The authors examined the effects of task complexity and posture on laterality and compared lateralization during different tasks in 9 captive grey-cheeked mangabeys (Lophocebus albigena) during spontaneous food processing and 3 experimental tasks. Comparisons with data of red-capped mangabeys (Cercocebus torquatus torquatus; semiterrestrial species) were used. Less than half the monkeys were lateralized for simple everyday activities, but 6 were lateralized for complex daily activities. Moreover, all the monkeys were lateralized when performing experimental tasks. Laterality at the group level was found for the bipedal task: Mangabeys were right-handed. Complexity of tasks increases laterality at the individual level. Significant differences between the 2 species of mangabeys were found, allowing us to confirm that postural constraints are a major factor in the emergence of group handedness.