Chimpanzee right-handedness reconsidered: Evaluating the evidence with funnel plots

Evidence for population-level right-handedness in nonhuman primates seems inconsistent and contradictory, and many hypotheses have been advanced to account for this volatility. Funnel plots (scatter plots of percent right-hand use vs. sample size) offer a straightforward graphical technique for assessing: 1) the strength and consistency of handedness, 2) whether variability is consistent with normal sampling variation, and 3) how likely reports of statistically significant handedness might have arisen due to chance (i.e., type I error). They are informative for both within- and among-population variation. Reexamination of within-population variation from a detailed and widely cited study reporting significant population-level right-handedness in 140 individual captive chimpanzees (Hopkins [1994] Dev. Psychobiol. 27:395-407) revealed several puzzling patterns: 1) funnel plots showed higher percent right-hand use among individuals for which fewer observations were recorded, 2) when individuals with fewer than 25 observations were excluded, statistical support for population-level right-handedness either became marginal (P = 0.043, when computed as average percent use of the right hand) or disappeared (P = 0.62, when computed as proportion of individuals using the right hand more than the left, whether they did so significantly or not), and 3) the proportion of statistically ambilateral chimpanzees actually increased with increasing number of observations per individual, rather than decreased as would be expected for true population-level right-handedness. In addition, funnel plots of among-population variation from an earlier meta-analysis (McGrew and Marchant [1997] Yrbk. Phys. Anthropol. 40:201-232) suggested that the four reports of significant right-handedness, out of 37 estimates from 14 studies, were likely those that achieved statistical significance simply due to chance. Funnel plots, and the more refined statistical tests they suggest, confirm that the current evidence for population-level right-handedness in chimpanzees remains equivocal.