A large aspect of David Starr Jordan's academic research was about the science of eugenics.

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His major contribution to eugenic theory was his belief that war had negative dysgenic impacts on racial health. After studying a few famous wars, Jordan concluded that war results in the death of brave men and the survival of cowardly men. Thus, Jordan believed that war resulted in a worsening of the race, as the cowardly men lived on to survive. 

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Jordan's writings also popularized eugenic theory for a general audience. His popular 1911 Heredity of Richard Roe taught the basics of eugenics to the common reader: "if the fittest do not serve as parents, the next generations will not inherit fitness."

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In this text, Jordan also endorsed the sterilization and reproductive restriction of disabled people, citing the work of Charles Davenport, a fellow eugenicist and acquaintance. He emphasized the necessity of "limitation by public authority of the marriage of the defective, the insane, and the criminal."

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David Starr Jordan both researched eugenic theories and popularized eugenics to a wider audience.