New Scientist is carrying this article detailing recent findings about the relationship between large brains and certain types of primate behavior. Turns out, larger brains confer the ability to be a sneaky bastard, which is advantageous in a complex social network such as the ones that nearly all primates live in. Includes this amusing example:
Byrne [one of the authors of the study] has himself observed a young baboon dodging a reprimand from its mother by suddenly standing to attention and scanning the horizon, conning the entire troop into panicking about a possible rival group nearby. "We were rather shocked that baboons could do anything quite as subtle as that," he says.