A SCREENING OF NEWBORN MALES IN SINGAPORE FOUND THE INCIDENCE OF THE 47,XXY CHROMOSOME ABNORMALITY IN THE GENERAL POPULATION TO BE ABOUT 1 IN 3,000 AND THAT OF THE 47,XYY ABNORMALITY TO BE ABOUT THE SAME OR LESS. THE SCREENING OF INMATES AT CHANGI PRISON FOUND ONE XXY MALE AND ONE XYY MALE, AN INCIDENCE NOT SIGNIFICANTLY GREATER THAN THAT IN THE GENERAL POPULATION. BOTH CHROMOSOMALLY ABNORMAL INMATES WERE OF LOW INTELLIGENCE AND FROM SOCIOECONOMICALLY DEPRIVED BACKGROUNDS. SEVEN OTHER CASES OF XYY CONFIGURATION AND SIXTY-TWO CASES OF XXY CONFIGURATION WERE BROUGHT TO THE ATTENTION OF THE GENETICS LABORATORY THAT PERFORMED THE SCREENING EITHER FOR MENTAL RETARDATION OR INFERTILITY, NOT BECAUSE THE SUBJECTS HAD COMMITTED CRIMES. IT SEEMS THAT THE INCREASED INCIDENCE OF XXY AND XYY ABNORMALITIES FOUND IN WESTERN STUDIES OF PRISONERS IS DUE TO SELECTION BIAS: THESE STUDIES GENERALLY HAVE BEEN PERFORMED WITH INMATES OF PRISON PSYCHIATRIC FACILITIES RATHER THAN WITH INMATES IN THE GENERAL PRISON POPULATION. A DANISH STUDY OF OVER 4,000 MALES SUPPORTS THE VIEW THAT CHROMOSOMALLY NORMAL AND ABNORMAL MALES ARE EQUALLY LIKELY TO COMMIT CRIMES IF THEY ARE MENTALLY RETARDED AND SOCIOECONOMICALLY DISADVANTAGED. A LIST OF REFERENCES IS INCLUDED.