In a study of 404 eight-year-old boys and girls from Finland (one Finnish- and one Swedish-speaking group) Poland, and Chicago (one white and one black group) blacks were found to be more physically, verbally, and indirectly aggressive (passive-aggressive stuff) than the other groups. This was true for boys and girls, and regardless of whether self- or peer-estimations were used. (The black kids were inner-city).
The racial gap was larger for girls. This is consistent with the analysis of adults in the MIDUS study I described earlier.
According to this study, broad heritability for aggression among adults is around 70%.
(Karin Osterman, Kaj Bjorkqvist, Kirsti M.J. Lagerspetz, Ari Kaukiainen, L. Roweli Huesmann, and Adam Fraczek. 1994. Peer and Self-Estimated Aggression and Victimization in 8-Year-Old Children From Five Ethnic Groups. AGGRESSIVE BEHAVIOR, 20, 411-28.)