Picture an American president. Young and handsome, he’s atop the political heap with a smart wife and an adored daughter by his side, but the reports, both true and rumoured, of his hypersexuality long ago became part of his public persona.

Now imagine that a government shutdown requires most paid White House staff to go home, forcing high government officials into contact with unpaid – and nubile – interns. Now visualise a curvy young intern striking up a romance with the ‘Leader of the Free World’. Soon, he’s receiving her oral favours while on the phone with congressmen, and she’s putting on a show for him involving a cigar and a navy blue Gap dress.

If this ever came out, if he lied about it to the press or, God forbid, under oath in court, he might even face criminal problems. A politician could not survive this, right? He’d probably have to resign lest he drag his party down with him.

You know how President Bill Clinton’s affair with a White House intern drove the Republican Congress to impeach him. What remains poorly remembered is that Republicans did not profit at the ballot box in the next election. Despite predictions that the moral crusade against Clinton would depress his party’s turnout, Democrats gained five seats in the House of Representatives and held in the Senate. Meanwhile, Clinton ended that year with a 78 per cent job approval rating.

More than a decade later, the lessons of Monica-gate are clear: what sells tabloids does not necessarily move voters. Nevertheless, few in the US have mastered the correct way to employ negative attacks in politics. As a rule, voters focus on details that will inform their choice at the ballot box, while your average political insider will fixate on a photograph of a Congressman’s staffer.

This makes intuitive sense. The details of a juicy political scandal stick in one’s memory a generation later. In 1997, political scientist Bruce Pinkelton looked at this phenomenon and called such details “emotionally arousing”. He found numerous studies about non-political advertising, showing that consumers give more weight to negative information about a choice than they do to positive information. Basically, negative details are easier to remember.

There are a million examples of this in recent history, not least the tampon as which Prince Charles said he wished to be reincarnated. But the stickiness of certain salacious details in the memory does not explain why a sex scandal is more likely to force a politician to resign than to be defeated for re-election.

So, what works? When it comes to sorting the wheat from the chaff, US voters have proven themselves more discerning than the press. The press might pant over a governor’s love affair with his mistress, but voters instinctively evaluate negative attacks on three criteria: credibility, relevance and tone.

That’s why the ludicrous rumours surrounding Barack Obama’s place of birth were pervasive but unable to stop him from winning the presidency. Billboards sprang up in middle America, demanding that the president release his birth certificate. At least 24 Congress members offered legislation requiring presidential candidates to prove their citizenship before taking office. It was mass hysteria – but only within the confines of the Republican Party.

Tone is subjective, but the racist undertones were clear, from posters depicting Obama with African face decorations to the overwhelming impression that these were white people questioning the legitimacy and basic ‘American-ness’ of a black man. It became hateful and personal, repellent to anyone who did not already share the ‘birthers’ viewpoint. The racist tone also undercut their credibility.

Taken at face value, the question of whether Obama was born in Kenya met the relevance standard – the US Constitution requires presidents to be native-born – but the above factor and others blunted the effectiveness of this attack with a wider audience.

In 1998, professors Paul Freedman and Dale Lawton at the Sorensen Institute for Political Leadership at the University of Virginia set out to study exactly what kinds of attacks voters find relevant and what they don’t take into account at the ballot box.

They grouped typical criticisms into three categories: fair, moderate and unfair. At least two-thirds of respondents agreed that the ‘fair’ attacks were fair, and up to two-thirds deemed the unfair ones irrelevant. Everything in the middle was ‘moderate’.

Topics that respondents highlighted as ‘unfair’ were: behaviour of family members (only 8 per cent thought this was fair censure), personal lives of party leaders (19 per cent), past personal troubles (26 per cent) and past extramarital affairs (28 per cent). These scandals have sex appeal, but do not engage voters in a constructive way.

In the middle category, only 37 per cent thought political actions of the party’s leaders was fair. Current extramarital affairs drew 45 per cent and current personal troubles got 56 per cent.

Here’s the gold standard of political attacks as far as public perception is concerned: taking money from special interests (71 per cent); voting record (71 per cent); business record (76 per cent); and, at 81 per cent, talking one way and voting another.

But you don’t need the eggheads to tell you what makes some attacks stick and some slide off the TV screen. From Obama defeating birthers to Clinton surviving his impeachment hearings, stupid, personal, irrelevant condemnation has proven ineffective at changing voters’ minds.

One incredibly ill-informed truism that persists is that political rhetoric tends toward negativity because political attacks work. In fact, they only work when you’re speaking reasonably and credibly about what voters actually want to discuss. Because if you’re not doing that, you’re spreading rumours. And while that might boost your TV ratings, it doesn’t win elections.

Case studies

MyDavidCameron.com – The image manipulation website that mocked Conservative 2010 campaign posters went viral, even penetrating mainstream news. The site’s popularity meant newer, Photoshop-unfriendly posters were released by the Conservatives.

Lib Dem ‘VAT Bombshells’ – Mirroring the 1992 Conservative ‘Labour Bombshell’ posters, the blunt attack that the Tories would raise VAT by £13.4bn is still used against Nick Clegg for raising VAT when in power.

Anti-Gordon Brown – Saatchi posters – Attacking Brown directly with “I took millions from pensions…” posters showed that Conservatives believed he was Labour’s weakest link. While parodied extensively, it highlighted voters’ reluctance to support Brown.

No to AV ‘dead baby’ – A crying baby with the “She needs a maternity ward, not AV” tagline suggested that money spent on reform drained resources directly from NHS services. The poster received numerous complaints, but was ultimately successful.

Nick Griffin for ‘Yes to AV’ – The Yes to AV campaign took a different approach by naming Nick Griffin, someone most voters disliked, as a supporter of No to AV. The poster declared: “Say no to the BNP. Vote Yes on May 5th.”