Donald, David Herbert (1995). 《Lincoln》. New York: Touchstone. 85쪽.
firstladies.org
Historians have suggested that Robert Smith Todd and Elizabeth Parker were double first cousins: his paternal aunt was married to her father, and her paternal aunt was married to his father.Mary Todd BiographyArchived 2012년 5월 9일 - 웨이백 머신
Emerson, Jason. "Mary Todd Lincoln."The New York Times, Dec. 13, 2010. Accessed Nov. 17, 2012
web.archive.org
Historians have suggested that Robert Smith Todd and Elizabeth Parker were double first cousins: his paternal aunt was married to her father, and her paternal aunt was married to his father.Mary Todd BiographyArchived 2012년 5월 9일 - 웨이백 머신