AbstractSixteen individuals having a 47, XYY chromosome were perfectly identified in a sample drawn from the general population. The findings confirmed previous reports that XYY individuals are taller than average. However, no major evidence was found for contact with law‐enforcement agencies. Detailed psychological tests (including TAT and Rorschach) were conducted in a double blind study on seven XYY subjects together with a control group, matched for age height, educational level and socioeconomic status. The chromosomal abnormality tended to be associated with a psychological profile somewhat different from that found in the control group. The XYY individual appeared to have a certain degree of psychomotor difficulty, a lower level for emotional control and a tolerance threshold for inappropriate responses (eg. aggression) which was lower than that of controls. Use of immediate gratification and low control over frustration were also inferred from the test data. The self concept of the XYY group in general, was found to be less mature and often associated with high levels of anxiety. There appeared to be a lack of integration of aggression into the XYY subjects' perception of reality.