Thailand is widely known as "the Land of Smiles." But the wooden faces of two young Thai women working at Bangkok's international airport drew widespread frowns. Dressed in traditional brocade costumes, the women were supposed to welcome tourists with flowers and broad smiles. But a video of their indifferent expressions while greeting Chinese visitors went viral, prompting their employer to fire them.

Their experience reminds me of a friend who was invited to speak at a Thai university about disarming Afghan tribal leaders. Her lecture, delivered with the seriousness the issue demanded, was frequently interrupted by a comic commentary from the back row. The joker was the Thai academic who had invited my friend.

Later, the academic explained that jokes are a good way of imparting knowledge. Students switch off if there are no smiles, he said. This piece of wisdom helped my friend see the Land of Smiles in a different light.

That story has helped me when engaging with Thais in public settings. Shortly after arriving in Thailand 16 years ago, I read that office culture placed much stock in the "three S's," which stand for sabai, saduak and sanook (happy and comfortable, enjoying things because they are convenient, and having a good time).

The implication is that an office environment should include ample space for the Thai smile, or yim. One also gets that sense in other public settings, where Thais appear most comfortable if they can engage with locals and foreigners alike with light banter and bright smiles.

At times, though, I have wondered which particular smile my hosts have whipped out from their bag of yims. I read once that there are 13 types of Thai smiles. Further research suggests there is one to conceal your emotions at awkward moments, another that is appropriate for debtors, one for use when heartbroken and, of course, the fake smile that transcends cultures.

Students of the subject say it goes deeper, explaining why the country has elevated the smile to a badge of patriotic duty. They locate the first hints of its future importance in the wake of a revolution in 1932 that ended the absolute monarchy. In a tumultuous period of political transition Field Marshal Plaek Pibulsonggram, a military dictator, issued 12 cultural mandates intended to "civilize" Thais.

Records show that it was during those years that public references to the country as the Land of Smiles first appeared. Usage peaked in the mid-1940s, then dropped off, but re-emerged in the early 1960s, when the country was under another dictator, Field Marshal Sarit Thanarat. After another lull the slogan was picked up again in the mid-1980s to promote Thailand as a happy destination for tourists.

Given the importance of the smile in Thailand, it is no surprise that the army commander who staged the country's most recent coup justified his putsch as a way of restoring happiness. Gen. Prayuth Chan-ocha, prime minister and junta chief, even composed a song, "Returning Happiness to the People," to get the country smiling.

Set to music played by the Royal Thai Army Band (accompanied offstage by arbitrary arrests, repeated cancellations of promised elections and a new constitution that entrenches army influence), Prayuth's ditty promises Thais: "Let us be the ones who step in, before it is too late/The land will be good soon/Happiness will return to Thailand." As the lyrics suggest, Thais are still expected to keep smiling, even in the face of deep political divisions.

David Streckfuss, an American academic who studies Thai political culture, says societal pressure to smile remains a distinct cultural marker. "What was just a nickname made up in the 1930s has turned into national policy, reaching higher levels in times of dictatorships," he said. "Add the excessive zeal for tourism, and you have real-life implications for not smiling enough ... or not [in] the correct way."

This social pressure was clearly reflected in the response of one of the former flower girls, who were expected to smile 2,000 times a day, at the rate of five smiles a minute, for just 300 baht ($9.35) a day.

"That day was really hot, and some Chinese tourists mocked me, but I didn't know they were taking a video and that I'd be widely criticized," she said, adding: "I feel stressed and sorry because internet users verbally attack me. I'd like to apologize to society for my inappropriate action."

For these unfortunate flower girls, the Thai smile turned out to be no laughing matter.

Marwaan Macan-Markar is an Asia regional correspondent of the Nikkei Asian Review. He is based in Bangkok.