If Medical School Professor and Professor of Sociology Nicholas Christakis, the co-author of the new book Connected: The Surprising Power of Our Social Networks, sat with your friends in Pfoho, where he is the House Master, he could tell you a lot about yourself—how likely you are to become obese, and maybe even whether you would be an efficient choice for a flu vaccine.

Earlier this year, Time magazine named Christakis one of the 100 most influential people in the world. Connected’s co-author, James Fowler ’92, an associate professor of political science at the University of California, San Diego, provided the first evidence of the positive “Colbert bump” in polls for politicians appearing on the Colbert Report.

The book has received favorable reviews in the press, and popular Harvard Professor of Psychology Daniel Gilbert raved about their new book: “We think we are individuals who control our own fates, but as Christakis and Fowler demonstrate, we are merely cells in the nervous system of a much greater beast,” he wrote. “If someone you barely know reads Connected, it could change your life forever.”

More on fat, flu, and Facebook after the jump.

The book combines years of the research by the authors with other studies to support its ideas. One of their most striking findings—from a large study that began in 1948 in Framingham, Massachusetts—is that obesity is greatly determined by social networks (some other researchers have questioned this interpretation). According to Connected, “If a mutual friend becomes obese, it nearly triples a person’s risk of becoming obese.” Because of imitation and shared expectations called “norms,” even friends who are 1,000 miles away produce this strong effect. But don’t get ready to dump your friends just yet—people who do are even more likely to become obese, the authors found.

Your social networks, and in particular how popular you are, may also affect how good a candidate for a flu vaccine you are. Vaccinating an entire population may be less efficient than choosing people at random, asking them to name their friends, and then vaccinating those friends. The friends are likely to come into contact with many people, so vaccinating them might do the most good, the authors argue. "You can achieve the same level of protection for the population at one-third the cost doing an intervention like this," Fowler said, according to CNN. He and Christakis hope to do a preliminary experiment monitoring the spread of the H1N1 virus on a college campus.

Social networks are powerful, but, the authors argue, online sites do not seem to be increasing how many close friends students have. In a study of Facebook pages at an unnamed university, students traded posts and photo tags with six and seven people, not far from sociologists’s estimate that people have four to six close friends in real life.

The British anthropologist Robin Dunbar estimated that humans would have social groups of around 150, and Fowler said that the average Facebook user has about 110 friends on the site, which fits the relatively small groups humans evolved in. He and Christakis found that there is a “Three Degrees Rule”—networks are influential to point of the friends of a friend of a friend, but not very much beyond that.

So, next time you order another late night burger at the grill, think of all the people you are affecting. And next time you meet a few potential friends, hike with them to Pfoho to seek the advice of House Master Christakis.

Photo courtesy Nicholas A. Christakis/Wikimedia Commons