THE THREE VARIABLES TESTED WERE AGGRESSIVENESS, INTELLIGENCE, AND HEIGHT. SINCE XYY'S TEND TO BE TALL, THE STUDY SAMPLE WAS DRAWN FROM ALL MEN IN THE TOP 15 PER CENT OF THE HEIGHT DISTRIBUTION OF THE DANISH MALE POPULATION BORN IN COPENHAGEN BETWEEN JANUARY 1, 1944 AND DECEMBER 31, 1947 INCLUSIVE. A TOTAL OF 4139 MEN WERE INTERVIEWED AND TESTED FOR SEX CHROMOSOME ANOMALIES. DATA WAS ALSO RECORDED ON HEIGHT, CONVICTIONS FOR CRIMINAL OFFENSES, LEVEL OF INTELLECTUAL FUNCTIONING AS INDICATED BY SCORES ON AN ARMY SELECTION TEST AND EDUCATIONAL ATTAINMENT, AND PARENTAL SOCIAL CLASS AT THE TIME OF THE SUBJECT'S BIRTH. TWELVE XYY'S AND 16 XXY'S WERE IDENTIFIED, WITH THE REMAINDER OF THE SAMPLE SERVING AS CONTROLS. ANALYSIS OF STUDY DATA REVEALED THAT XYY'S WERE NO MORE LIKELY TO COMMIT CRIMES OF VIOLENCE THAN XY'S AND THAT THERE WAS A STATISTICALLY SIGNIFICANT DIFFERENCE IN XYY'S CONVICTED OF CRIMES, THE HYPOTHESIS THAT INTELLIGENCE IS A MEDIATING VARIABLE IN THE RELATION BETWEEN THE PRESENCE OF AN EXTRA Y AND ANTISOCIAL BEHAVIOR WAS SUPPORTED. THE HYPOTHESIS THAT HEIGHT MAY BE AN INTERVENING VARIABLE WAS NOT CONFIRMED. REFERENCES ARE INCLUDED.