Hand preferences for a coordinated bimanual task were assessed in 109 chimpanzees (Pan troglodytes). Hand preference was evaluated for 4 test sessions using bouts and frequencies of hand use to compare the sensitivity of each level of analysis in evaluating individual variation in handedness. Overall, significant population-level right-handedness was found using several different measures of hand use. Handedness indices based on bouts and frequencies were highly and significantly correlated. Moreover, hand preferences were consistent across tests despite efforts to situationally bias preference during each test. Taken together, these data do not support the view that bouts are a better level of analysis for evaluating hand preference. The results further suggest that hand preferences for coordinated bimanual actions are not influenced by situational factors and may reflect an inherent specialization of the left hemisphere for motor skill.The issue of whether nonhuman primates exhibit population-level handedness has been a topic of historical and recent debate (Ettlinger, 1988;Fagot & Vauclair, 1991;MacNeilage, StuddertKennedy, & Lindblom, 1987;Marchant & McGrew, 1991;Ward & Hopkins, 1993;Warren, 1980
NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript sample display the same directional bias in hand use. The historical view held that handedness was bimodally distributed in nonhuman species including primates (Warren, 1980), but this view has recently been challenged by a host of behavioral Studies (see Bradshaw & Rogers, 1993;Hook-Costigan & Rogers, 1997;Hopkins, 1996;Hopkins & Morris, 1993;Lehman, 1993;Ward, Milliken, & Stafford, 1993, for reviews). Of specific interest to this article is the evidence of population-level right-handedness in chimpanzees and other great apes. Some have argued that there is a 2:1 ratio of right-to left-handed individuals (Corballis, 1997;Hopkins, 1999b;Hopkins & Pearson, 2000). In contrast, others have argued that the evidence is weak for population-level right-handedness in apes and that some Studies reporting evidence of population-level right-handedness are methodologically or statistically flawed (McGrew & Marchant, 1997).Presently, the debate over whether great apes (and other nonhuman primates) exhibit population-level hand preference centers on two issues, both of which are subject to different interpretation. One issue is whether population-level right-handedness is restricted to captive populations of apes, and this issue is not addressed in this article. The second issue, which is addressed in this article, is whether the use of frequencies contrasted with bouts of lateralized behavior reflects the same or different manifestations of lateral bias at the individual and population level. Specifically, Marchant (1994,1997), as well as others (Boesch, 1991;Byrne & Byrne, 1991), have argued that bouts are a better measure of laterality in hand use than the use of the raw frequencies. The central premise of this argument, outlined by McGrew and ...