Why do I love my wife, Melinda?

The sociologist Nicholas Christakis would probably give way more practical answers than I would. He’d argue that our emotional connection gives us a greater incentive to work together to ensure the survival of our kids (and our bloodlines). If we’re ever attacked, our larger, combined family unit is more likely to successfully defend ourselves. We’re also more likely to share food and supplies with one another, upping our chances of living through a tough winter.

(My own reasons for loving Melinda are a lot less practical.)

In his terrific book Blueprint, Christakis explains that humans have evolved to work together and be social. Although this instinct originally developed because it made us more likely to live longer, our need to form groups has had a huge impact on human history.

Early in the book, Christakis says, “The human ability to construct societies has become an instinct. It is not just something we can do—it is something we must do.” He believes that this instinct has led to eight common traits that—with very few exceptions—you can find in every society on earth. These eight traits form what he calls the “social suite:

1. Individual identity
2. Love for partners and children
3. Friendship
4. Social networks
5. Cooperation
6. Preference for your own group
7. Some form of hierarchy
8. Social learning and teaching

Most of the book is devoted to explaining how each of these traits is found in seemingly disparate peoples, from the Roman Empire to the Turkana people of Kenya. Our world has gone from being small groups of hunter-gatherers that were closely related to the modern world where you can live in a city with millions of other people. The fact that the social suite has remained constant despite those changes is amazing.

But the social suite alone is only part of the story. If you want to explain human behavior, there’s a lot going on. You’ve got the genetics you were born with. You’ve got hormones running through the body. You’ve got your childhood and how that shaped you. And you’ve got learned behaviors—the understanding of what’s allowed and what’s not allowed that is passed to you through societal norms.

Blueprint focuses mostly on the last part. If you want a more complete picture, I recommend the book Behave by Robert Sapolsky. My older daughter suggested I read it (Sapolsky was one of her favorite professors in college), because it’s a super in-depth look at why humans act the way they do. He’s giving you a framework down to the biological and hormonal level, while Christakis focuses more on person-to-person interactions.

Behave is really long, though—nearly 700 pages!—and the incredible level of detail isn’t for everyone. It almost feels like a very well-written textbook. Blueprint is a lot more accessible for a general audience. I recommend starting with Christakis and then, if your interest in the subject is piqued, moving onto Sapolsky.

One of the things that makes Blueprint so readable is all of the fascinating examples and stories that Christakis uses. The book begins by looking at a bunch of different shipwrecks that resulted in people getting stranded on deserted islands. Each group had to develop their own form of society, and some were more successful than others. The ones that performed the best (“best” meaning that a high percentage of its people survived to be rescued) embodied some or most elements of the social suite. The societies that didn’t have these elements fell apart quickly, with several even devolving into cannibalism.

I was particularly fascinated by one section of the book where Christakis compares chimpanzees and bonobos. Chimps are super aggressive with each other and sometimes will kill members of their own group to assert dominance. Bonobos, on the other hand, are largely peaceful and playful creatures. They’re one of the only species other than humans who engage in sex for pleasure, not just for procreation purposes.

Christakis offers a couple theories for why the two types of apes are so different, even though they look so similar. Chimpanzees were more likely to share territory with gorillas and had less access to food than the bonobos, so aggressive chimps might have had a better chance to survive. Or maybe bonobo females—like humans—evolved to value cooperation over aggression when choosing a mate.

I was a little surprised to learn that the field of comparing the social behaviors of humans with other species isn’t more developed. Christakis appears to be at the frontier of this, and Blueprint only gives you a surface level understanding.

I also wish he had gone deeper on the main conclusion of his book: that every human being on earth has more in common than not. It raises big questions, like how we can leverage that commonality to get things done. Can we really get 7 billion people to work together and solve big problems like climate change? Are our similarities powerful enough to overcome the few differences we do have?

Christakis doesn’t answer these questions, but he does imply that the answer is yes by showing that we have an innate capability and need to cooperate. A lot of people are fascinated by the differences between us—but the differences are actually pretty minor compared to the similarities. In that regard, Blueprint is a fundamentally optimistic book.

I didn’t expect to finish a book about behavior feeling more hopeful, but Christakis surprised me. It’s easy to feel down reading news headlines every day about how polarized we’re becoming. Blueprint is a refreshing reminder that, when people say we’re all in this together, it’s not just a platitude—it’s evolution.