Differences in cognitive abilities among primates are concentrated on G: Phenotypic and phylogenetic comparisons with two meta-analytical databases

Abstract

General intelligence has been shown to exist within and among species of mammals and birds. An important question concerns whether it is the principal source of differences in cognitive abilities between species, as is the case with comparisons involving many human populations. Using meta-analytic databases of ethological observations of cognitive abilities involving 69 primate species, we found that cognitive abilities that load more strongly on a common factor (which is here termed G, in line with the terminology developed in previous literature to describe aggregated measures of general intelligence) are associated with significantly bigger interspecies differences and bigger interspecies variance. Additionally, two novel evolutionary predictions were made: more G-loaded abilities would present (1) weaker phylogenetic signals, indicating less phylogenetic conservativeness, and (2) faster rates of trait evolution, as it was hypothesized that G has been subjected to stronger selection pressures than narrower, more domain-specific abilities. These predictions were corroborated with phylogenetic comparative methods, with stronger effects among catarrhines (apes and Old World monkeys) than within the entire primate order. These data strongly suggest that G is the principal locus of selection in the macroevolution of primate intelligence. Implications for the understanding of population differences in cognitive abilities among human populations and for the theory of massive modularity applied to intelligence are discussed.