Nicholas A. Christakis (b. 1962) is an American physician and sociologist, internationally recognized for his work on social networks and other factors affecting health, health care and longevity. He is a Professor of Medical Sociology in the Department of Health Care Policy at Harvard Medical School; Professor of Sociology in the Department of Sociology in the Harvard Faculty of Arts and Sciences; and an Attending Physician in the Department of Medicine at the Harvard-affiliated Mt. Auburn Hospital.

Christakis attracted international media attention in 2007 with the publication in the New England Journal of Medicine of the results of a study in Framingham, MA, which showed that obesity can spread from a person to person, through social networks, much like a virus during an epidemic. (Watch the research video here, or the interview video here.)

Over the next two years, working with a former Harvard graduate student and now Professor at UCSD James H. Fowler and a team of researchers in his Harvard Medical School group, Christakis published a series of articles arguing that social networks can transmit not only obesity but also other health states and behaviors, including smoking and happiness . In 2008, the Christakis Group at Harvard Medical School was awarded an $11 million grant from the National Institutes of Health (NIH) for the study of networks and neighborhoods. In 2009, his group extended the study of social networks to genetics, publishing in the PNAS: Proceedings of the National Academy of Sciences a finding that social networks may be heritable; that an increase in the twins' shared genetic material corresponds to the differences in their social networks.