CHROMOSOMES ARE THIN THREADS OF GENETIC MATERIAL CONTAINING HEREDITARY INSTRUCTIONS FOR THE GROWTH AND REPRODUCTION OF EVERY CELL IN AN ORGANISM. INSTEAD OF THE NORMAL COMPLEMENT OF 46 CHROMOSOMES, THE CELLS OF SOME MENTALLY RETARDED, TALL CRIMINALS WERE FOUND TO CONTAIN 47, THE EXTRA ONE BEING A Y CHROMOSOME. WHEN THE ASSOCIATION OF THE EXTRA Y CHROMOSOME WITH TALL STATURE, MENTAL RETARDATION, AND AGGRESSIVE BEHAVIOR WAS FIRST MADE, IT HAD A PROFOUND EFFECT ON THE SCIENTIFIC COMMUNITY, THE LEGAL AND MEDICAL PROFESSION, AND THE PUBLIC AT LARGE. STUDIES SOON BEGAN TO APPEAR, AND IT WAS LATER DISCOVERED THAT SOME OF SOCIETY'S MOST NOTORIOUS MURDERERS (E.G., ROBERT PETER TAIT, DANIEL HUGON, JOHN FARLEY) HAD THE EXTRA Y CHROMOSOME. OTHER STUDIES, ESPECIALLY THAT OF A MAN INSTITUTIONALIZED SINCE HIS LATE TEENS FOR A LIFE-LONG MANIFESTATION OF INORDINATELY AGGRESSIVE BEHAVIOR, SEEM TO ADD STILL MORE SUPPORT TO THE RELATIONSHIP BETWEEN THE EXTRA CHROMOSOME AND AGGRESSION. HOWEVER, THE EXISTENCE OF AN XYY SYNDROME IS ARGUABLE, ESPECIALLY IN LIGHT OF THE FACT THAT MANY MEN WITH AN EXTRA Y CHROMOSOME HAVE BEEN FOUND TO LEAD ESSENTIALLY UNREMARKABLE, QUIET LIVES. DATA FROM STUDIES ALL OVER THE WORLD, WHILE GENERALLY AFFIRMING THE AGGRESSIVE TENDENCIES OF XYY MALES, HOLD VARIOUSLY ON THE INCIDENCE OF XYY'S AMONG THE POPULATIONS OF OFFENDERS INCARCERATED BOTH IN PRISONS AND MENTAL INSTITUTIONS. WHATEVER INCIDENCE MAY EVENTUALLY BE DETERMINED, IT IS SAFE TO PREDICT THAT PERSONS WITH AN EXTRA Y CHROMOSOME WILL CONSTITUTE AN INSIGNIFICANT PROPORTION OF THE PERPETRATORS OF VIOLENT CRIMES. REFERENCES AND TABULAR DATA ARE INCLUDED.