The prevalence of aggression as a predominantly male behavior pattern has often been noted. The nature and perceived extent of sex differences in aggression have recently revived interest in speculation that such differences are attributable to factors inherent in the biological basis of sex differentiation. This paper will critically examine the empirical and theoretical basis for Maccoby and Jacklin's contention that males are biologically predisposed toward aggressive behavior. The literature to be examined includes: (1) cross-cultural studies of children's behavior, (2) the behavior of nonhuman primates, (3) sex hormones and aggression, and (4) early learning influences in child development. Contrary to Maccoby and Jacklin's assertions, these first 3 research traditions are found to indicate no biological predisposition toward aggression in human males. Furthermore, the pattern of existing evidence suggests that the gender-dimorphic nature of aggression is reliably observable in children's spontaneous behavior only after the age of 5 years. This finding presents great difficulty for the biological theory as proposed by Maccoby and Jacklin. Suggestions are presented for an interactional theory of the etiology of observed sex differences in adult aggression.