Other commonly accepted answers to "Yali's question" are either "assumed biological differences"(Diamond 18), "changes in geographical climate" (Diamond 22), or "European guns, infectious diseases, steel tools and manufactured products"(Diamond 23). Diamond writes off the explanation of assumed biological differences as an underlying feeling of genetic superiority by European peoples. Diamond says that explanation of changes in geographical climate, too, fails to survive scrutiny because "…complex irrigation systems did not accompany the rise of centralized bureaucracies but followed after a considerable lag." (Diamond 23). Diamond says that the explanation about guns germs and steel is on the right track but incomplete because it only offers a proximate explanation.