THE TWIN METHOD OF STUDYING THE INFLUENCE OF GENETIC FACTORS ON BEHAVIOR CAPITALIZES ON THE FACT THAT MONOZYGOTIC TWINS HAVE ESSENTIALLY IDENTICAL GENETIC STRUCTURES WHILE DIZYGOTIC TWINS ARE NO MORE SIMILAR GENETICALLY THAN SIBLINGS. THE GREATEST WEAKNESS OF THE METHOD FOR DRAWING CONCLUSIONS RELATED TO GENETICS IS THE FACT THAT TWINS ARE USUALLY RAISED TOGETHER. THE THE ADOPTION METHOD ENABLES THE SEPARATION OF GENETIC AND ENVIRONMENTAL FACTORS. NUMEROUS TWIN AND ADOPTION STUDIES ARE REVIEWED THAT LEND CREDENCE TO A HYPOTHESIS INVOLVING GENETIC FACTORS IN THE ETIOLOGY OF PSYCHOPATHY AND CRIMINALITY. IF THIS HYPOTHESIS IS CORRECT, IT IS LIKELY THAT THE OPERATION OF GENETIC INFLUENCES CAN BE RELATIVELY NONSPECIFIC OR CAN BE VIA SOME PHYSIOLOGICAL PREDISPOSING FACTOR, OR BOTH. FEAR REDUCTION THROUGH INHIBITING AGGRESSIVE RESPONSES WAS TESTED IN A LONGITUDINAL STUDY OF 311 INDIVIDUALS THAT INVOLVED SKIN CONDUCTANCE TESTING (ELECTRODERMAL RECOVERY). THE POSSIBILITY OF INHERITING PSYCHOPHYSIOLOGICAL BEHAVIOR WAS ALSO EXPLORED, ALONG WITH THE IMPORTANCE OF THE FAMILY MILIEU. STUDY DATA INDICATED THAT GENETICS MAY PLAY A PARTIAL PREDISPOSING ROLE IN THE ETIOLOGY OF ANTISOCIAL BEHAVIOR, WITH GENETIC INFLUENCES MORE MARKED WHERE ADOPTEES OR THEIR FATHERS ARE HIGHLY RECIDIVISTIC. WHEN THE ENVIRONMENTAL PUSH TOWARD ANTISOCIAL BEHAVIOR WAS APPARENTLY MINIMAL, PSYCHOPHYSIOLOGICAL VARIABLES MOST CLEARLY EXPRESSED THEMSELVES. REFERENCES AND SUPPORTING DATA ARE INCLUDED. (DEP)