AbstractFluctuating asymmetry (FA) represents non‐directional deviations from perfect symmetry in morphological characters. Prenatal stressors contribute to the imprecise expression of symmetrical phenotypes and display of agonistic behavior in children and adults. Because prenatal stress affects neurological function and overt behavior, and FA is often used as a marker for prenatal stress, we hypothesized that high FA would be associated with elevated levels of human reactive aggression. Data were collected from 100 males and females (average age = 20.1) on FA of 11 bilateral traits (second, third, fourth, and fifth digit length, palm height, wrist diameter, elbow width, ear height, ear width, foot breadth, and ankle circumference). Additional relationships were also investigated among FA, testosterone (T), and type of provocation to test a comprehensive aggression model. Experimental participants solicited donations for a fictitious charity organization via telephone and selected follow‐up letters after the calls. High FA and T values were independently associated with elevated reactive aggression (force of terminating the call) under low provocation in males, and under high provocation in females. In the absence of phenotypical markers, i.e., FA and T, sex differences in response to provocation disappeared and a “passive‐aggressive” response emerged. Both males and females selected hostile follow‐up letters, but showed low reactive aggression when terminating the call under high provocation. This pattern was reversed under low provocation. Taken together, these data suggest that individuals' phenotype and intensity of provocation are important determinants of individual and sex differences in aggression. Am. J. Hum. Biol. 16:458–469, 2004. © 2004 Wiley‐Liss, Inc.