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Children are back in school this fall, and more “grown-ups” are back in the office. For our children, the classroom is an opportunity to develop lifelong friends, social skills and confidence. For adults, workplaces offer opportunities to collaborate and build meaningful relationships. I’m certainly energized by working with more of my colleagues in person. Yesterday in my office, I happened upon one of my closest colleagues. We have spoken via video dozens of times in the last few months. Nevertheless, it still felt like a reunion with an old friend. There was a warmth and joy that cannot be achieved online.

As I’ll explain in this month’s post, the relationships we build at every stage of life are critical to our health, well-being and even the economy.

What Is a Friend?

Philosophers, politicians, authors, scientists and artists have explored this question for centuries. How should we understand the nonfamilial, nonromantic relationships that hold communities together, drive new ideas and solutions, ensure survival and allow us to thrive?

According to the , friendship is “a distinctively personal relationship that is grounded in a concern on the part of each friend for the welfare of the other, for the other’s sake, and that involves some degree of intimacy.” The defines a friend as “one attached to another by affection or esteem” and also explains the meaning of the word ‘friend’ in the context of social media — as in a person to whom you are connected on Facebook.

Pop culture reflects our insatiable interest in friendship. Some of the most successful and enduring television shows focus on a group of friends, like Three’s Company, Cheers, The Golden Girls, Seinfeld, Living Single, Friends, Will and Grace, How I Met Your Mother and The Big Bang Theory. Countless books about friendship, like those written by J.R.R. Tolkien in the 1930s, continue to capture our imaginations even today. I remember reading The Hobbit as a child and was delighted to learn that Tolkien’s literary ambitions were enriched by his .

If we seem to know instinctively that friendships are important, it may be because friendship is in our nature. Studies suggest that other mammals also show signs of friendship. Take chimpanzees. Jan Engelmann of the Max Planck Institute for Evolutionary Anthropology “Humans largely trust only their friends with crucial resources or important secrets. In our study, we investigated whether chimpanzees show a comparable pattern and extend trust selectively toward those individuals they are closely bonded with. Our findings suggest that they do indeed, and thus that current characteristics of human friendships have a long evolutionary history and extend to primate social bonds.” Studies of my favorite of all animals, dogs, have shown that dogs will choose bonding time with their human even over a tasty food treat.

The Power of Friendship

Economists tend to consider friendship in relation to , which “is often measured through participation rates in different types of associational life, and self-reported levels of trust.” Like human capital, which I explore here��, social capital in childhood impacts our health and, as a recent suggests, our income in adulthood. For the study, researchers analyzed the social networks of 72.2 million people (a total of 21 billion friendships) using Facebook data. They found, as explains, “that the degree to which the rich and poor were connected explained why a neighborhood’s children did better later in life, more than any other factor.” In other words, if most of your friends expect to attend college or pursue high-income careers, then you’re more likely to as well.

This research hits home for me. I grew up in a middle-income family in a small town in Oregon. Only a few of my relatives had attended college, and no one in my sphere had gone to graduate school. However, we had many family friends who were doctors and lawyers and people with strong academic pedigrees. I suspect these associations were part of what set me on a trajectory to Yale, Stanford and many fine institutions I’ve been privileged to be a part of.

Given what researchers have learned about how our friendships impact our behaviors and feelings, this makes sense. Using data collected by the , the authors of a published in The New England Journal of Medicine found that “The spread of obesity in social networks appears to be a factor in the obesity epidemic. Yet the relevance of social influence also suggests that it may be possible to harness this same force to slow the spread of obesity. Network phenomena might be exploited to spread positive health behaviors, in part because people's perceptions of their own risk of illness may depend on the people around them.”

In 2008, the same researchers analyzed the Framingham data again. They found that ��too, even considering the possibility that happy individuals may tend to associate with other happy individuals. According to the study, if a friend who lives a mile or less away from you becomes happy, you are 25% more likely to be happy.

Data gathered on individuals over their lifetimes as part of the Harvard Study of Adult Development��suggests that “Close relationships, more than money or fame, are what keep people happy throughout their lives. Those ties protect people from life’s discontents, help to delay mental and physical decline, and are better predictors of long and happy lives than social class, IQ or even genes. That finding proved true across the board among both the Harvard men and the inner-city participants.”

A lack of protective, close relationships like friendships can undermine our health and the U.S. economy. The AARP estimates that social isolation drives . Cigna estimates that loneliness among adults costs the U.S. economy ��. The good news is that there are many ways to foster friendships and combat isolation. As communities, we can support projects that increase shared space and programs that foster cross-generational connections. As individuals, we can proactively reconnect with old friends or join a social group. that social interactions with people who are acquaintances rather than close friends have a positive impact on well-being.

“The Only Cement That Will Ever Hold the World Together”

The United States is not alone when it comes to loneliness. At least two countries, and the , have designated Ministers of Loneliness. Other countries have established programs that help university students board in the homes of older adults. In , President Woodrow Wilson, whose friendship with Justice Louis Brandeis had a on American politics, said that "Friendship is the only cement that will ever hold the world together.”

I feel very fortunate to have consequential friendships of my own. I had the good fortune of attending high school with a remarkable group of friends. Seven or eight of us have remained close for over 40 years. We talk regularly, take vacations as “guy trips,” and as couples, and we still call on each other when we need support. We have all known each other longer than we have known our wives. I value these relationships more than words can describe.

If you’re interested in learning more about friendships and social capital, check out these resources:

• Use at Opportunity Insights, a Harvard-based nonprofit, to explore what researchers discovered about connectedness in your county and state.
• Take a virtual tour of the J. Paul Getty Museum exhibit “” to see how artists have depicted friendship across centuries and cultures.
• Visit KidsHealth^® to read expert advice for and about friends and friendship.