Dale Carnegie’s best-selling How to Win Friends and Influence People originated from a popular nighttime lecture he used to deliver at the YMCA. The book lists six ways to make people like you: be interested in others, smile, remember a person’s name, be a good listener, talk in terms of the other person’s interests, and make the other person feel important. Novelist Sinclair Lewis summed up Carnegie’s advice: “Smile and bob and pretend to be interested in other people’s hobbies precisely so that you may screw things out of them.”

A fourteenth-century-bc peace treaty recorded on a cuneiform tablet contains an invocation to Mithra, the Indo-Iranian god of the sun as well as of oaths and mutual obligation. In the Roman Empire, Mithras was honored as the god of loyalty to the emperor, while in Indian Vedic texts, he represents friendship and benevolence and, along with the god Varuna, is entrusted with the maintenance of the gods’ essence.

In Confucianism the five cardinal human relationships (wulun) are love between fathers and sons, righteousness between rulers and subjects, difference between husbands and wives, seniority between older and younger brothers, and trust between friends. Though at the bottom of this hierarchy, friendship is the only relationship not determined by ranking or kinship. 

Researchers at Yale and UC San Diego found that among a sample of almost two thousand subjects, none of them related, pairs of friends were significantly more likely to share gene variants than pairs of strangers; on average, close friends were the genetic equivalent of fourth cousins, making them “functional kin.” “Not only do we form ties with people superficially like ourselves,” said sociologist Nicholas Christakis, one of the study’s authors, “we form ties with people who are like us on a deep genetic level.”

At Uyun al-Hammam, an ancient graveyard discovered in northern Jordan, the remains of foxes were found buried alongside human remains, leading to speculation that humans may have kept red foxes as pets around sixteen thousand years ago, several millennia before animals were believed to have been domesticated. At one point, a human corpse had been disinterred and relocated. “Because the link between the fox and the human had been significant,” said one archaeologist studying the site, “the fox was moved as well.”

In a letter from Deir el-Medina, an Egyptian village of artisans working on pharaonic tombs during the period of the New Kingdom, Nakhtsobk, the self-described “scribe of the necropolis,” complains to Amennakhte, a workman, about being neglected. “It is only to me that you don’t send anything whatsoever, really this is a rotten day,” Nakhtsobk writes. “What offense have I done against you? Aren’t I your old eating companion?” In another letter from the same village, the sender, possibly Nakhtsobk, writes dejectedly, “It is I who write to you continually, but you never write to me.”

Ahead of a visit from Hannah Arendt in 1971, Mary McCarthy purchased anchovy paste, which she knew Arendt enjoyed. When McCarthy pointed out where to find it in her cupboard, Arendt looked displeased. “She had a respect for privacy, separateness,” McCarthy later wrote. “I knew I had done something wrong in my efforts to please. She did not wish to be known, in that curiously finite and, as it were, reductive way. And I had done it to show her I knew her—a sign of love, though not always—thereby proving that in the last analysis I did not know her at all.”

According to a study published in the American Sociological Review, from 1985 to 2004 the mean number of “close confidants” (people with whom one can discuss important matters) Americans had dropped by nearly a third, from 2.94 to 2.08 people. Similarly, the number of those who said they could not discuss important matters with anyone more than doubled, to nearly 25 percent. “The general image,” the researchers wrote, “is one of an already densely connected, close, homogeneous set of ties slowly closing in on itself.”