This hypothesis, found in the Prologue to Guns, Germs and Steel, occurs in the context of Diamond's observation that psychologists have not been able to establish any substantive difference in IQ between white and nonwhite peoples. To this, Diamond adds his observation, based on decades of working with New Guineans, that they actually might be more intelligent than people in Western societies. He cites two potential causes for this.
First, in societies where people live in concentrated population areas, massive epidemics tended to be a major cause of death. These diseases affected intelligent and less intelligent people at the same rate. In New Guinea, however, the major causes of death were "murder, chronic tribal warfare, accidents, and problems in procuring food." Diamond concludes that "intelligent people are likelier than less intelligent ones to escape these causes of high mortality," thus selecting people of higher intelligence for survival in New Guinea.
The second cause is non-genetic, and relates to the amount of stimulation New Guinean children have in relation to Western children, who spend much of their childhood, known to be crucial to intellectual development, being "passively entertained" by television and other technologies:
In contrast, traditional New Guinea children have virtually no such opportunities for passive entertainment and instead spend almost all their waking hours actively doing something, such as talking or playing with other children or adults.
New Guineans are thus both more genetically inclined to be more intelligent and "superior in escaping the devastating developmental disadvantages under which most children in industrialized societies now grow up." Thus the central question in Diamond's book, namely how some societies came to become more powerful and capable of conquest and spreading their influence than others, is not related to some intrinsic, genetic Western superiority in intellect. Indeed, it may be the opposite in many cases.
Source: Jared Diamond, Guns, Germs, and Steel: The Fates of Human Societies (New York: W.W. Norton, 1999), 20-21.