Diamond is an evolutionary biologist, interested in adaptations, and he proceeds to speculate that New Guineans exhibit these heightened-mental-activity traits in response to their environment, which presents them with constant and varied challenges. He gets away with it because (1) this statement is buried inside his main thesis, which is in accord with the fundamental dogma (Western superiority is just an accident of geography), and (2) his superior racial group is non-white, and it's okay to call whites inferior