THE SUBJECTS RANGED IN AGE FROM 14 TO 20 YEARS AND IN HEIGHT FROM 153 TO 175 CENTIMETERS. ALL HAD COMMITTED SERIOUS OR REPEATED OFFENSES. NONE WERE MENTALLY RETARDED. NO OBVIOUS PHYSICAL ABNORMALITIES WERE DETECTED. THE SCREENING REVEALED NO CHROMOSOME DISORDERS AMONG THE SUBJECTS, AN INDICATION THAT CHROMOSOME ABNORMALITY DOES NOT CONTRIBUTE SIGNIFICANTLY TO THAILAND'S DELINQUENCY PROBLEM. A COMPARISON OF THE FINDINGS OF 12 CHROMOSOME STUDIES IN DIFFERENT COUNTRIES SHOWS THAT THE XYY CHROMOSOME ABNORMALITY IS FOUND MORE FREQUENTLY IN GROUPS SELECTED BOTH FOR UNUSUAL HEIGHT AND MISBEHAVIOR THAN IN GROUPS SELECTED FOR MISBEHAVIOR ALONE. THE DIFFERENCE IN FREQUENCY IS SIGNIFICANT AND SUGGESTS THAT THE XYY PHENOTYPE USUALLY IS ASSOCIATED WITH A COMBINATION OF UNUSUAL HEIGHT, SOME MENTAL IMPAIRMENT, AND AGGRESSIVE OR ANTISOCIAL BEHAVIOR. SUPPORTING DATA AND A LIST OF REFERENCES ARE INCLUDED. (LKM)