Psychiatry Research, 48:229-242 Elsevier 229 Heritability of Irritable Impulsiveness: A Study of Twins Reared Together and Apart Emil F. Coccaro, C.S. Bergeman, and Gerald E. McClearn Received July 22, 1992; revised version received Januar_v 26. 1993; accepted February 18, 1993 Abstract. The heritability of self-reported personality traits related to impulsive- ness, irritability, and the inhibition of assertive or aggressive behavior was examined in up to 500 healthy monozygotic and dizygotic twin pairs raised together or apart. Two factors related to “(lack of) assertiveness/aggression” (Factor 1) and “impulsive irritability”(Factor II) were examined using traditional and model-fitting procedures. Results of model-fitting procedures were consistent with a genetic, but not a shared environmental, influence for both factors. Fur- ther analysis suggested a nonadditive genetic influence for Factor I1 and an additive influence for Factor I. Bivariate model-fitting analyses suggest that self- reported “irritable impulsiveness” and “(lack of) assertiveness/aggressiveness” show substantial, though different, genetic influences. Key Words. Aggression, genetics, personality. A relationship between indices of central neuronal system function and of impulsive- aggressive behavior has been demonstrated repeatedly in humans over the past 15 years. Specifically, inverse relationships between indices of brain serotonin (5hydroxytryptamine, 5-HT) and of suicidal or impulsive-aggressive behavior have been described in psychiatric patients as well as nonpatient volunteers (Coccaro, 1989; Coccaro and Astill, 1990). Temporal stability in indices of central 5-HT function (Coppen et al., 1972; Post et al., 1980; Traskman-Benz et al., 1984; Coccaro et al., 19906) and of impulsive aggression (Olweus, 1979) suggest that these two variables may represent biological and phenomenological manifestations of a stable psychobiological trait in humans. In addition, an increased frequency of depression (Sedvall et al., 1980) alcoholism (Rosenthal et al., 1980; Linnoila et al., 1989), and impulsive-aggressive behaviors (Linnoila et al., 1989; Coccaro et al., in press) in family members of individuals with indices of reduced central 5-HT system function suggests that an abnormality in central 5-HT, and related psychopathology, may be heritable in humans. Emil F. Coccaro, M.D., is Associate Professor and Director, Clinical Neuroscience Research Unit, Department of Psychiatry, Medical College of Pennsylvania at Eastern Pennsylvania Psychiatric Institute, Philadelphia, PA. C.S. Bergeman, Ph.D., is Assistant Professor, Department of Psychology, University of Notre Dame, IN. Gerald E. McClearn. Ph.D.. is Professor, Center for Developmental and Health Genetics. Pennsylvania State University, State College, PA. (Reprint requests to Dr. E.F. Coccaro. Clinical Neuroscience Research Unit, Department of Psychiatry, Medical College of Pennsylvania at Eastern Pennsylvania Psychiatric Institute, 3200 Henry Ave., Philadelphia, PA 19129, USA.) 0165 178 1 / 93/ $06.00 @ 1993 Elsevier Scientific Publishers lreland Ltd