Connected: The Surprising Power of Our Social Networks and How They Shape Our Lives, by Nicholas A. Christakis and James H. Fowler (Little, Brown, 352 pp., $25.99)

Before Facebook, few of us asked others, explicitly, to be our friends. We didn’t monitor how many friends we had as an indication of our status or scroll through listings of friends of friends to pad our own list.

Yet the history of humanity is a history of social networking all the same, according to Nicholas A. Christakis and James H. Fowler, authors of Connected: The Surprising Power of Our Social Networks and How They Shape Our Lives. “Our connections affect every aspect of our daily lives,” they write. “How we feel, what we know, whom we marry, whether we fall ill, how much money we make, and whether we vote all depend on the ties that bind us.” And the burgeoning field of network research is revealing that “our connections do not end with the people we know.” Social networks take on lives of their own, transmitting information, germs, and habits between people who are nearly as tangentially linked as actors in the old parlor game Six Degrees of Kevin Bacon. “Friends of friends of friends can start chain reactions that eventually reach us,” the authors argue, “like waves from distant lands that wash up on our shores.”

Christakis, a professor at Harvard Medical School, and Fowler, a professor of political science at the University of California, San Diego, gained fame a few years ago for some important research into this new field. For years, the Framingham Heart Study has tracked the vital stats and psychological states of residents of Framingham, Massachusetts; researchers have studied this data set for what it says about health habits. But Christakis and Fowler went further. In order to follow up with the Framingham subjects over a period of years, it turned out, the original researchers had asked for the names of family and friends and kept detailed notes of these connections. Christakis and Fowler pieced together these notes to reconstruct the social networks of thousands of connected friends, spouses, neighbors, and siblings. Crunching the numbers from the Framingham health data and using computer graphics, they observed how traits spread through the social network over time: the modern epidemic of obesity advanced, smoking receded to the peripheries, and both happiness and misery appeared to be contagious.

Controlling for environmental factors and the tendency of birds of a feather to flock together—happy people prefer hanging out with other happy people—Christakis and Fowler found that we really do emulate those we care about, whether we mean to or not. Being connected to a happy person, for instance, makes you 15 percent more likely to be happy yourself. “And the spread of happiness doesn’t stop there,” they note. It radiates out for three degrees of separation, so that, say, your sister’s best friend’s husband’s mood exerts a greater influence on your personal happiness than an extra $10,000 in income would. If he gains 50 pounds, it will be that much harder for you to stay slim, as the frame of reference for what’s “normal” changes through your network. Or, on the positive side, if he quits smoking, your chances of kicking the habit improve, too, even if you’ve never met him.

The authors spend much of the book discussing how social networks might have arisen through evolution, what the rise of the Internet means for social connections, and the implications for both our personal lives and public policy. For instance, public health workers can more effectively stop the spread of sexually transmitted diseases if they know what kind of network they’re dealing with: a hub and spoke (e.g., a prostitute with many clients) or a more transitive “ring” network where people have few partners, but many of these partners overlap (which could happen at a small high school). On another front, they point out that voting makes little sense for an individual—one vote never decides an election—but is far more rational in a network context. As with happiness and obesity, the decision to vote has repercussions through three degrees of connections. When you vote, your spouse, colleagues, and friends, and their spouses, colleagues, and friends and so forth, are more likely to vote as well, and vice versa. Since liberals and conservatives tend to form their own social networks, this means that your decision to vote can increase the likelihood of hundreds of other people voting for the same candidate. This explains—at least on an unconscious level—why people stand in line in the rain in November.

The authors view just about everything through a network lens, including religion. “God can actually be seen as a part of the social network,” they write. Since God is presumably connected with everyone, believing in him would literally allow you to feel connected to the rest of humankind with just one degree of separation: “through God everyone is a ‘friend of a friend’” (even if he doesn’t yet appear to be on Facebook).

Connected is a fascinating and, for two academics, surprisingly well-written book. Christakis and Fowler keep tongues firmly planted in cheek as they describe other studies; for instance, researchers studying grooming patterns across networks in different species learned that “the model that best predicted the network structure of U.S. senators was that of social licking among cows.” Their book suffers from two main drawbacks, however. First, though the cover blurb from Stumbling on Happiness author Daniel Gilbert announces that “Connected could change your life forever,” the book never explains how. The authors scrupulously avoid straying into self-help territory that would describe how to become better connected or how to strengthen the ties you already have. Possibly this is because they view the worth of one’s network position as dependent on circumstances. Being on the inside (that is, well-connected) is better if you want to find a job, but being on the outside is more advantageous in the midst of a swine flu epidemic. But I did find myself wondering exactly how I would use this knowledge about “the surprising power of our social networks and how they shape our lives” to improve my own life.

Second, the authors raise, but then mostly avoid answering, some profound questions. “Embedded in social networks and influenced by others to whom we are tied, we necessarily lose some of our individuality,” they write. “If we unconsciously copy the good deeds of others to whom we are connected, do we deserve credit for those deeds? And if we adopt the bad habits or evil thoughts of others to whom we are closely or even loosely tied, do we deserve blame? Do they?” This idea that individual actions, from stealing cars to donating organs, may not really be a function of personal choice is disconcerting, to say the least, and deserves more space than a casual discussion on page 305.

Still, Connected is an important book, a scientific look at the ties that bind us together. “The great project of the twenty-first century—understanding how the whole of humanity comes to be greater than the sum of its parts—is just beginning,” they write. As a first step in that project, Connected is well worth reading.