SOME CONCLUSIONS DRAWN FROM SCOTTISH, ENGLISH, AUSTRALIAN, AND AMERICAN STUDIES OF XYY'S USING INSTITUTIONALIZED AND NONINSTITUTIONALIZED POPULATIONS ARE THAT THE PENAL RECORDS OF THE XYY'S SHOW SIGNIFICANTLY FEWER CRIMES OF VIOLENCE AGAINST PERSONS THAN THOSE OF THE CONTROLS; THE MANIFESTATION OF THE DISTURBED BEHAVIOR, AS MEASURED BY THE AGE AT FIRST CONVICTION, APPEARS TO BE EARLIER IN THE XYY'S THAN IN THE CONTROLS; THE FREQUENCY OF CRIME AMONG SIBLINGS OF THE XYY'S IS SIGNIFICANTLY LOWER THAN AMONG THOSE OF THE CONTROLS; THE BEHAVIOR DISORDER RATHER THAN INTELLECTUAL INCOMPETENCE IS ASSOCIATED WITH THEIR CRIMINALITY; AND REPEATED AND PROLONGED ATTEMPTS AT REHABILITATION AND TREATMENT OF THESE INDIVIDUALS HAS NOT BEEN SUCCESSFUL. SEVERAL CONSIDERATIONS MAKE THESE CONCLUSIONS PREMATURE. FOR EXAMPLE, THE LACK OF SIBLING INVOLVEMENT IN CRIME MAY BE APPLIED TO THE 11 46,XY CONTROLS USED IN THE CARSTAIR STUDY; ONLY 7 OF THE 18 FAMILIES STUDIED HAD MORE THAN 1 MEMBER WITH A CONVICTION RECORD. THE PRESENT STAGE OF RESEARCH ON EVALUATION OF REHABILITATIVE TREATMENT IN INSTITUTIONS MAKES IT PREMATURE TO CONCLUDE THAT A SPECIFIC GROUP IS NONTREATABLE. A NUMBER OF METHODOLOGICAL AND CONCEPTUAL CONSIDERATIONS HELPFUL IN INTERPRETING RESULTS ARE PRESENTED. THE CRITERIA USED IN EVALUATING THE Y CHROMOSOME AND THE NUMBER OF CELLS COUNTED TO DETERMINE THE KARYOTYPE VARY CONSIDERABLY AMONG LABORATORIES; MANY OF THE STUDIES WERE NOT CARRIED OUT IN A DOUBLE BLIND DESIGN; IT IS DIFFICULT TO OBTAIN SAMPLES MATCHED FOR ALL SIGNIFICANT CHARACTERISTICS; AND NO STANDARD FORMULA DESIGNATES WHICH CHARACTERISTICS ARE SIGNIFICANT. WITHOUT EXACT KNOWLEDGE OF THE INCIDENCE OF THE XYY IN THE GENERAL POPULATION, THE ARGUMENT CONCERNING THE ELEVATED FREQUENCY OF THESE INDIVIDUALS IN INSTITUTIONS HAS LITTLE MEANING. RESEARCH IS NEEDED REGARDING COMPARISONS OF THE DEVELOPMENT OF ANTISOCIAL BEHAVIOR IN GROUPS OF 46,XY AND 47,XYY INDIVIDUALS, THE MECHANISMS WHICH PRODUCE THE DOUBLE Y CONDITION, AND THE DEFINITE SPECIFICATION OF DIFFERENT CLASSES OF AGGRESSIVE BEHAVIOR. REFERENCES ARE PROVIDED. (DAG)