A social network "like a group, is a collection of people, it includes something more: a specific set of connections between people in the group. These ties, and the particular pattern of these ties, are often more important than the individual people themselves. They allow groups to do things that a disconnected collection of individuals cannot. The ties explain why the whole is greater than the sum of its parts. And the specific pattern of the ties is crucial to understanding how networks function." So write Nicholas Christakis, a professor at Harvard University with joint appointments in the Departments of Health Care Policy, Sociology, and Medicine, and James Fowler, an associate professor at the University of California, in the Center for Wireless and Population Health Systems and Department of Political Science. Social networks can be a large multigenerational family, coworkers in an office, a college dormitory, an entire community, Facebook friends, and Twitter followers.

Christakis and Fowler explain that certain rules of connection and contagion govern all social networks. These comprise what they call "the anatomy and the physiology of the human superorganism."

1. We Shape Our Network
2. Our Network Shapes Us
3. Our Friends Affect Us
4. Our Friends' Friends' Friends Affect Us
5. The Network Has a Life of Its Own
6. Six Degrees of Separation and Three Degrees of Influence

In a series of thought-provoking chapters, Christakis and Fowler delineate with fascinating vignettes and scientific research the manifold ways in which social networks influence the spread of happiness, the search for sexual partners, the maintenance of health, the functioning of markets, and the struggle for democracy. In the process, they have fashioned a science of social networks which offers us new and interesting slants on the interconnected world we live in and often take for granted.

Here are some of the tantalizing topics covered in the book:

• Emotions spread from person to person as we imitate the facial expressions of others and often come to feel as others do.

• Being surrounded by happy people has a positive effect on our feelings of well-being.

• Loneliness is both a cause and a consequence of becoming disconnected.

• A couple is the simplest of all possible social networks. Men who lose their wives are between 30 and 100 percent more likely to die during the first year of widowhood.

• If your friend's friend's friend gains weight, you have a tendency to gain weight, regardless of whether you know that person.

• Connections that make us happy also make us suicidal as in suicide contagion.

• More than 80 percent of American teenagers use the Internet and nearly half do so every day. Well over 75 percent use e-mail, instant messaging, or other online communication technologies.

• The average user on Facebook has approximately 110 friends.

Christakis and Fowler explore the question of what in human nature lies behind the need for connecting with others and then, in a wide-ranging chapter, present an overview of the complicated technological worlds of social networking sites, dating sites, collective information sites, and online games. The authors conclude that slowly we are taking our real lives online. They also are convinced that social networks can transmit positive and desirable outcomes when people tap into their natural reserves of altruism, reciprocity, charity, and other pro-social activities.

"The great project of the twenty-first century — understanding how the whole of humanity come to be greater than the sum of its parts — is just beginning," conclude Christakis and Fowler. "Like an awakening child, the human superorganism is becoming self-aware, and this will surely help us to achieve our goals."