...by focusing on mother-baby relationships, Denworth misses the fact that social brains can and do develop in the absence of mammal-style parental care ... Exploring friendship’s opposite — loneliness — throws the topic into poignant relief, especially when Denworth asserts that what’s striking about loneliness 'is not that those who suffer from it are peculiar, but that they are so ordinary' ... Her account of research suggesting an association between loneliness and autoimmune disease, and her description of a dismal state known as the 'loneliness loop,' are some of the most informative — and from a public health perspective, important — passages in the book.

... [a] deft exploration of the biology of a 'fundamental bond' ... Ms. Denworth does an admirable job of outlining the sources of our yearning for it. She has a solid command of the complex material before her and a seemingly effortless ability to make it not just digestible but engaging ... not just a biological account of friendship but a quick and painless tutorial in the history of evolutionary science, which is revealed to us through the prism of a subject all of us can relate to...Given how much there is to be said about friendship, maintaining this sharp focus must have required discipline ... [Denworth] sensibly resists trying to wrap her arms around research on (to cite a few examples) the effect of our sprawling built environment, our increased tendency to sort ourselves by our politics and preferences, or our growing ethnic diversity—all of which have ramifications for friendship and could fill additional books. For there is more than enough in the life sciences and adjacent fields to keep us busy in a single volume; indeed, the author’s parade of scientists and findings, though carefully presented, inevitably blurs in places as it goes by, trying our patience as even our best friends can sometimes do ... the author is judicious in weighing scientific evidence, most clearly with respect to the effects of videogames and social media on people’s social lives ... In these polarized times, when some readers may object to the very premise that there is something called human nature that can’t be wished away, Ms. Denworth sticks to the science, calmly telling us the truth no matter what we think we need to hear. What else are friends for?

In an accessible and enlightening style, she takes us with her on her journeys to primatology research sites ... She avoids knee-jerk railing against social media and its overuse ... By highlighting the importance of human connection, Denworth has crafted a worthy call to action.

In a personable and accessible style, Denworth lays out her argument, exploring the biological underpinnings and the evolutionary history of friendship ... After reading Denworth's treatise on friendship, you may want to immediately call your best friend, or make a new one. Recommended for fans of human biology and nonfiction browsers.

...critical and convincing ... Denworth’s work achieves the best of science writing by making complicated concepts clear. She uses intelligent observation, empathy, and curiosity to offer a friendship manifesto that will absolutely affect readers’ own personal approaches to friendship.

Science writer Denworth takes a broad look at the origins and functions of friendship in her intriguing debut ... Denworth draws several striking conclusions ... Her reporting is peppered with personal asides about how she and her family members have navigated various relationships. While this enlivens her work’s more technical facets, it does potentially give the impression of putting anecdotal experiences on a par with evidence-based studies, thus undercutting the importance of the latter. Science enthusiasts may find Denworth’s survey wider than it is deep, but it does provide an effective introduction to its subject.

In addition to examining the scientific underpinnings of friendship, Denworth capably demonstrates how loneliness, an increasing hazard as Americans age and lose friends and family, is truly a health- and life-threatening condition, and there are things to be done to avoid it. Convincing evidence that evolution endowed us with a need for friends, support, comfort, stimulation, and, ultimately, happiness.