THE PURPORTED LINK BETWEEN AN XYY CHROMOSOMAL CONSTITUTION AND CRIMINALITY CAME TO LIGHT IN 1965 WHEN RESEARCHERS AT WESTERN GENERAL HOSPITAL IN EDINBURGH, SCOTLAND, PUBLISHED THEIR FINDINGS ON 197 MENTALLY ABNORMAL INMATES UNDERGOING TREATMENT IN A SPECIAL SECURITY INSTITUTION. ALL HAD DANGEROUS, VIOLENT, OR CRIMINAL PROPENSITIES, AND SEVEN WERE FOUND TO BE XYY'S. SINCE THE PUBLICATION OF THESE FINDINGS, DOZENS OF OTHER REPORTS OF SIMILAR XYY SYNDROME OFFENDERS HAVE SURFACED. THE EXTRA Y CHROMOSOME SEEMS TO POSSESS AN ELEVATED AGGRESSIVNESS POTENTIAL, WHEREAS THE X APPEARS TO CONTAIN A HIGH GENTLENESS COMPONENT. IT APPEARS PROBABLE THAT THE ORDINARY AGGRESSIVENESS OF A NORMAL XY MALE IS DERIVED FROM HIS Y CHROMOSOME AND THAT THE ADDITION OF ANOTHER Y MAY, UNDER CERTAIN CONDITIONS, FACILITATE THE DEVELOPMENT OF AGGRESSIVE BEHAVIOR. HOWEVER, AS WITH ANY CHROMOSOME, THIS DOES NOT MEAN THAT THE GENES ARE RESPONSIBLE FOR THE END EFFECT. GENES, CHROMOSOMES, OR HEREDITY ARE NOT TO BE INTERPRETED AS EQUIVALENT TO FATE. ON THE CONTRARY, HEREDITY IS THE EXPRESSION, NOT OF WHAT IS GIVEN ONE'S GENES AT CONCEPTION, BUT OF THE RECIPROCAL INTERACTION BETWEEN THE INHERITED GENES AND THE ENVIRONMENT TO WHICH THEY HAVE BEEN EXPOSED. BECAUSE THE Y CHROMOSOME CARRIES RELATIVELY FEW GENES, MAJOR ABNORMALITIES DO NOT OCCUR IN XYY INDIVIDUALS, ALTHOUGH THEY DO TEND TO SHARE CERTAIN CHARACTERISTICS: TALLNESS, FACIAL ACNE, AND MENTAL DULLNESS. TALLNESS MAY BE THE MOST SIGNIFICANT FACTOR BECAUSE AN XYY CHILD, PREMATURELY TALL, MAY BE SUBJECT TO THE TAUNTS AND JEERS OF HIS PEERS, AND AS SUCH VARIOUSLY DRIVEN EITHER TO AGGRESSION OR WITHDRAWAL. THE FACT THAT THE XYY COMPLEMENT IS KNOWN TO BE ASSOCIATED WITH PERSISTENT ANTISOCIAL BEHAVIOR RAISES AT LEAST TWO QUESTIONS THAT ANY REASONABLE SOCIETY MUST CONSIDER: (1) SHOULD ALL INFANTS AT BIRTH OR SHORTLY AFTER BE CHROMOSOMALLY TYPED, AND (2) HOW SHOULD KNOWN XYY INDIVIDUALS BE DEALT WITH. TABULAR DATA ARE INCLUDED. (KBL)