNORMAL, WITH IQ'S (INTELLIGENCE QUOTIENTS) OF 60 TO 80. THEIR CRIMINAL RECORDS AND BEHAVIOR IN THE HOSPITAL INDICATED THAT THE XYY PATIENTS WERE LESS VIOLENT THAN WERE CONTROL PATIENTS (WHO ALSO SUFFERED FROM SEVERE PERSONALITY DISTURBANCES). THE XYY PATIENTS BEGAN THEIR CRIMINAL ACTIVITIES AT A VERY EARLY AGE, AN AVERAGE OF 5 YEARS EARLIER THAN CONTROLS. THE XYY PATIENTS HAD NO SIGNIFICANT FAMILY HISTORY OF CRIME OR MENTAL ILLNESS. THEIR CRIMINAL BEHAVIOR HAD PROVED RESISTANT TO CONVENTIONAL TRAINING AND TREATMENT. THE FINDINGS SUGGEST THAT THE EXTRA Y CHROMOSOME RESULTED IN SEVERELY DISORDERED PERSONALITIES IN THESE SUBJECTS, WHICH LED THEM INTO CONFLICT WITH THE LAW. A LIST OF REFERENCES AND SUPPORTING DATA ARE PROVIDED. (LKM)