President Herbert Hoover once praised a group of PR professionals. “You have taken over the job of creating desire,” he said, “and have transformed people into constantly moving happiness machines—machines which have become the key to economic progress.” It was 1928; the Great Depression began the following year.

Chinese Taoist philosophers Zhuangzi and Hui Shi took a walk on a bridge over the Hao River in the fourth century bc. “The minnows swim about so freely,” said Zhuangzi; “such is the happiness of fish.” Hui Shi responded, “You are not a fish, so whence do you know the happiness of fish?” “You are not I,” Zhuangzi replied, “so whence do you know I don’t know the happiness of fish?”

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.

A 2018 study run at Buttercups Sanctuary in Kent, England, found that goats are sensitive to human emotions and strongly prefer to sniff smiling, happy faces rather than frowning ones.

“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.”