There's no credible evidence behind the theory that autism is triggered by the measles-mumps-rubella (MMR) vaccine, scientists have concluded after reviewing 31 studies, many of which they found flawed by unreliable reports of outcomes, incomplete descriptions of the children studied, and other sources of possible bias. And those were the good studies–the researchers tossed out almost 5,000 others with even more blatant defects.

The report was released this week by the Cochrane Library, an international, independent consortium of scientists. The group regularly and rigorously evaluates medical research to determine the effects of treatments. This time, they tackled the MMR-autism link, one fueled by a recent book by lawyer Robert F. Kennedy Jr., recurrent congressional hearings by Rep. Dan Burton, an Indiana Republican, thousands of worried parents, an undisputed rise in the number of autism cases over the past decade, and one disputed 1998 medical journal article claiming that mercury in early childhood vaccines–it's not in the MMR shot–is a toxin that can alter a child's immune system and set the stage for a neurological problem.

But it's a notion based on unreliable research, says Vittorio Demicheli of the Servizo Sovrazonale di Epidemiologia in Italy, the new report's lead author. Even the 31 studies of vaccine effects that were relatively free of bias did not point to any link. Nor was there any sound connection to Crohn's disease, a digestive disorder also sometimes blamed on the vaccine.

But possible defects contaminated so much of the research that the authors end their report with a scolding for the medical research community, saying that studies were so sloppy they could barely prove MMR vaccines prevented their targeted diseases–although, they are quick to point out, the fact that mass immunization has coincided with mass elimination of these diseases in many, many countries makes it hard to argue that vaccines don't work.

Better studies is also the rallying cry from autism advocacy groups, who don't think distrust of the vaccine is going to go away.

"The medical establishment relies too much on epidemiological studies, and what's really needed are reviews of the biological systems impacted by vaccines," says Peter Bell, CEO of Cure Autism Now. "That's one big reason these questions about vaccines persist."

Bell also notes that such studies of vaccine biology could explain–as either cause or coincidence–why some parents have noted that autism symptoms appear soon after their kids get an MMR shot. "Doctors have always dismissed this," he says. "But usually parents' instincts that something might be wrong are pretty good ones."