You can find the answer to this in the Prologue. The answer begins on p. 19 in the paperback edition of the book. There, Diamond says that this idea is simply wrong and that New Guineans are in fact more likely to be more intelligent than Europeans. Diamond gives two reasons for this. First, he says, intelligence has not helped Europeans survive and reproduce for thousands of years. Therefore, natural selection has not been "picking" the smartest Europeans and allowing them to live. By contrast, intelligence has helped New Guineans survive. Second, New Guineans are much more active than Europeans. They do not sit around watching TV. Instead, they are always doing things that require at least some thought. For these reasons, Diamond says, they are likely to be more intelligent than Europeans, not less.