Police stopped a man on the street last month because he fitted the description of a serial rapist who had been terrorising the Harlem district of New York. Vincent Heyward, 21, agreed to provide a DNA swab that was later found to link him to all four rapes in the area between August 1 and September 7, and he turned himself in.

The increasing use of DNA technology has helped to drive reported rapes in America to a 20-year low. Latest FBI figures estimate that 89,000 women reported being raped in 2008 — a rate of 29 women for every 100,000. That compares with the 1992 peak of 109,062, or 43 women in 100,000.

“I think DNA is definitely a big contributor to the drop in rape,” said Scott Berkowitz, the president of the Rape, Abuse & Incest National Network, a victims group.

"It makes it easier to catch criminals and it makes it easier to catch them earlier in their careers before they can rape many women," he said. "Many rapists are career criminals who are not just committing rapes."

The FBI has begun collecting samples from suspects awaiting trial rather than after conviction — a practice that has been adopted by 15 states — with its database of 6.7 million profiles expected to increase by about 1.2 million a year by 2012 from the present 80,000.

The practice has provoked protests from privacy advocatesand Human Rights Watch has estimated that there is a backlog of some 400,000 cases of evidence waiting to be tested.

Congress passed the so-called Debbie Smith Act in 2004 to provide funds to clear the blacklog. The legislation was named after a Virginia woman who was raped in 1989, but the rapist remained at large because his DNA went untested for six years.

Congresswoman Carolyn Maloney led a recent effort to ensure adequate funds were made available.

"Every unprocessed rape kit represents a victim who has been denied justice and a predator who remains at large," she said. "I was proud to author the original Debbie Smith Act, which has been called the most important anti-rape legislation ever passed by Congress."

DNA evidence is particularly useful in combating rapes, where the perpetrator inevitably comes in close contact with his victim.That means that rape trials no longer become a simple case of “he said, she said” — once a factor that deterred prosecutors from bringing accused rapists to court.

Mr Berkowitz doubts that would-be rapists are deterred by the prospect of DNA testing, but he said there was evidence that rapists have changed their behaviour by using condoms or taking other steps.

Many rapists, however, do not realise that DNA can be obtained from hair or skin left on a victim as well as semen. “There have been all kinds of cases where they have tried to clean up after themselves, but ended up leaving DNA evidence anyway,” Mr Berkowitz said.

Mr Heywood, who had no prior arrests, has been charged with approaching four women from behind and robbing them of small sums before raping them.

He has also been tied to the attack on a 24-year-old woman in the SoHo are of New York after she punched him in the nose. DNA collected from blood splatters on the wall reportedly matched his genetic profile.

He has denied the four Harlem rapes on women ranging in age from 23 to 69. In a court appearance he tried to run out of court before being tackled by court officers.

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