(CNN) -- Meet "network man." He has basic desires of his own, but has many arbitrary preferences, such as in music or clothes, that have been influenced by the people he knows.
A new book finds that our likes and dislikes are influenced by our social networks, both online and off.
Network man's likes and dislikes, in turn, affect the behavior of his friends, and their friends, and their friends. For example, when he gets into an obscure indie rock band, he shares an album with his friend, who likes it so much that he recommends it to his cousin, who spreads the word to her friends.
This is the view of human behavior put forth in "Connected," a new book by Dr. Nicholas Christakis, professor at Harvard University, and James Fowler, associate professor at the University of California, San Diego.
Examining years of research of their own and from others, the authors conclude that social networks, both offline and online, are crucial in understanding everything from voting patterns to the spread of disease.
People have profound influences on each other's behavior within three degrees of separation, the authors find. That means that your friends, your friends' friends, and your friends' friends' friends may all affect your eating habits, voting preferences, happiness, and more. At the fourth degree, however, the influence substantially weakens. Read about their research on how happiness is contagious
These are not small effects. "If a mutual friend becomes obese, it nearly triples a person's risk of becoming obese," the book said. Even geography doesn't matter; you're still at risk for gaining weight if a friend 1,000 miles away gets bigger. Listen to James Fowler talk about social networks »
Why is that? Obesity seems to spread in networks because of behavioral imitation -- you copy what people close to you are doing -- and shared expectations called "norms," the authors said. When you see people close to you gaining weight, that makes you readjust your own idea of what is an acceptable body size. Read more about how "overweight" is relative
Still, this does not mean you should get rid of your overweight friends, Fowler said. In fact, the researchers re-examined the same data set and found that people who dumped their friends who gained weight were even more susceptible to obesity.
"On the one hand, yes, our work showed that if you keep your friend, you are going to be susceptible to their bad behaviors," he said. "On the other hand, time and again, what our work shows is that every friend makes you healthier and happier." Watch CNN's Elizabeth Landau talk about the book »
Finding the hubs of social networks can be invaluable from a public health point of view, the authors say. For example, instead of vaccinating everyone in a population against a disease, it may be just as effective to choose people at random and ask them to name their closest friends, then vaccinate those friends.
The idea is that if someone names you as a friend, you are likely to be more central to the network than a participant chosen at random. That means you're probably less socially isolated, and more likely to come into contact with a lot of people, than someone randomly selected.
"You can achieve the same level of protection for the population at one-third the cost doing an intervention like this," Fowler said.
In fact, the authors hope to do a preliminary experiment monitoring the spread of the novel H1N1 virus on a college campus, Fowler said. Using the "friend" method for the vaccine against that virus, distributed for the first time this week, would not be a bad idea, although it is still a new concept that needs to be tested, Fowler said.
"What you are seeing is a lot of people who are experimenting with these ideas in trials, and trying to figure out how can we use all of this great new information about social networks to make everybody's life better," he said.
Social networks were crucial to understanding how sexually transmitted diseases broke out among teenagers in 1996 in Rockdale County, Georgia, a quiet upper-middle-class suburb near Atlanta.
An investigation found that a collection of young girls, mostly under 16, had been having sex with various clusters of boys, the book said. This epidemic of syphillis and other diseases stopped when the network changed, the authors argue.
"They actually figured out a good intervention there, which was to break apart this central group of girls that were essentially promoting the spread of this norm of a heightened sexuality at a very young age," Fowler said. "By essentially quarantining them, the norm couldn't spread anymore, and pretty soon they were able to get control of the epidemic."
It turns out online social networks haven't necessarily expanded the number of friends the average person has. Christakis and Fowler looked at all of the Facebook pages of students at a particular unnamed university and tried to figure out who was close friends with whom based on "picture friends." Their reasoning: Two people who post and "tag" each other in photos on Facebook are likely to be more socially close than those who do not.
They found that, on average, students had between six and seven close friends on Facebook, which is not far from sociologists' estimate that most people have four to six close friends in real life.
Moreover, the expected size of social groups in humans, based on their large brain size and the behavior of other primates, should be about 150, according to British anthropologist Robin Dunbar. At the time "Connected" was finalized, the average Facebook user had 110 registered friends on the site, Fowler said.
The "Three Degrees Rule" may have evolved with the human species, the authors argue. Having a social network in which influence spreads is useful, as is knowing who is a potential ally or enemy based on friends' associations. But humans evolved in small groups, not large collectives, so during early human history there may not have been people who were four degrees removed from anyone, they write.
The Iranian blogosphere, despite having a government that blocks access to several Web sites, expresses many viewpoints on the Internet. In June, Iranian citizens used Twitter to organize and protest against what they viewed as an unfair election. Read more about the power of retweeting
"In closed societies, the sheer value of being able to spread information quickly is going to be the reason why social networks play such a strong role," Fowler said.
An important caveat is that political organizing through social networks, just like choosing financial investments, depends on what you believe other people believe, he said. Bubbles in the stock market. for example, are created when a lot of people believe a lot of other people highly value a company's stock.
"The same thing happens with these protests," he said. "You're kind of afraid to go out and do it yourself, but if you think that other people are going to go show up, then you're more likely to show up yourself."
While social networking sites such as Friendster have risen and fallen, online social networking in general is here to stay, Fowler said. He likened it to the introduction of the telephone, which had its initial skeptics too.
"This is just yet another way through which humans exert their inherent natural tendency to try to connect to other people that they care about," he said.