I was saying something flippant, as I often do, when I found myself pulling up short. Glibness can be a seductive tone and even a check to the ego. Whatever you’re worrying about is almost certainly survivable and will probably be forgotten tomorrow. But this week, as I went about lightly mocking an American writer to a friend, for the crime of taking herself too seriously, it didn’t sound good. In fact, it sounded like something lately risen to the level of national crisis.

There is a tonal trap British people fall into, particularly when they position themselves against “worthy” Americans, which is the off-the-top-of-the-head casual genius. This attitude is, we know, intimately wedded with our ideas of class. The icily superior toff demonstrates wealth by wearing jumpers with holes at the elbows, and brains by never revising. There he or she is, making us laugh, scorning the sweaty efforts of the lower orders.

Knowing one’s stuff is the preserve of Americans and bores (though not, obviously, the current president of the United States)

Who can fail to be charmed by the Mitfords, or Evelyn Waugh, or Edward St Aubyn’s Patrick Melrose, who, apart from his heroin addiction, wears the enviable look of a man who always knows what to say? Rather him than Leonard Bast – that solid, plodding, swotting nonentity who in spite of his efforts never gets it quite right, only to be killed in a freak library accident.

If there is something admirable about the British – or rather, English – fondness for cavalierness, it is a certain embedded resilience, a firm rebuff to neurosis. The impulse not to make a 10-course meal out of things can, within its limitations, be delightful and entail some grit.

But then here comes Boris Johnson taking it to such extremes that it becomes something else entirely. And it’s not only him. I was reminded of the curse of indifference again this week watching The Great Hack, the Netflix documentary about the Cambridge Analytica scandal in which the former head of the firm, Alexander Nix, comes off in precisely the Johnsonian style. It is an understatement to say his conduct has been dubious, but that is not how his colleagues remember him. “Fun,” is what Brittany Kaiser, a former associate who blew the whistle on Nix, said, looking back with regret.

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Jolliness is close to godliness. Knowing one’s stuff is the preserve of Americans and bores (though not, obviously, the current president of the United States). The cleverer and better bred one is, the less one gives the appearance of care. Taken to its natural conclusion, this tone assumes the form of a moral sinkhole of a man who has never found a situation so serious he can bestir himself to be adequately briefed on it, as his record at the Foreign Office suggests.

Johnson has now ruined this mode for the rest of us. This week I left something to the last minute and was consoling myself by boasting about my brilliance in the face of bad time management. It wasn’t fun or funny, and it didn’t sound particularly brilliant. Maybe it’s time for the rise of a new earnestness.

• Emma Brockes is a Guardian columnist based in New York