Brain Size• Brain size has increased threefold in the

last 3 million years.

Social Intelligence• OOne cause for greater brain size:

– sSocial intelligence:– the ability to process information about the

behavior of others and to react adaptively to their behavior.

• AAs group size grows, so do complexities

• PPrimates live in social groups that are more complex than those of other animals

Social Intelligence

• One ability that would have conferred a great advantage to survival and reproduction is the ability to predict other individuals’ behavior.

• “The world of higher primates – of monkeys, apes, and humans – is quintessentially a game of social chess, a keen intellectual challenge. The challenge is keener yet than the ancient board game itself, because the pieces not only unpredictably change identity – knights becoming bishops, pawns becoming castles, and so on – they occasionally switch colors to become the enemy…”

• (Leaky and Lewin 1992)

Theory of Mind: A chimpanzee is as likely to beg for food from a person who is unable to see the gesture as from one who is watching

Clark’s nutcracker

Astatotilapia burtoni

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