A vicious head-butt by one of soccer's greatest players. Thuggish behavior by hockey and basketball stars. Allegations, proven or rumored, of doping-related cheating by many of the world's best runners, swimmers, baseball players — and now by the much-hailed winner of this year's Tour de France.
       ''It becomes an erosion of trust that can be devastating to fans, especially to kids,'' said New York-based sports psychologist Stanley Teitelbaum. ''What they're learning is, 'Don't trust the player. Don't trust the sport.'''
       When American Floyd Landis won the Tour last weekend after a comeback in the final mountain stage depicted as perhaps the event's greatest all-time feat, it seemed a cause for unadulterated celebration — a triumph against all odds by a plucky, likable cyclist who overcame severe chronic pain in his weakening hip to reach the peak of his sport.
       Within three days, he was suspended by his team, and strenuously pleading his innocence in the face of findings that — after that critical mountain stage — he had tested positive for elevated levels of testosterone.
       Landis may yet be vindicated, but the Tour de France will remain saddled with immense image problems in the wake of other doping scandals that kept many of the top cyclists from even entering this year's race.
       Earlier this month, the World Cup — soccer's premier event — ended on a sour note with the ejection of French captain Zinedine Zidane for violently head-butting an Italian defender. Earlier in the tournament, star England forward Wayne Rooney was sent off for stomping on a Portuguese player's groin.
       In North America, stars in both the NHL and NBA have been tarnished by violent outbursts — notably the on-ice assault by Vancouver's Todd Bertuzzi and the charge into the stands by several Indiana Pacers during a game with Detroit.
       In the NFL, training-camp holdouts are a rite of summer, when it seems that every team has at least one player refusing to show up for work because he doesn't like the terms he signed, sometimes as little as one year before.
       But doping — as a form of cheating — is perhaps the sin that troubles fans the most. Even with the Barry Bonds saga unresolved, his legacy seems tarnished for good, along with Mark McGwire, Sammy Sosa, Rafael Palmeiro and other sluggers linked in the public's mind to steroids use.
       In track and field, dozens of world-class athletes — dating back to sprinter Ben Johnson's stripped-away gold medal at the Seoul Olympics in 1988 — have been implicated in doping scandals, or linked to them, including superstars such as Marion Jones. On Saturday came word that reigning Olympic and world 100-meter champion Justin Gatlin had tested positive for testosterone or its precursors after a race in April. The co-world record holder in the 100, Gatlin had positioned himself as a leader in trying to prove track and field is a clean sport.
       The cheating, of course, has seeped into other levels of sport, with overage players infiltrating Little League championships and disabled athletes busted for doping in the Paralympics. But the deepest disappointment seems to come when the biggest stars — the ones many fans view as heroes — prove flawed.
       ''People are disillusioned in all areas of our life, not just sports, but also political leaders, Hollywood personalities,'' said Richard Lustberg, a psychologist who runs the Web site psychologyofsports.com.
       ''But a lot of people find sports more important than politics and pay more attention to it,'' he said. ''We attribute qualities to an athlete that weren't really there in the first place, and then when that athlete turns out to be a mere mortal, we're just crushed.''
       Some commentators contend that no athlete — however successful — should be regarded as a hero solely for on-field performance. Their argument is that hero worship should be reserved for athletes' non-sports achievements — Roberto Clemente dying in a plane crash en route to aid earthquake victims, Ted Williams and Pat Tillman volunteering for military duty they could easily have avoided.
       Nonetheless, there are some contemporary athletes whose achievements and comportment to date elevate them to some sort of hero status — examples might include Roger Federer in tennis, Mariano Rivera and Derek Jeter in baseball, and, perhaps most notably, Tiger Woods.
       For years, Woods has set himself apart as the greatest golfer of his era, perhaps of all-time. Now, after his father's death and his tearful reaction to winning the British Open, respect for his skills is coupled with a new degree of empathy.
       ''That gave me chills, even though I'm not a golf fan,'' said University of Michigan psychologist Christopher Peterson, a sports enthusiast who specializes in the study of character.
       Peterson questioned whether the seeming demise of the sports hero was anything new.
       ''It looks as if things just keep getting worse, but I'm not convinced that's really true,'' he said, suggesting that intensive round-the-clock coverage of sports on the Internet and cable TV as well as in print media has produced more exposures of athletes' misdeeds.
       ''I'm sure Babe Ruth did all sorts of nasty things — and Ty Cobb,'' Peterson said.
       Teitelbaum, author of ''Sports Heroes as Fallen Idols,'' said today's athletes should not be let off the hook.
       ''A lot of heroes don't seem to get it — that their misbehavior has an enormous impact on the fans that idolize them and the kids who are emulating them,'' he said. ''Charles Barkley says he doesn't want to be a role model, but it comes with the territory. Sports stars have to take responsibility for being role models.''
       As the Landis case unfolded, Kansas City Star sports columnist Jason Whitlock was among many commentators suggesting that fans stop placing athletes on a pedestal.
       ''It's unhealthy. It's improper. It's a position they don't want or deserve,'' Whitlock wrote.
       ''They're entertainers. They're no different from Jim Morrison or Justin Timberlake or the movie star who goes in and out of rehab.''
       However, Peter Roby, director of the Center for the Study of Sport in Society at Northeastern University, urged fans not to view sports more harshly than other realms of modern life.
       ''Every day people are accused of cheating. We see it in the media with cases like Enron, Martha Stewart, Tyco, and unfortunately sport,'' he said. ''In all cases the people involved have lost perspective, and stop thinking about the people that rely on them like employees, investors, fans, teammates and family.''