Beyond ethics, it would seem to defy common sense for cyclists to break the rules in the middle of such a crackdown.

The lure of fame and fortune — embodied in the yellow jersey — are amazingly powerful, of course. But doping and cycling experts also say the cheating goes on because of a general disdain among riders and teams for the drug-testing process.

There's the wink-wink encouragement of team managers, and no one to help them say "no." Many believe there are plenty of loopholes in the anti-doping rules, and there's a notion that technology is advancing so fast that drug testing can't keep up.

Some scoff at the potential ill-effects of performance enhancers. Throw in the stresses of training for one of the world's most demanding athletic events, and you've got an atmosphere conducive to cheating.

"They think they can get away with it," said Dr. Ramsus Damsgaard, a leading Danish doping expert. "Whenever anybody presents them with a new substance, they feel comfortable using it."

The credibility of cycling's marquee event has once more been tarnished by doping cheats this year: Three riders were kicked out after testing positive for the banned blood booster EPO. Two of those — promising young rider Riccardo Ricco of Italy and Moises Duenas Nevado of Spain — spent a night in jail for police questioning.

The cases are yet another black eye for a sport damaged by Floyd Landis' positive test for testosterone in the 2006 Tour, depriving the American of his title, and an array of doping busts last year.

The dragnet has gotten tighter. France has enacted a law that makes possession of doping products illegal. The country's anti-doping agency is conducting checks at the Tour — not the International Cycling Union as in years past. The agency's chaperones escort eight riders, including the stage winner and overall leader, to doping controls after every stage.

The riders who tested positive "thought the tests wouldn't be rigorous. They got it wrong," agency chief Pierre Bordry said. "I didn't think they'd be so clumsy. What surprised them is that we did what we said we would."

Damsgaard, who wants new testing protocols, said the World Anti-Doping Agency rules are too conservative about EPO tests, and that telltale signs of doping are often passed over when samples go under the microscope. As a result, some riders believe they can squeak by — and sometimes they're right.

The temptation of EPO, cycling's designer drug, can be enormous.

"The fact is that the products today work well: EPO can increase the body's ability to absorb oxygen by 10 to 15 percent," said Alain Groslambert, a sports psychologist who works with the French Cycling Federation.

But it's not without risks. He pointed to a study by a colleague at the University of Franche-Comte in eastern France that showed cardiovascular problems in mice and rats that were injected repeatedly with EPO in doses that would be a rough proportional equivalent to what an athlete might use.

"When we tell young athletes aged 22, 23, or 24 that they are putting their health in danger (by using performance enhancers), they laugh," Groslambert said.

Brian Gilley, an assistant professor of anthropology at the University of Vermont, agreed. He is studying attitudes on doping among cyclists under 23 — many from the United States, France, Italy or Belgium — on a WADA grant.

Among his U.S. participants, "about 20 percent said, 'Oh, I would dope if I could be on a ProTour team,' or 'I would dope if I could win the Tour de France,'" Gilley said.

Gilley said young riders face huge pressures to dope, and their decisions often depend on their upbringing, class and their career alternatives to cycling.

"It's almost as if they're saying success in the sport, given the pressures, is impossible without doping," he said.

Gilley says the "institutions" of cycling — the ICU, Tour organizers, and teams — deserve some of the blame. They benefit from dazzling rider performances and take the high moral ground when cheats are caught.

"If Ricco went uncaught, it would be, 'We saw this rider flower at the Tour de France. ... This is where heroes are made,'" said Gilley. "But then, when he gets caught, they get to say, 'we're doing all these things right.'"

Tour organizers and UCI officials have trumpeted the positive doping tests this year as a sign the sport is serious about revealing cheats.

There's also pressure to please fans, said Dorian Martinez, who runs the association Ecoute Dopage_ a telephone hotline set up in France nine years ago to help athletes avoid doping and navigate complex sports rules.

For cyclists, doping is more often tied to recovering from a punishing season stocked with races, he said. "Not to go faster, but to recover faster — and to exist" in the public eye, he said.

In the Tour's earliest days, riders juiced up on cocaine, wine, even strychnine, to get a lift in the nearly inhuman three-week race. The sport's culture has historically fostered use of banned pick-me-ups. And since the old days, the racing calendar has grown, putting cyclists through even more strain.

"We don't accept that champions can have a moment of weakness, which could push them to dope," Martinez said. The demand to excel is relentless, making a one-time decision to use drugs unlikely.

Some of the least helpful people to talk to about doping are current and former riders. There's long been a code of silence about the topic.

Milram rider Erik Zabel admitted last year that he tried EPO in the first week of the 1996 Tour, but never again. The German rider, who won six green jerseys as the Tour's best sprinter, refused to speak to an Associated Press reporter.

Team CSC owner Bjarne Riis, meanwhile, has taken to finger-pointing, calling one rider who tested positive this year "a scandal."

Yet Riis admitted last year to having used EPO between 1993 and 1998, including on the way to his 1996 Tour victory. He told the AP he didn't know what motivates today's cheaters.

"Ask them," he said.