He then examines the debate on nature versus nurture and finds that the heritability of IQ and the ratio of intellectual giftedness of people within a civilization were "at best a possible, but not a necessary, element determining the degree of advancement of a race."
He states that "a direct relation between physical habitus and mental endowment does not exist," giving examples of different human sizes and shapes that do not correlate with intelligence.
He also discusses the role of environment and intelligence, especially nutrition, as well as the degree of racial admixture and "domestication" within societies and its effects on the "mentality of man."
He debunks claims that "primitive" humans have no impulse control, no attention span, no originality of thought, no power of reasoning, and are not capable of sustainability.
Boas says the primary difference between primitive and civilized society is a shift from irrationality to rationality caused by "an improvement of the traditional material that enters into our habitual mental operations."