Spending time with others improves our health, emotionally and physically. Our risks of depression and early mortality are reduced. The effects are so robust that social connection with others, good friends or casual acquaintances, is now recognized by scientists as a public health issue of equal importance to eating well and not smoking.

We humans are biologically built to seek friends, and we can see suggestions of our evolutionary past in the social behavior of some animals.

In “Friendship: The Evolution, Biology, and Extraordinary Power of Life’s Fundamental Bond,” journalist Lydia Denworth explores the science behind friendship. In an accessible and enlightening style, she takes us with her on her journeys to primatology research sites in Puerto Rico and Kenya, and to cutting-edge biology and neuroscience laboratories in the United States. She discovers that female baboons in Kenya who establish stable social bonds with friends and kin have more babies and live longer. Numerous species of animals, ranging from elephants to zebra fish, show evidence of friendships as measured by the degree of the long-term cooperation between pairs of individuals.

In a human study, researchers drew on a group of graduate students connected by social networks. They put each of the 42 participants into a brain scanner (an fMRI machine) and had them watch video clips. The scientists discovered that they “could predict which participants were good friends and which were not by matching up the way their brains perceived and responded to the world around them.” The findings suggested that friends’ neurons fire more similarly than the neurons of more distant acquaintances. That result posed the question: Does the brain change as friendships develop, or do friends with similar brain patterns come together in the first place? Preliminary results suggest it may be the former: that friends’ brains gradually align.

Denworth reports on the connection between loneliness and health. One long-running study of 229 older Chicago residents found that loneliness, determined through physical exams and extensive interviews, was associated not only with poor sleep and greater social withdrawal but also with increased mortality and depression. Blood analysis of 14 participants revealed that genes in the loneliest people were more inclined toward inflammatory responses than antiviral responses.

Denworth introduces us to Paula Dutton, a woman whose life became one of near-isolation after her parents died, her marriage broke up and she retired from her job. After a severe panic attack, she realized she had to make some social connections. She joined a group called Generation Xchange that brings volunteers into underresourced South Central Los Angeles schools. Dutton volunteered most weekdays in a first-grade classroom. Denworth describes how Dutton lights up as students joyfully share their work with her. UCLA epidemiologist Teresa Seeman conceived and runs Gen X, as it’s known. “It is an educational nonprofit,” writes Denworth, “wrapped in a community health initiative with a loneliness intervention program beating quietly but steadily at its heart.”

In a chapter titled “Digital Friendship,” Denworth asks whether connections forged on social media amount to real friendship. She avoids knee-jerk railing against social media and its overuse. She contends that if Facebook is used to enhance connection with those we spend time with in person, or to share aspects of our lives with people who live far away, it may support rather than undermine intimacy.

I found one aspect of “Friendship” to be disturbing. Denworth describes a variety of cruel animal experiments mostly without comment. We read of rats “subjected to blaring light, deafening noise, and extremes of hot and cold,” kittens whose eyes were covered to “disrupt the development of normal vision,” monkeys that were prevented from ever being able to see another face, human or nonhuman. Of the monkeys’ treatment, Denworth writes: “For the record, the animals had lots of attention, activity, and toys. They were well-nourished, and lovingly bottle-fed as infants. The only thing their worlds lacked was faces.” Clearly, the science that Denworth witnesses in pursuit of understanding friendship is at times brutal.

By highlighting the importance of human connection, Denworth has crafted a worthy call to action. “Friendship is not a choice or a luxury;” she tells us, “it’s a necessity that is critical to our ability to succeed and thrive.” Embrace friendship, invest in it, work at it, she urges. Your life will be the better for it.


The Evolution, Biology, and Extraordinary Power of Life's Fundamental Bond

By Lydia Denworth

Norton. 297 pp. $26.95