Does variation in sample size explain individual differences in hand preferences of chimpanzees (Pan troglodytes)? An empirical study and reply to Palmer (2002).

Abstract

Despite the recent evidence of population-level handedness in nonhuman primates, continued debate persists over the analysis and interpretation of findings on handedness, particularly in great apes (Hopkins, 1999; McGrew and Marchant, 1997). Palmer (2002) recently questioned the analysis and interpretation of findings on hand preference in nonhuman primates, and in particular, results suggesting population-level right-handedness in chimpanzees. Specifically, Palmer (2002) reanalyzed hand-preference data for bimanual feeding in chimpanzees reported by Hopkins (1994) and argued, based on his analysis using funnel-graphs, that there were unusual patterns in the data that raised questions on the validity of the results for these data. With respect to the findings reported by Hopkins (1994), Palmer (2002) argued that a fundamental problem with the data was in the relationship between the number of individual data points collected for each subject and the distribution of hand preferences in the sample. Palmer (2002) quartiled the sample (N = 140) on the basis of the total number of observations used to derive the individual handedness scores (see his Table 1), and showed that as the number of observation increased, the number of ambiguously handed subjects increased. According to Palmer, this pattern of results should not have been evident under basic statistical assumptions of increasing effects with increasing sample size. In short, the data regressed toward a mean of zero, or no hand preference, with increasing sample size. Palmer (2002) also created funnel graphs of individual variation in percent right-hand use as a function of sample size, and argued that the patterns did not conform to patterns that would normally reflect nonsampling error in the data of Hopkins (1994). Lastly, Palmer (2002) applied funnel-graph analysis to 14 selective studies on hand preference in chimpanzees, and argued that most studies found no evidence of population-level handedness. TABLE 1 Distribution of hand preference as a function of sex and rearing history With respect to the data of Hopkins (1994), Palmer (2002) fairly and representatively indicated the inherent problems with the variation in percent right-hand use as a function of sample size. Our main response to this criticism is that the bulk of the youngest subjects make up the group comprising the largest N and most ambiguously handed subjects. In other words, sample size is confounded by subject age, and it is clearly legitimate to assume that there may be age differences in the expression and magnitude of hand preference in chimpanzees. If the youngest subjects are removed from the analysis, one-sample t-tests still reveal population-level right-handedness for bimanual feeding. The effect is not terribly robust, and this is largely due to the fact that there are so many ambiguously handed subjects. The fact that there were so many ambiguously handed subjects was clearly noted as a limitation of this measure in Hopkins (1994). Further, in Hopkins (1994), it is emphasized that measures involving coordinated actions of the hands may better elicit population-level hand preferences in chimpanzees than the bimanual feeding measure employed in that study. Although we recognize the limitations of the findings by Hopkins (1994), we do not believe that all studies of chimpanzee handedness are subject to the same criticism, and in this paper we report on evidence that contradicts the conclusions of Palmer (2002). Specifically, we developed a measure of hand preference for coordinated bimanual actions, referred to as the TUBE task (Hopkins, 1995; Hopkins et al., 2001). This measure of hand use also elicits a population-level hand preference in captive chimpanzees, but unlike the feeding task used by Hopkins (1994), the majority (90%) of chimpanzees show a significant hand preference (either left or right). Palmer (2002, personal communication) has been equally critical of these data, in terms of the application of funnel plots to the distribution of hand preference in relation to sample size. To address this issue of whether individual differences in sample size influence the distribution of hand preference in chimpanzees, we conducted an experiment in which we assessed hand preference for the TUBE task. This experiment differed from previous studies in that the exact same number of observations was collected in each subject. If the degree of hand preference found in chimpanzees is determined by sampling biases associated with N, then no population-level hand preferences should be found for this measure when sample sizes are identical for all subjects. In contrast, if the reported effects of population-level right handedness are consistent and not influenced by sample size, then population-level right-handedness should be found for this measure of hand use in this sample of chimpanzees.