You probably saw the story about the lost wallets, which shone briefly and hopefully last month through the fog of our depressing moment in history. To recap, researchers left 17,000 wallets in 355 cities around the world, some containing large or small amounts of money – and contrary to expectations, people proved more likely to contact the owner when cash was involved. Better still, while 51% of people reported wallets containing about £10 (compared to 40% for empty wallets), the rate rose to 72% when the sum was £75. True, there were national differences in honesty – annoyingly, this is another area where the Scandinavians surpass the Brits – but the headline was clear: the death of human decency had been wildly exaggerated. What kind of grouchy misanthrope could possibly find something negative in that result? The answer, I regret to inform you, is: whatever kind of grouchy misanthrope I am.

To be clear, should you find a lost wallet, I think it’s best to return it. But a second, survey-based phase of the study suggests that kindness and altruism weren’t the only motivations involved; another major factor was “an aversion to viewing oneself as a thief”. People want to feel morally good about themselves – which explains the higher rate at which wallets with greater sums of money were reported, because you’re more of a thief if you keep £75 than £10. As one of the researchers put it, “Keeping a found wallet means having to adapt one’s self-image, which comes with psychological costs.” If you’re still wondering why there might be anything bad about people being deeply invested in thinking of themselves as moral, I can only respond: have you spent five minutes on Twitter recently?

The question is rhetorical (obviously nobody should spend five minutes on Twitter, ever). But surely one central reason for the rancour of modern political discussion is precisely this desire to maintain a moral self-image. That’s what’s inaccurate in the rightwing accusation of “virtue signalling”, which implies that the world – online, at least – is full of cynics pretending to subscribe to fashionable dogmas in order to make themselves popular. But it’s also what’s wrong with the leftwing tendency to assume that anybody who disagrees with you is secretly revelling in being evil. In fact, as the psychologists Jillian Jordan and David Rand have shown, the strategic and sincere aspects of morality can’t be so easily disentangled. People “signal” their virtue even when there’s nobody around – and they genuinely feel the virtue that motivates the signal.

All of which is a reminder that politics, and other zones of disagreement, are vastly trickier than many of us would like to admit. Arguments are rarely won by calling out the other side’s insincerity, because they’re not being insincere in the first place. And the same urge to feel like an upstanding citizen that motivates someone to report a lost wallet might be what motivates them to support positions you detest. Maybe the lost wallet study does show that people still strive to act decently. But we shouldn’t forget that, beyond the basics of not stealing each other’s cash, we still can’t agree on what decency is .

Listen to this

We don’t believe what we believe for the reasons we believe, Daniel Kahneman explains in an episode of the On Being podcast.

oliver.burkeman@theguardian.com