Monkeys and apes who are good at deceiving their peers also have the biggest brains relative to their body size. [...] Richard Byrne and Nadia Corp, psychologists at St Andrews University in the UK, have found more direct evidence for this after studying records of primates deceiving each other for personal gain.
Deception amongst primates is well documented. Sometimes a female gorilla will mate with a male surreptitiously to avoid a beating from a more dominant male. Or monkeys might feign disinterest in tasty food so that others do not come and steal it.
Byrne 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.
[...]Bush babies and lemurs, which have a relatively small neocortex, were among the least sneaky. The most tactically deceptive primates included macaques and the great apes - gorillas, chimpanzees, bonobos and orang-utans - which also have the largest neocortex.