Why rape victims need flawless pasts

The surprising thing about the case of Dominique Strauss-Kahn and the alleged sexual assault of a hotel chambermaid isn't that all charges have been dropped. It's that he was arrested and charged in the first place, given how unlikely it was that he would be convicted. Even if DSK hadn't been one of the world's most powerful men, the chances of his being found guilty and going to prison were always low, as they are for most men who find themselves accused of rape or sexual assault.

Nationwide figures for the US are hard to track down but an analysis of Department of Justice statistics by two of the country's leading sexual assault experts in 2009 found that conviction rates hadn't improved since the 1970s; their study suggested that only 2 per cent of rapes reported to police in the US ended in a defendant being sent to jail. Across a number of countries, a rising number of reported rapes has not led to a corresponding increase in convictions.

There is no mystery about this. The hunt for the ideal rape victim is never-ending but fruitless, for the simple reason that it requires unimpeachable conduct on the part of the victim in every area of her life, past and present. Women who have been drinking, who know their alleged attacker or who've ever told a lie to a public official, even about an unrelated matter, are not victims that prosecutors want to put before juries.

DSK's semen on Nafissatou Diallo's uniform and the carpet of his suite proved beyond doubt that a sexual encounter had taken place, while medical evidence and Diallo's evident distress appeared to support her claim that it wasn't consensual. On Monday, when Manhattan district attorney, Cyrus Vance, asked a judge to dismiss the charges, he used an ambiguous formulation to explain his change of heart: "The nature and number of the complainant's falsehoods leave us unable to credit her version of events beyond a reasonable doubt, whatever the truth may be about the encounter between the complainant and the defendant [my italics]." This is an absolutely classic outcome, signalling not the vindication of the defendant but the prosecution's judgement that the accuser would not make a good witness.

Vance's words suggest that his decision was based not on Diallo's account of the alleged assault, but lies she told when she arrived from Guinea and claimed asylum in the US. Such behaviour is not uncommon when would-be immigrants are trying to improve their chances of being allowed to stay; Ayaan Hirsi Ali, the Somali-born Dutch MP, resigned from Parliament five years ago after admitting that she lied on her application for asylum in the Netherlands. She later moved to the US where her views are treated with respect.

If Diallo's persuasive account of a violent sexual assault is to be dismissed because she lied to get into the US, the implications for other immigrants are alarming. Are prosecutors really saying that anyone who has lied on an asylum application cannot be considered a credible witness in an unrelated matter, no matter how many years later and regardless of forensic evidence supporting their claims?

This is surely setting the bar too high, as well as sending a message that some potential victims cannot expect the protection of the law. It's well known that sexual predators often choose their victims with care, selecting girls and women who are "vulnerable" in some way – black, poor, working-class – and likely to make less convincing witnesses.

In the case of DSK, there is also a nagging question of double standards. Fairness demands that his past conduct should also be examined with equally rigorous standards, and the picture that's emerged is far from edifying. The French writer Tristane Banon has given a graphic account of what she claims was an attempt by DSK to rape her when she went to interview him in 2003. It is possible that DSK is the victim of a truly dreadful coincidence, becoming the victim of slander by women who don't know each other on two continents. But it is also possible that he's a sexual predator who targets women who are reluctant to report him or unlikely to make a good impression on a jury.