All too human? Chimpanzee and orang-utan personalities are not anthropomorphic projections

Ratings of chimpanzee, Pan troglodytes, and orang-utan, Pongo pygmaeus and Pongo abelii, personality reveal dimensions resembling those found in humans. Critics have argued that this similarity derives from anthropomorphic projection or other rater-based effects. We developed two forms of data reduction analyses to determine whether these dimensions can best be explained by the inherent tendencies of the animals (e.g. orang-utans that are curious are playful) or anthropomorphic projections of raters (e.g. believing that orang-utans that are curious should be playful). We found that personality dimensions derived after differences between rater means and rater*item interactions had been removed from ratings replicated the previously discovered dimensions. Conversely, we found a different set of dimensions when analysing items from which differences between animal means and animal*item interactions had been removed. Finally, we used multilevel factor analysis to examine whether the published structure was replicated when we extracted factors based on the within-level animal differences in item scores effects while allowing between-rater differences to covary freely. Again, the personality dimensions were similar to those described in previous studies. These analyses can be used in combination with interrater reliability, temporal stability, and correlations between personality and other external variables to validate animal personality ratings. These analyses confirmed that personality similarities between humans and great apes are best explained by genetic and phylogenetic affinity and not by anthropomorphic artefacts.