Jeremy Paxman has been encouraging people to throw rubbish out of their car windows. He didn’t mean to. He didn’t want to. But he did.

Yesterday morning the Newsnight presenter produced an article in The Guardian bemoaning the “uglification” of Britain. He rejected the assertion of the Keep Britain Tidy campaign that “litter levels in England have fallen to a five-year low”. “How can they claim the country is so clean,” asks Paxman “when the evidence of our eyes suggests quite the reverse?”

The TV man conducted an informal survey on a quiet stretch of country road. He hadn’t gone 500 yards before counting 100 pieces of rubbish. “Most — sandwich wrappers, McDonald’s bags, crisp packets and endless plastic bottles — had been deliberately jettisoned.”

How can I argue that this passionate and in many ways highly admirable attack on littering encourages people to litter? Let me tell you a story.

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Actually it’s not my story. It was told to the Prime Minister’s advisers by the social psychologist Professor Robert Cialdini when he went to 10 Downing Street recently to discuss environmental issues.

One of the professor’s students visited the Petrified Forest National Park in Arizona with his fiancée, a notably honest woman, someone who wouldn’t borrow a paperclip without returning it. As they entered, the couple encountered a sign cautioning against stealing petrified wood. “Our heritage is being vandalised by the theft of 14 tons of wood every year.” The fiancée’s reaction was quite unexpected. “We’d better get ours now,” she whispered.

Unwittingly the sign provided visitors with two pieces of information that made them more likely to steal wood. The first was that the forest was being depleted rapidly, wood was running out, you better get a move on. They may as well have put up a sign reading: “Hurry now, while stocks last.” Nothing moves goods quite as rapidly as the idea that the product is scarce, as any retailer will tell you.

The other information provided by the sign was that it was quite normal to steal wood. Lots of people steal wood, it’s commonplace, go on, you’ll not be different from the rest.

Information about social norms — how other people behave — is an extremely powerful influence on behaviour. It’s the reason why bandwagons get going in by-elections. And the information need not be accurate to alter people’s conduct. Less than 3 per cent of the park’s visitors had ever stolen wood, contrary to the impression given by the sign.

So when the Paxman article appeared, he doubtless hoped that we would be shamed into tidier ways. But, sad to report, the attitude of many of his readers will be to open their windows and toss out some more rubbish. I’ve always been a tidy person, they’ll think, but I read a piece in the paper by that clever bloke off of University Challenge that says that these days no one else is bothered much with tidiness. I don’t see why I should go all the way to the bin, I’ll just drop my Twix wrapper on the pavement like all the rest do.

The Keep Britain Tidy campaign leads its website with this claim: “Half of us boast impeccable habits.” This may be impossibly optimistic for your tastes, but it certainly demonstrates a solid grasp of the principles of social psychology.

Almost every day in the media there is a Paxman-type story — an attempt to persuade people to behave differently by telling us all how bad things are getting. Over the past fortnight, for example, there have been countless articles about the decline in marriage. And every one of them encourages a further decline. If you wanted to increase marriage rates you would be emphasising how usual it is to get married, how despite all you’ve heard it’s still the norm. People like you get married and stay together, that’s the message you want people to hear. If you make deserting your children seem like a normal thing to do, more will do it. Same with drink-driving, shoplifting, drug-taking, gang membership, whatever.

Last week I called myself a social responsibility militant, picking up a phrase of David Cameron’s that describes his policy of altering behaviour through persuasion rather than the law. I argued that laws are often ineffective. There is a wealth of data showing that if you, say, make wearing a seat belt compulsory, drivers buckle up before speeding up and killing others. Persuade drivers to take safety seriously and you may get somewhere.

The petrified forest story and the example of Jeremy Paxman’s article show why, despite all the data on the clumsiness of the law, politicians continue to prefer legislative initiatives. It’s because legislating is so much easier.

There is a correct criticism of Mr Cameron’s idea of using the bully pulpit provided by political office to encourage responsible behaviour. But it is not the common one — that it is the soft option, the policy of a man with no policy. The correct criticism is that it is an incredibly difficult path to tread. And the correct question is whether Cameron’s Conservatives are really going to be able to tread it.

It’s hard to craft a message that succeeds in altering behaviour in the direction you want. It’s easy to go wrong. A message that works (announcing that everyone seems to be getting married these days, for instance) may seem complacent to your supporters and you will get little credit for any success.

And it is not just crafting the message that is hard — it is finding the right messengers. The social responsibility agenda requires a Tory party sufficiently diverse that when it says “people like us behave responsibly” it is talking about more than a narrow group. And it requires ministers to behave responsibly themselves, to avoid the charge of hypocrisy.

David Cameron’s bully pulpit idea will be a hard taskmaster. Too hard? I wonder.

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