Science

"It is a well-known paradox that humans are certainly the most cognitively advanced species but nevertheless show a smaller average brain size, by weight or volume, than elephants and whales. One attempt at a resolution of this paradox invoked the encephalization quotient (EQ), a measure of brain mass normalized essentially by body mass (Jerison, 1973). Many researchers found EQ appealing because it served well to distinguish humans from other species, but this measure turns not to provide an acceptable rank order of other species (Deaner, Isler, Burkart, & van Schaik, 2007; MacLean et al., 2014); for example, monkeys are more encephalized than the undoubtedly intelligent great apes (Deaner, van Schaik, & Johnson, 2006). Furthermore, the notion of an EQ may not be applicable to individual differences within the human species, for both birds and mammals exhibit within-species brain-body scaling that is weaker and more variable than in other vertebrates (Tsuboi et al., 2018)."