A STANDARD QUESTIONNAIRE WAS USED TO OBTAIN SOCIOLOGICAL AND CRIMINOLOGICAL DATA ON THE PAIRED SUBJECTS--THESE DATA WERE SUPPLEMENTED BY INFORMATION FROM POLICE, PRISON, PAROLE, AND PROBATION RECORDS--AND THE WECHSLER ADULT INTELLIGENCE SCALE, THE EYSENCK PERSONALITY INVENTORY, AND THE FOULDS AND CAINE EXTRAPUNITIVE-INTRAPUNITIVE SCALE WERE ADMINISTERED TO ALL SUBJECTS TO GATHER INFORMATION ON PSYCHOLOGICAL STATUS. DIFFERENCES BETWEEN PAIRS WERE EVALUATED BY THE WILCOXON TWO TAILED SIGNED RANKS TEST. DIFFERENCES FOUND REGARDING INTELLIGENCE LEVELS AND THE EXTENT OF PREVIOUS PSYCHIATRIC HISTORY FOR XYY'S AND THEIR PAIRED CONTROLS WERE SIGNIFICANT. CONTROLS HAD HIGHER INTELLIGENCE QUOTIENTS AND LESS INCIDENCE OF PREVIOUS PSYCHIATRIC TROUBLE. DIFFERENCES BETWEEN FAMILY AND SCHOOL HISTORY OF SUBJECTS AND CONTROLS WERE NOT STATISTICALLY SIGNIFICANT, AND DIFFERENCES IN EMPLOYMENT HISTORIES, THOUGH INCONCLUSIVE, SEEM TO SUGGEST THE XYY'S HAD BEEN DOING LESS SKILLED LABOR THAN CONTROLS. PHYSICAL EXAMINATIONS REVEALED TREMOR IN TWO XYY'S BUT NONE IN CONTROLS. THE CONCLUSIONS ARE MITIGATED BY THE SMALLNESS OF THE SAMPLE, BUT RESULTS SUGGEST THAT THE XYY CONSTITUTION MAY BE ACCOMPANIED BY THE FOLLOWING: A PAST HISTORY OF MENTAL ILLNESS USUALLY DESIGNATED AS PSYCHOPATHY; LOWER THAN AVERAGE INTELLIGENCE; HIGHER FREQUENCY OF CONVICTIONS; AND LESS ACHIEVEMENT IN COMPARISON WITH FATHER AND BROTHERS. STUDY DATA AND REFERENCES ARE PROVIDED. (DAG)