Buzz Aldrin’s reaction to the conspiracy theorist who told him the moon landings never happened was understandable, if not excusable. The astronaut punched him in the face.

Few things in life are more tiresome than engaging with cranks who refuse to accept evidence that disproves their conspiratorial beliefs — even if violence is not the recommended response. It might be easier to dismiss such conspiracy theorists as harmless eccentrics. But while that is tempting, it is in many cases wrong.

As we have seen during the Covid-19 pandemic and in the mob assault on the US Congress last week, conspiracy theories can infect the real world — with lethal effect. Our response to the pandemic will be undermined if the anti-vaxxer movement persuades enough people not to take a vaccine. Democracies will not endure if lots of voters refuse to accept certified election results. We need to rebut unproven conspiracy theories. But how?

The first thing to acknowledge is that scepticism is a virtue and critical scrutiny is essential. Governments and corporations do conspire to do bad things. The powerful must be effectively held to account. The US-led war against Iraq in 2003, to destroy weapons of mass destruction that never existed, is a prime example.

The second is to re-emphasise the importance of experts, while accepting there is sometimes a spectrum of expert opinion. Societies have to base decisions on experts’ views in many fields, such as medicine and climate change, otherwise there is no point in having a debate. Dismissing the views of experts, as Michael Gove famously did during the Brexit referendum campaign, is to erode the foundations of a rational society. No sane passenger would board an aeroplane flown by an unqualified pilot.

In extreme cases, societies may well decide that conspiracy theories are so harmful that they must suppress them. In Germany, for example, Holocaust denial is a crime. Social media platforms that do not delete such content within 24 hours of it being flagged are fined.

In Sweden, the government is even establishing a national psychological defence agency to combat disinformation. A study published this week by the Oxford Internet Institute found “computational propaganda” is now being spread in 81 countries.

Viewing conspiracy theories as political propaganda is the most useful way to understand them, according to Quassim Cassam, a philosophy professor at Warwick university who has written a book on the subject. In his view, many conspiracy theories support an implicit or explicit ideological goal: opposition to gun control, anti-Semitism or hostility to the federal government, for example. What matters to the conspiracy theorists is not whether their theories are true, but whether they are seductive.

So, as with propaganda, conspiracy theories must be as relentlessly opposed as they are propagated.

That poses a particular problem when someone as powerful as the US president is the one shouting the theories. Amid huge controversy, Twitter and Facebook have suspended Donald Trump’s accounts. But Prof Cassam says: “Trump is a mega disinformation factory. You can de-platform him and address the supply side. But you still need to address the demand side.”

On that front, schools and universities should do more to help students discriminate fact from fiction. Behavioural scientists say it is more effective to “pre-bunk” a conspiracy theory — by enabling people to dismiss it immediately — than debunk it later. But debunking serves a purpose, too.

As of 2019, there were 188 fact-checking sites in more than 60 countries. Their ability to inject facts into any debate can help sway those who are curious about conspiracy theories, even if they cannot convince true believers.

Under intense public pressure, social media platforms are also increasingly filtering out harmful content and nudging users towards credible sources of information, such as medical bodies’ advice on Covid.

Some activists have even argued for “cognitive infiltration” of extremist groups, suggesting that government agents should intervene in online chat rooms to puncture conspiracy theories. That may work in China but is only likely to backfire in western democracies, igniting an explosion of new conspiracy theories.

Ultimately, we cannot reason people out of beliefs that they have not reasoned themselves into. But we can, and should, punish those who profit from harmful irrationality. There is a tried-and-tested method of countering politicians who peddle and exploit conspiracy theories: vote them out of office.