CONCORDANCE IS DEFINED AS PERSISTENT MANIFESTATION OF SIMILAR TRAITS IN BOTH TWINS. AN UNSELECTED SAMPLE OF 138 PAIRS OF SAME-SEXED MALE TWINS AGE 40-50 YEARS, WHO WERE OBTAINED THROUGH THE NATIONAL TWIN AND CRIMINAL REGISTERS OF NORWAY, WERE PERSONALLY TESTED FOR IDENTICALNESS, INTERVIEWED, AND INVESTIGATED. EMPLOYING A BROAD CONCEPT OF CRIME - INCLUDING VIOLATION OF THE MOTOR VEHICLE LAW AND TREASON DURING WORLD WAR TWO - CONCORDANCE WITH RESPECT TO REGISTERED CRIME WAS SLIGHTLY HIGHER IN IDENTICAL THAN IN FRATERNAL TWINS. (22.4 PER CENT TO 18.0 PER CENT). WITH A MORE STRICT CONCEPT OF CRIME, CONCORDANCE WAS 25.8 PER CENT IN IDENTICAL TWINS AND 14.9 PER CENT IN FRATERNAL TWINS. SINCE IDENTICAL PAIRS EXPERIENCE A MORE SIMILAR UPBRINGING THAN FRATERNAL PAIRS, GROUPS OF BOTH TYPES OF TWINS WHO BY AND LARGE HAD BEEN EXPOSED TO THE SAME TYPE OF ENVIRONMENTAL INFLUENCES IN CHILDHOOD AND ADOLESCENCE WERE COMPARED. IN THIS COMPARISON, THE DIFFERENCE IN CONCORDANCE ALMOST COMPLETELY DISAPPEARED. RESEARCHERS SUGGEST THAT THESE FINDINGS SUPPORT THE VIEW THAT HEREDITARY FACTORS ARE OF NO SIGNIFICANT IMPORTANCE IN THE ETIOLOGY OF COMMON CRIME. A REVIEW OF THE OLDER LITERATURE INDICATES THAT THESE OBSERVATIONS AND CONCLUSIONS ARE AT VARIANCE WITH MOST OF THE EARLIER TWIN STUDIES IN CRIMINALITY BECAUSE PREVIOUS STUDIES IN THE FIELD, DUE TO SAMPLING ERRORS AND UNRELIABLE IDENTICALNESS DIAGNOSIS, GAVE RESULTS IN WHICH THE GENETIC FACTOR WAS OVER-ESTIMATED. REFERENCES ARE INCLUDED. (AUTHOR ABSTRACT MODIFIED)