Studies on laterality have been developed in many animal species. These studies contribute to a better understanding of the origins of human brain asymmetry and of its adaptive value. Two main theories have been developed to explain primate handedness: Mac Neilage et al.’s “postural origins theory” (1987) and Fagot and Vauclair’s “complexity task theory” (1991). To test these theories, we studied manual laterality in 19 red-capped mangabeys (10 males and 9 females – 7 juveniles and 12 adults) in different experimental or natural situations. In experimental conditions, tasks with different postures and degrees of complexity were used. An analysis based on the number of lateralized individuals, the number of left and righthanders, the handedness index (HI) and the strength of laterality, was performed. Adults were right-handers for hanging task, but no significant bias was found for juveniles. Moreover the strength of adults’ laterality was higher for hanging and bimanual tasks. Females were righthanders for hanging task; males were left-handers for bimanual task. The number of righthanders and left-handers increased for adults and juveniles respectively, with task complexity. The number of right-handed females increased with the task complexity. The number of lateralized subjects and the strength of laterality, whatever sex and age, increased with task complexity.