Heard the one about Zimbabwe? A policeman stops a motorist and asks for a donation: terrorists have kidnapped the former Sir Robert Mugabe, and have vowed to soak him in petrol and set him alight if the ransom is not paid.

“How much are other people giving?” the motorist asks.

“On average about two or three litres.” It may not be new, or even funny, but the joke represents one of the few points of light on the dark landscape of Zimbabwe. Mugabe and his thugs have killed off any meaningful election, food shortages are acute, inflation is heading for 1.5 million per cent, but one currency in Zimbabwe is steadily increasing in value - jokes.

Unreported amid the horrors is the growth of underground anti-government humour. Jokes about Mugabe are a crime; anyone saying or writing anything insulting to the Government is liable to be arrested. Yet the jokes are spreading, by text message, e-mail and by word of mouth. The www.nyambo.com website is dedicated to Zimbabwean humour. (“Nyambo” is Shona for “jokes”.) Question: What did Zimbabweans use for light before candles? Answer: Electricity.

There is no sound more terrifying to a tyrant than a collective snigger. “Every joke is a tiny revolution,” George Orwell wrote. The moment of truth for the Romanian tyrant Nicolae Ceausescu came when he looked out over the balcony during a rally in Bucharest and heard not the regimented chanting of a cowed people but the unmistakable susurrus of rebellion, a welling, mocking laughter that signalled the end.

Jokes alone cannot topple dictators, but anti-regime humour is the most subtle form of revolt, the slow erosion of a despot's dignity, a survival mechanism, a cathartic snook cocked at the stupidity, cruelty and hypocrisy of life under the boot.

Autocrats have seldom managed to suppress humour, although most have tried. Satire is banned in North Korea, the world's most humourless land. Earlier this month, police arrested Zarganar, Burma's most famous comedian, who has attracted a wide following by mocking its military rulers. Zarganar had led efforts to distribute humanitarian aid to victims of Cyclone Nargis. Official reports accused “unscrupulous elements” of exaggerating the country's problems.

“Humour,” as Joseph Goebbels remarked, “has its limits.” He was wrong, of course, for humour has no limits, and an uncanny way of seeping through cracks of the most vicious dictatorship.

Iraqis laughed behind their hands at Saddam Hussein, Romanians secretly teased Ceausescu (Why does he hold a May Day rally each year? To see how many people have survived the winter) and the French Revolution was preceded by a spontaneous upsurge of ribald humour at the expense of the monarchy.

Perhaps the most extraordinary proof of how humour can survive and even flourish under oppression is the spread of jokes under Soviet communism. In a fascinating new study entitled Hammer and Tickle, published by Weidenfeld & Nicolson, Ben Lewis explores the wealth of subversive humour during the long, bleak decades of communism.

People gathered, treasured and exchanged jokes, the “music of the oppressed”, in Lewis's words - jokes about the endless shortages, official corruption, and the chasm between official pronouncement and crushing everyday reality: laughter in the face of unhappy truth.

Question: What stage comes between socialism and communism? Answer: Alcoholism.

Humour did not defeat communism, but it helped, chipping away at the plinth of dignity and omniscience on which the entire, ludicrous structure was perched. Ronald Reagan used to insist on telling anti-Soviet jokes to Mikhail Gorbachev at every meeting, to make a point - that the jokes made about him did not threaten the entire political system.

Party bosses understood the danger, and attempted to co-opt humour itself. Stalin encouraged jokes about Trotsky. Soviet ideologues invented “positive humour”, a genre designed to emphasise the virtues of communism, and hilariously unfunny.

The most chilling moment in The Lives of Others, the brilliant 2007 film about East Germany's surveillance society, comes when a Stasi boss overhears a young underling telling a mild joke about Erich Honecker: he bids him repeat the joke, laughs heartily and then takes down his name and rank.

Perhaps the same sort of thinking lies behind Robert Mugabe's amusing dress sense - wear a ludicrous shirt and see who dares laugh, the Emperor's New Clothes in reverse.

Hitler authorised a book of cartoons in 1933 respectfully satirising himself, apparently in the belief that if humour was tolerated, up to a point, it might be controlled.

A thin but resilient vein of humour persisted, even in the death camps, where a mordant Jewish wit survived. What is the difference between a Jewish optimist and Jewish pessimist: Jewish pessimists are all in exile; Jewish optimists are all in concentration camps.

In July 1944 Father Josef Möller was sentenced to hang by a Nazi court for “one of the most vile and dangerous attacks directed at our confidence in our Führer”.

He had told two parishioners this joke. A fatally wounded German soldier asked his chaplain to grant a final wish: “Place a picture of Hitler on one side of me, and a picture of Goering on the other side; that way I can die like Jesus - between two criminals.”

Möller's last joke, Holocaust humour, the Soviets mocking their own plight and the thousands of Zimbabweans exchanging grim laughter in the face of brutality - these mark the strange point in history where courage and comedy combine. The very best jokes do not just make us laugh.

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