Kullberg, Judith S., and J. David Singer. "Bringing Neuroscience into Political Science: The Caregiving System and Human Sociopolitical Evolution." Moving Beyond Self-Interest: Perspectives from Evolutionary Biology, Neuroscience, and the Social Sciences. : Oxford University Press, 2011-10-26. Oxford Scholarship Online. 2012-01-19. Date Accessed 20 Jun. 2013 <http://www.oxfordscholarship.com/view/10.1093/acprof:oso/9780195388107.001.0001/acprof-9780195388107-chapter-016>.
Kullberg, J., & Singer, J.(2011-10-26). Bringing Neuroscience into Political Science: The Caregiving System and Human Sociopolitical Evolution. In Moving Beyond Self-Interest: Perspectives from Evolutionary Biology, Neuroscience, and the Social Sciences. : Oxford University Press. Retrieved 20 Jun. 2013, from http://www.oxfordscholarship.com/view/10.1093/acprof:oso/9780195388107.001.0001/acprof-9780195388107-chapter-016.
Kullberg, Judith S., and J. David Singer. "Bringing Neuroscience into Political Science: The Caregiving System and Human Sociopolitical Evolution." In Moving Beyond Self-Interest: Perspectives from Evolutionary Biology, Neuroscience, and the Social Sciences. : Oxford University Press, 2011-10-26. http://www.oxfordscholarship.com/view/10.1093/acprof:oso/9780195388107.001.0001/acprof-9780195388107-chapter-016.
Kullberg JS, Singer JD. Bringing Neuroscience into Political Science: The Caregiving System and Human Sociopolitical Evolution. In: Brown SL, Brown RM, Penner LA. Moving Beyond Self-Interest: Perspectives from Evolutionary Biology, Neuroscience, and the Social Sciences. Oxford University Press; 2011. http://www.oxfordscholarship.com/view/10.1093/acprof:oso/9780195388107.001.0001/acprof-9780195388107-chapter-016. Accessed June 20, 2013.