Although the social brain hypothesis has found near-universal acceptance as the best explanation for the evolution of extensive variation in brain size among mammals, it faces two problems. First, it cannot account for grade shifts, where species or complete lineages have a very different brain size than expected based on their social organization. Second, it cannot account for the observation that species with high socio-cognitive abilities also excel in general cognition. These problems may be related. For birds and mammals, we propose to integrate the social brain hypothesis into a broader framework we call cultural intelligence, which stresses the importance of the high costs of brain tissue, general behavioral flexibility and the role of social learning in acquiring cognitive skills.
The cultural intelligence hypothesis recognizes that in some lineages domain-general cognitive abilities evolved that could be trained especially on socio-cognitive challenges, in addition to the specialized cognitive adaptations already present. In these animals, maturing individuals acquire vital cognitive skills, be they social or ecological, through social learning. This idea makes many of the same predictions as the social brain hypothesis but can also account for the third kind of grade shifts left unexplained by the latter. Obviously, it applies only to those species where domain-general cognitive abilities are well developed.