First documented in the Divine Farmer’s Materia Medica Classic of the first or second century, the bark of the mimosa tree, known as he huan pi (“collective happiness bark”), is thought to harmonize the emotions and render a person worry-free. The manual advises that “protracted taking may make the body light, brighten the eyes,” and induce one to feel “as if one had acquired whatever one desired.” Modern practitioners sometimes call it “herbal Prozac.”

A study of U.S. Supreme Court oral transcripts from 2004 to 2005 tabulated every instance the court reporter described a justice’s remarks provoking laughter. It found that Antonin Scalia had caused seventy-seven such episodes, while Clarence Thomas had caused zero.

In 2016, Better Business World Wide hired “mystery shopping providers” to evaluate customer service around the world. The results were compiled in a Smiling Report, which found that Ireland scored the highest: 100 percent of customers received a smile. Shoppers in both Spain and Switzerland were greeted with a smile 97 percent of the time, while the lowest score was recorded in Hong Kong, where smiles occurred in only 48 percent of customer interactions.

“One enterprising seventeenth-century systematizer,” noted classicist Mary Beard in a discussion of the different ways the sounds of laughter have been recorded throughout history, “attempted to classify the variants in these sounds and relate them to the different temperaments, hi hi hi indicating melancholics, he he he cholerics, ha ha ha phlegmatics, and ho ho ho hotheads.”

Not long before his death in 961, Umayyad caliph Abd al-Rahman III testified that over his fifty years of reign, during which “riches and honors, power and pleasure, have waited on my call,” he had “diligently numbered the days of pure and genuine happiness.” Al-Rahman had counted only fourteen. “O man,” he lamented, “place not thy confidence in this present world!”