Proc. Natl Acad. Sci. USA 10.1073_pnas.0901780106 (2009)

A plausible hypothesis states that socialization in mammals puts them under selective pressure to evolve larger brains.

John Finarelli of the University of Michigan in Ann Arbor and John Flynn of the American Museum of Natural History in New York compared relative brain sizes for the group Carnivora, which includes cats, dogs, bears and weasels. They looked at 289 examples, 125 of them extinct, and found that brain-size changes through evolutionary time do not correlate well with the development of sociality.

The authors suggest that the 'social brain hypothesis' falls apart when looking at carnivore groups, extinct and otherwise, beyond modern canids (wolves, jackals, foxes and the like).