Who we know determines who we are. Three new books reveal how much heroes and even distant acquaintances influence us.

Surprisingly, a $10,000 raise may not make you as content as simply knowing a friend of a friend of a friend is happy, says sociologist Nicholas A. Christakis and political scientist James H. Fowler in their landmark book Connected: The Surprising Power of Our Social Networks and How They Shape Our Lives (Little, Brown, 2009). The authors draw on the famous Framingham Heart Study to show that, although we may not realize it, those on the fringes of our social networks dramatically affect our moods, political leanings and even waist size.

In Heroes: What They Do and Why We Need Them (Oxford University Press, 2011), psychologists Scott T. Allison and George R. Goethals argue that a single larger-than-life individual, such as Lucretia Mott, America's first feminist, or Martin Luther King, Jr., has the power to influence a diverse group. We identify with these heroic figures because they make us feel good about ourselves.

Peer pressure is not just about gateway drugs, awkward school dances and eating disorders—it can also motivate positive social change. In Join the Club: How Peer Pressure Can Transform the World (W. W. Norton, 2011), MacArthur fellow and award-winning journalist Tina Rosenberg demonsrates how peer pressure has helped minority students raise their grades and reduced teen smoking in the U.S. —Ferris Jabr