Image caption A family on a liner off Gibraltar by Charles Pears, promoting the Empire Marketing Board

Propaganda has a bad name. It is what repressive regimes use to glorify their leaders, motivate their citizens and demonise their enemies. It is about lies and distortion, manipulation and misrepresentation.

But it is also, according to the British Library, about alerting people to the risks of disease, about making sure children learn how to cross the road safely and about building a perfectly legitimate sense of common purpose among the citizens of a democracy.

The library's new exhibition, Propaganda: Power and Persuasion, takes a broad definition of its subject.

Ian Cooke, the exhibition's co-curator, defines it as any communication designed to change the way we think or to alter our behaviour.

Media playback is unsupported on your device Media caption Watch three examples of historical public information films

So the exhibition encompasses public information films like the 1970s Green Cross Code road campaigns, featuring footballer Kevin Keegan and actor David Prowse (Darth Vader in the original Star Wars trilogy), and the campaign alerting Britons to the dangers of Aids, voiced by John Hurt, and those "coughs and sneezes spread diseases" posters which have been around for decades.

'A tool'

And it includes banknotes, postage stamps, a display of model Eiffel Towers and Nelson's Columns and a huge photographic blow-up of Mount Rushmore, illustrating the ways states use iconography and subliminal propaganda to brand themselves and promote a sense of belonging and common purpose among the citizenry.

From 1930s Britain there is a spectacular poster of a family aboard a liner off Gibraltar by Charles Pears, promoting the Empire Marketing Board.

Image caption An American war bond stamp issued to raise funds during World War I

"You can think of propaganda as really just a tool, something that's quite neutral, and used by all states," says Mr Cooke.

"One argument is that in democracies you need propaganda more because you have to govern by consent, so you need to bring public opinion with you... so the comparison with more ideologically-driven, authoritarian, or totalitarian states doesn't always have to apply."

But inevitably, much of the content is less benign.

There is what is said to be the world's most reproduced painting, Chairman Mao Goes to Anyuan, declared a model of Cultural Revolution art in 1967.

It was painted when Mao was 74, but shows him as a young man striding atop a cloud-shrouded mountain to win a miner's strike in 1922. A similar painting from 1949 shows the young Joseph Stalin reading a Georgian epic poem. Mountains feature in that one too.

In the same vein there is a spectacular life-size portrait of Napoleon, in laurel wreath and coronation robes, painted to hang in the council chamber at Montpellier shortly before the emperor's fall. It was returned a year later to the artist, the bill unpaid.

And there is a repellent anti-Semitic poster from Nazi-occupied France, with the legend "Derriere tout: le Juif".

Nuclear advice

Propaganda has a habit of dating badly. One of the most entertaining but also chilling exhibits is a 1950s film called "Duck and Cover", featuring an animated character called Bert the Turtle and a jaunty little song, which sets out to tell American schoolchildren what to do in the event of a nuclear attack - crawl under the schoolroom desk and cover their heads with their hands, apparently.

The art of propaganda is not telling lies but rather seeing the truth you require and giving it mixed up with some truths the audience wants to hear Richard Crossman, Politician

Propaganda is often at its least subtle at times of conflict.

Some of the most direct and compelling exhibits come from the two world wars, including James Montgomery Flagg's 1917 recruiting poster, "I Want You for US Army", in which Uncle Sam, with white top hat and penetrating stare, points directly at the viewer; and a British fund-raising poster from 1915 showing a hapless German soldier felled by an enormous British coin, "Lend Your Five Shillings to Your Country... and Crush the Germans."

But even in wartime propagandists are capable of a lighter touch. Two of the most surprising exhibits are a pair of fashionable women's headscarves from World War II, one showing a map of London bomb-sites with the slogan "London can take it again", recycling the US broadcaster Ed Murrow's famous phrase from the Blitz of 1940 to sustain morale and rally support for the war in the face of a renewed bombing campaign.

The best propaganda, the exhibition implies, is that which distorts least.

It opens with a selection of quotations.

"The art of propaganda is not telling lies but rather seeing the truth you require and giving it mixed up with some truths the audience wants to hear," according to Labour politician Richard Crossman.

Image caption The exhibition runs until 17 September

"The propagandist is a man who canalizes an already existing stream. In a land where there is no water, he digs in vain," according to author Aldous Huxley.

And in one display case is a typewritten memo from newspaper baron Lord Northcliffe, a leading British propagandist of World War I, whose rules for successful propaganda include "What can be done by open means must not be done by occult means", "Lies are the least effective form of propaganda" and "Propaganda that looks like propaganda is third-rate propaganda".

The exhibition, which runs until 17 September, ends with an installation showing the torrent of twitter messages inspired by events like the London 2012 Olympics and Barack Obama's inauguration.

'Fake grassroots'

Classic propaganda involves Them telling Us what to think or how to behave. So can it survive in a world of social media, which is supposedly all about us telling each other spontaneously what we think? Apparently it can.

Image caption An anti-Semitic poster from Nazi-occupied France

Eliane Glaser wrote Get Real, a book about communications in the modern world. She says governments and politicians use the informality of social media to deliver a carefully-planned message.

"They emphasise the role of direct engagement with consumers taking part in the marketing process, and they talk of two-way communications with consumers, which sounds very egalitarian," she says.

"But what it really means is that consumers are now circulating entertaining viral adverts on Facebook and companies are using 'Astroturf' or fake grassroots techniques to create the appearance of a broad base of support for their product or message, and I think these Astroturf techniques are becoming endemic both in politics and marketing."

So when you were tweeting enthusiastically during the televised opening ceremony of the London 2012 Olympics, it seems that you were in reality complicit in a piece of propaganda designed to make us all feel good about ourselves and our country.